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Surprising History of Glowing Fish

Black Dragonfish (Idiacanthus).
Black Dragonfish (Idiacanthus).
Photograph by Matt Davis

During the Cretaceous period, while flowers and tyrant dinosaurs were spreading over the land, and pterosaurs and birds were taking over the skies, in the oceans, fish were starting to glow.

Today, some 1,500 fish species are bioluminescent—able to make their own light. They have luminous fishing lures coming out of their heads, glowing stripes on their flanks, bright goatees dangling from their chins, flashing headlamps beneath their eyes, or radiant bellies that cancel out their silhouettes to predators watching from below.

They evolved their glow in a variety of ways. Some came to generate it on their own, through chemical reactions within their own cells. Others formed partnerships with luminous bacteria, developing organs for housing these microscopic beacons.

Despite the obvious diversity of bioluminescent fish, no one knew exactly how often these animals evolved their self-made light. “We thought it might be a dozen or so just by eyeballing a list,” says Matthew Davis from St. Cloud State University, “but the actual number was considerably higher.”

Together with John Sparks and Leo Smith, Davis built a family tree of ray-finned fish—the group that includes some 99 percent of fish species. By marking out the bioluminescent lineages, they report Wednesday in the journal PLOS ONE that these animals independently evolved their own light at least 27 times.

Illuminated Netdevil (Linophryne arborifera). Credit: Leo Smith
Illuminated Netdevil (Linophryne arborifera). Credit: Leo Smith

Of those 27 origins, 17 involve partnerships with glowing bacteria, which the fish took up from the surrounding water. Deep-sea anglerfishes housed the microbes in their back fins, which they transformed into complex lures. Ponyfishes kept the microbes in their throats, and controlled the light they produced by evolving muscular shutters and translucent windows.

But to Steven Haddock from the Monterery Bay Aquarium Research Institute, these partnerships are fundamentally different from cases where fish evolved their own intrinsic light. “If one species of bacteria evolves the ability to glow, then is eaten and proliferates in the guts of four different fishes, you could argue that bioluminescence evolved once in the bacterium,” he says. “To me, this is much less interesting than fishes that have their own chemical and genetic machinery.”

Those intrinsic light-producers have come to dominate the open oceans. There are around 420 species of dragonfishes, most of which have long bodies and nightmarish faces armed with sharp teeth. They include the bristlemouth, the most common back-boned animal on the planet; hundred of trillions of them lurk in the deep ocean. The lanternfishes are similarly prolific; the 250 or so species account for around 65 percent of the fish in the deep sea by weight. “They’re among the most abundant vertebrates on the planet in terms of mass, but the average person doesn’t know anything about them,” says Davis.

Silver Hatchetfish (Argyropelecus). Credit: Leo Smith
Silver Hatchetfish (Argyropelecus). Credit: Leo Smith

These groups aren’t just diverse, but unexpectedly so. In the relatively short time they’ve been around, they’ve accumulated far more species than is normal, and far more so than lineages that got together with glowing bacteria. Why?

Davis thinks it’s because they can exert greater control over their light. While ponyfishes have to rely on body parts that obscure the continuous glow of their microbes, lanternfishes and dragonfishes can turn their glows on or off, using nerves that feed into their light organs. That means they can flash and pulse. They can use their light not just to lure prey or hide from predators, but to communicate with each other.

Many scientists think that deep-sea fish could use bioluminescence as badges of identity, allowing them to recognises others of their own kind and to mate with partners of the right species. This might also explain why these fish became so extraordinarily diverse, in an open world with no obvious features like mountains or rivers to separate them.

“Biodiversity in the deep sea used to be viewed as somewhat of a paradox given the apparent lack of genetic barriers,” says Edie Widder from the Ocean Research and Conservation Association. Davis’s study hints at an answer. After evolving their own light, some fish may have effectively built luminous towers of Babel—different flashing dialects that split single communities into many factions. (Something similar may have happened among electric fish in the rivers of Africa.)

The same story applies to sharks. They’ve evolved bioluminescent at least twice, and these luminous species account for 12 percent of the 550 or so species of shark. And the groups whose light organs allow them to communicate with each other seem to be exceptionally diverse. As Julien Claes from the Catholic University of Louvain told me last year, “They’re some of the most successful groups of sharks. We discover new ones every couple of years.”

So forget great whites and makos, salmon and tuna, clownfish and angelfish. The most common and diverse fish in the world are the obscure ones that you probably haven’t heard about, swimming somewhere in the open ocean, basking in their own glow.


We Still Don’t Know What Killed the Biggest Shark of All Time

We just can’t let Carcharocles megalodon rest. From Peter Benchley’s JAWS to the dreck that regularly bobs up to the surface of basic cable “science” channels, we can’t seem to resist invoking the specter of a shark so large that it could easily engulf a person without a drop of blood spilled into the sea.

Art by Fernando G. Baptista; Research by Ryan T. Willians, Fanna Gebreyesus; Source: STEPHEN J. GODFREY, CALVERT MARINE MUSEUM
Art by Fernando G. Baptista; Research by Ryan T. Willians, Fanna Gebreyesus; Source: STEPHEN J. GODFREY, CALVERT MARINE MUSEUM

Despite our fascination with this enormous, extinct relative of today’s great white shark, there’s still a great deal we don’t know about the life and death of the biggest shark that ever lived. For starters, we still don’t know why the last of the megatooths died over 2.5 million years ago.

In the entire history of cartilaginous fish, Carcharocles megalodon was a huge success story. And that’s not just because of the predator’s size and inferred ferocity. This species patrolled the coasts of the Atlantic, Pacific, and Indian Oceans for about 20 million years. Few creatures can claim such a record. And that only makes the disappearance of the shark all the more puzzling.

Changes brought on by a cooling climate have been the focus of the traditional explanation for the monstrous shark’s demise. C. megalodon has often been thought of as a warm-water hunter, and so, the argument goes, as sea temperatures dipped at the end of the Pliocene the whales, seals, and other fatty mammals the shark relied upon migrated to chilled seas where the shark couldn’t follow. The pitiful selachian was simply left behind as cetaceans spouted off for the poles.

But was the great shark so restricted by temperature? To find out, paleontologist Catalina Pimiento and colleagues drew from the Paleobiology Database to analyze occurrences of C. megalodon over time in relation to climate.  Contrary to what had previously been thought, temperature probably didn’t freeze the shark into extinction.

Curator Jeff Seigel stands in the five–-foot mouth of a fossil shark jaw. The shark is called Carcharoles Megalodon and was large enough to swallow a small car. Photograph by Rick Meyer, Los Angeles Times, Getty
Curator Jeff Seigel stands in the five–-foot mouth of a fossil shark jaw. The shark is called Carcharoles Megalodon and was large enough to swallow a small car. Photograph by Rick Meyer, Los Angeles Times, Getty

The big picture looks something like this. During the shark’s early years, around 20 million years ago, C. megalodon primarily swam through waters of the northern hemisphere. Populations expanded around 15 million years ago to include every major ocean basin on the planet, the researchers write, but from there the sharks populations steadily declined.

All of this happened irrespective of climate. During times of major temperature spikes and dips, Pimiento and coauthors note, C. megalodon occurrences didn’t seem to show any direct response. Not to mention that the shark seemed fully capable of coping with a range of temperatures from 53 to 80 degrees Fahrenheit, and there have been waters in this range from the shark’s time until today. As Pimiento and coauthors write, “C. megalodon would have not been affected significantly by the temperature changes during the Pleistocene, Holocene and Recent.”

Populations of C. megalodon over time. From Pimiento et al., 2016.
Populations of C. megalodon over time. From Pimiento et al., 2016.

So if it wasn’t cooler waters, what drove the shark to extinction? There’s still no definitive answer. Even today, when we can witness species disappear, it’s often difficult to precisely retrace the road from the vanishing point back to the first signs of trouble. In the case of C. megalodon, though, Pimiento and coauthors have some ideas about possible killswitches.

Through hindsight, we can see that the road to extinction for the megatooth shark started in the middle of the Miocene. This coincided with two major events, as previously pointed out by paleontologist Dana Ehret as well as the authors of the new study. Against a background of crashing whale diversity during this time, the world saw the evolution of some stiff competition for C. megalodon: large sharks close to the ancestry of the great white and sperm whales that behaved and hunted more like today’s orcas. This trend continued only through the Pliocene, with fewer big baleen whales and an increasing array of predators that young megatooth sharks would have struggled against to get enough food down their throats. There was less food to go around for an expanding guild of predators who relied upon warm, blubbery prey.

The case isn’t closed yet, though. So much of what’s known about C. megalodon comes from teeth, the occasional vertebra, and some bite marks. Those pieces only reach so far in revealing the massive shark’s biology, including how much the fish actually relied on filter-feeding whales for food or the other predators it was striving against to survive.

We can be sure the megatooth shark is dead. The fish’s fossil record taps out by 2.5 million years ago, and we surely wouldn’t miss populations of fifty-foot-long sharks patrolling the global coastlines. But why the shark vanished is a secret still waiting to be dredged from the fossil record.


Pimiento, C., MacFadden, B., Clements, C., Varela, S., Jaramillo, C., Velez-Juarbe, J., Silliman, B. 2016. Geographical distribution patterns of Carcharocles megalodon over time reveal clues about extinction mechanisms. Journal of Biogeography. doi: 10.1111/jbi.12754

Paleo Profile: The Purgatoire River’s Whale Fish

Rhinconichthys as restored by Robert Nicholls.
Rhinconichthys as restored by Robert Nicholls.

If you could take a dip in the Cretaceous sea that covered Colorado around 92 million years ago, you might spot what would initially look like a pretty plain fish. Around six feet long, or a comparable to a mid-sized tuna, the streamlined swimmer would have a bullet-like profile. Until it opened its mouth. In one swift motion the long lower jaw would snap open, creating a planktivorous parachute to catch some of the ocean’s smallest morsels.

We know such a fish existed thanks to fossils discovered by paleontologist Bruce Schumacher and described with his colleagues earlier this year. They named the filter-feeder Rhinconichthys purgatoirensis, a new species of a genus that had previously been found in England and Japan. The Colorado species is significantly older than its evolutionary siblings, however, punting Rhinconichthys back in time as well as establishing that these fish were present in the Western Hemisphere through much of the Cretaceous.

Not that Rhinconichthys was the only filter-feeding fish around. Along with other research that has identified and named the giant Bonnerichthys and planktivorous sharks, Schumacher and coauthors point out that there was a wide array of filter-feeding fish throughout Cretaceous time. This is about more than species counts. Where there are big planktivores, there has to be enough plankton for them to eat. Before mantas, before whales, fish such as these were the largest creatures to strain the seas.

The skull of Rhinconichthys purgatoirensis. From Schumacher et al., 2016.
The skull of Rhinconichthys purgatoirensis. From Schumacher et al., 2016.

Fossil Facts

Name: Rhinconichthys purgatoirensis

Meaning: Rhinconichthys is a tribute to an unpublished name coined by 19th century paleontologist Gideon Mantell for specimens of this genus found in England, while purgatoirensis for Colorado’s Purgatoire River drainage where the new species was found.

Age: About 92 million years old.

Where in the world?: Rhinconichthys purgatoirensis was found in eastern Colorado, with other species turning up in England and Japan.

What sort of critter?: A filter-feeder belonging to an extinct group of ray-finned fish called pachycormiformes.

Size: Estimated at over six and a half feet in length.

How much of the creature’s body is known?: Rhinconichthys purgatoirensis is represented by a skull, pectoral girdles, and pectoral fins


Friedman, M., Shimada, K., Martin, L., Everhart, M., Liston, J., Maltese, A., Triebold, M. 2010. 100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas. Science. doi: 10.1126/science.1184743

Schumacher, B., Shimada, K., Liston, J., Maltese, A. 2016. Highly specialized suspension-feeding bony fish Rhinconichthys (Actinopterygii: Pachycormiformes) from the mid-Cretaceous of the United States, England, and Japan. Cretaceous Research. doi: 10.1016/j.cretres.2015.12.017

Previous Paleo Profiles:

The Unfortunate Dragon
The Cross Lizard
The South China Lizard
Zhenyuan Sun’s dragon
The Fascinating Scrap
The Sloth Claw
The Hefty Kangaroo
Mathison’s Fox
Scar Face
The Rain-Maker Lizard
“Lightning Claw”
The Ancient Agama
The Hell-Hound
The Cutting Shears of Kimbeto Wash
The False Moose
“Miss Piggy” the Prehistoric Turtle
Mexico’s “Bird Mimic”
The Greatest Auk
Catalonia’s Little Ape
Pakistan’s Butterfly-Faced Beast
The Head of the Devil
Spain’s Megatoothed Croc
The Smoke Hill Bird
The Vereda Hilarco Beast
The North’s Sailback
Amidala’s Strange Horn
The Northern Mantis Shrimp
Spain’s High-Spined Herbviore
Wucaiwan’s Ornamented Horned Face
Alcide d’Orbigny’s Dawn Beast
The Shield Fortress
The Dragon Thief

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Help! I’m Trapped in a Drop of Water

I am looking at a blob of water. Not pond water. Not pool water. Just ordinary H2O floating about in what appears to be zero gravity. And inside its wetness there’s a passenger, a goldfish, a very alive goldfish …


… that is trying desperately to escape—or so it seems. It flings itself at the blob’s edge, pushing it outward.


Then it tries to get out the back. That’s its head peeping through.


Then it charges and stretches the skin of the bubble almost to the breaking point.


But try as it might, it can’t get free. Poor little fish, I thought. It’s a prisoner.

I found it, the fish and its floating prison, on my favorite fluid dynamics blog, FYFD, which is curated by aerospace engineer Nicole Sharp. She had posted the video version produced by Professor Mark Weislogel and his students at Portland State University in Oregon. They used a “drop tower”— basically, a free-falling elevator—to create near-weightless conditions. So they’re the ones who packed the goldfish into its droplet-y cage. Here it is in full “get me out of here” motion.

What a struggle! As Nicole put it on her blog:

For years, I have wondered what a fish swimming in microgravity would look like. Finally, my curiosity has been rewarded. Here is a sphere of water in microgravity, complete with a fish. Personally, I am impressed that, despite the fish’s best efforts, the surface tension of the water is strong enough to keep it confined. This may not bode well for microgravity swimming pools at space hotels.

Got it. Fifty years from now, when I book my room at the Hotel Galactica a hundred miles from Earth, I’m not bringing a swimsuit. And anyway, who flies to the edge of space to go for a swim? Not me. Not you. We’d go for the view—obviously. So this video shouldn’t have bothered me. But after I saw it once, then again, then yet again, something about it didn’t seem … um … quite right.

EXCUSE ME, but …

What, exactly, is holding that fish inside the blob? Could it be the water? Here on Earth, fish have no trouble breaking through a pond surface to snatch a fly or a bug. Yes, the pond surface is a little resistant due to molecular or surface tension, but a goldfish is stronger than water. It whips its tail, propels itself up, and grabs lunch. No problem.

Is there something about zero gravity that changes that? I called my friend Henry Reich, author/illustrator and physics explainer over at Minute Physics. I showed him the video and asked, Is this fish trying to escape? And if it is, why can’t it get free?

“This fish is thrashing, yes,” he told me on the phone, “but I have a hunch it isn’t trying to escape. I think it’s testing its surroundings …”

Me: What do you mean “testing”?
Henry Reich: Well, I can’t think of any good reason why the fish couldn’t break through, even though it’s at zero gravity.
Me: So the surface isn’t holding it in?
HR: No, I don’t think so, and if you look at its fins and tail, you can see what’s really going on. Up close, you’ll see it’s paddling up to the edge, pushing forward and then, just when it could escape, its peddling back.
Me: You can see that?
HR: Yeah, look back at the video, and watch the fins. And the curled back tail. That’s what you’ll see …

I did. And I’m not sure if I saw what Reich saw. He sent me a video of goldfish swimming backward, and I couldn’t quite tell if my fish was doing what that fish did.

Drawing by Robert Krulwich
Drawing by Robert Krulwich

But just to go along with his notion, I asked Reich: If our goldfish was able to burst out of the blob—if physics didn’t prevent it—what’s making it stay? Henry said, OK, remember that a) I am not a goldfish, and b) This is just a wild guess, but …

HR: I think it’s scared.
Me: Scared?
HR: Yes.
Me: Of what? It’s a goldfish.
HR: Of the strangeness of being in a bubble of water.

For millennia, he went on to say, fish have evolved in rivers, lakes, ponds, seas—places where the surface was always “up,” or above them. Fish have no experience with surfaces that are underneath them or to the left or right.

So here’s this poor fish that finds edges in all the wrong places. It’s encountering a world that’s totally strange, and it’s poking about, testing, and discovering—uh oh—an edge here, and, oh my, an edge here too?

Pardon the unpardonable anthropomorphism, but this fish is freaking out. “It’s thinking, This is weird,” says Reich.

Drawing by Robert Krulwich
Drawing by Robert Krulwich

So maybe the bubble is not caging our fish? I have another physicist pal, Aatish Bhatia (whom I play with over at Noticing.co, where we solve puzzles together). He suggested that it’s possible the blob of water is pushing back on our fish—at least a little. Water at microgravity likes to be sphere-shaped. I might resist being splatted and stretched because, says Aatish, “The ideal shape of a water drop would be round … It’s the most compact shape possible.” So the water might be pressing back at the fish’s thrashes, but, like Reich, Aatish says do not pity this fish.

It’s no prisoner. It can break free. And then, lo and behold, Aatish proved it!

“Born Free, Free as the Wind Blows”

On October 23, 2014, Weislogel published a YouTube video from Portland State University. It was a lecture he gave, and I don’t know how Aatish found this, but 27 minutes in, up there on the big screen, is our entombed fish, the very one I saw—but with a different ending!

Apparently, the video on the fluid dynamics site was chopped, and in real life, our fish escapes! It flings itself out of the water blob, and breaks through! Here’s the moment:


But Wait …

My heart leapt at the sight, until I thought, Wait a second, where did the fish go? It’s in an elevator dropping six floors at 55 miles an hour in a near weightless state heading to the ground floor of the engineering building. When the elevator lands this fish is going to, um, land with it. Did they retrieve it? Give it a medal? A martyr’s burial?

I called Weislogel.

Not to worry, he told me. The fish (not a goldfish, actually, but a neon tetra) belonged to his son’s friend. This was a high school experiment done on campus, and there were a number of fish involved, all of them on loan “from somebody’s aquarium.”

The one in that video, he said, was the “most curious” of the bunch. Many stayed stock still during weightlessness. Some curled. This one poked, probed, and, because it was so lively, it probably made several trips. It was his favorite.

OK, but—what about the landing?

“No fish was harmed in the making of this video,” Weislogel said in an announcer’s voice. When gravity kicked back in, both the blob of water and the fish settled softly (“like a baby in the womb kind of thing”) into a receiving bowl placed at the base of the chamber. Nobody died. All were, gently, returned to the aquarium.

Of course. I completely forgot this experiment was conducted in Portland, Oregon, the town that’s made unpleasantness illegal. If this story ever becomes a plotline on the TV show Portlandia, each fish will have its own monogrammed landing pillow.

I should never have worried.

Special thanks to Henry Reich, who was coming off a plane from Cambodia when he got my “Help, what’s this fish doing?” message and, before getting on his next plane, called to tell me his nutty—but, as it turned out, totally accurate – opinion of what was going on. And thanks also to Aatish Bhatia, who’s my partner over at the other blog, and the guy I go to to when I can’t figure someting out, because he always can (and would have found the Higgs boson in two minutes somewhere on the Internet, no need for an atom smasher, if he’d only been asked). He can find anything.

AUV Shows What Great White Sharks Do All Day

A great white shark off Guadalupe Island. Photo by Terry Goss, CC BY 2.5.
A great white shark off Guadalupe Island. Photo by Terry Goss, CC BY 2.5.

No creature has a reputation more fearsome than the great white shark. Despite all we’ve learned about them, including how they really don’t have much interest at in all eating us, movies and basic cable documentaries still show them as “machines” that do little more than “swim and eat and make little sharks.” And that’s not to mention the various video games where your goal as a great white is to chomp everything in sight in as little time as possible.

But what do great white sharks really do all day? It’s easy for the mythology of these predators to overshadow their real biology because it’s difficult to spend an extended amount of time following and observing animals that live beneath the waves and can cross entire oceans. We mostly see these burly sharks when they’re near the surface, and, while ingenious, strategies like Crittercam have literally been limited in scope and what can be recorded. That’s why shark researcher Gregory Skomal and colleagues turned to a different technology to see what the great fish are up to.

Thanks to documentaries and celebrity sharks like “Deep Blue“, Guadalupe Island off the coast of Mexico has become known as a great white hotspot. Yet, despite the abundance of sharks and observers – including cage divers – in the area, no one has seen how these sharks go about getting their meals. At locales off California and South Africa great white sharks make the most of their natural countershading to hide their outline against the bottom while looking up to the surface for seals to surprise. Some propel themselves with such power that they actually launch themselves out of the water in the process. But no one has seen behavior like this at Guadalupe Island.

The great white sharks of Guadalupe Island feed on the fur seals, elephant seals, and sea lions that loll about in the shallows there. Sharks have been seen feasting on the blubbery mammals at the surface. But the initial strikes have never been seen. Given the waters around Guadalupe Island rapidly drop off from the shoreline, Skomal and coauthors write, some researchers expect that the sharks are attacking their prey at depth and follow the carcass up the water column as it bobs to the surface.

To find out, the ichthyologists turned to an autonomous underwater vehicle or AUV. It looks like a shark-seeking torpedo, except with cameras and scientific equipment rather than explosives. So after tagging a shark with a device that would allow the researchers to follow the location of the fish remotely, the researchers launched the AUV to follow the shark and hopefully catch one in pursuit of mammalian prey.

Unfortunately, none of the four sharks actively tracked in the study felt particularly peckish. Or, at least, the AUV couldn’t see it. Some of the predators dove much deeper than the limits of the AUV, sometimes spending the bulk of their tracked time down below. But Skomal and colleagues propose that they were able to catch a different sort of predatory behavior on camera.

Sharks testing out the AUV. From Skomal et al., 2015.
Sharks testing out the AUV. From Skomal et al., 2015.

Even though the team actively tracked four sharks over six sessions, the cameras picked up at least eight other great white sharks in the area. Some of these sharks apparently didn’t take too kindly to the AUV. Altogether, great white sharks approached the AUV 17 times, bumped it four times, and bit it nine times. These interactions sometimes went beyond gentle, exploratory mouthings. During one mission, the tracked shark was down near the bottom while another swam up from below and behind and bit the AUV for 11 seconds, returning to bite it four more times over the next eight minutes. Then a different shark came by a half an hour later and chomped the device so hard that she breached the AUV’s hull.

Being that none of us know the mind of a shark, it’s difficult to say for sure what these fish were doing. Were they truly considering the AUV as prey, or were they frustrated with the buzzing oddity making a ruckus on their turf? There may not be a conclusive answer, but Skomal and coauthors argue that at least some of these sharks were biting to kill. When great whites fight they tend to bite each other around the pectoral fins and heads, but they tend to target the rear of creatures they aim to eat – biting off the rear flippers is one way to immobilize a seal. The fact that most of the sharkbite damage to the AUV was to the rear suggests that the sharks were trying to use the same strategies they employ to kill seals, regardless of what their motivation was.

But it’s not as if all the sharks in the area immediately descended on the AUV to snap it in half. For the most part, the observed sharks just… swam around. They followed the coast, dove deep for a while, swam by other sharks, and just generally propelled their bulk through the water with swishes of their crescent-moon tails. Some were even curious about the AUV, popping up and down from deeper water to investigate the weird object. In a mammal, we’d probably have no qualms about calling this curiosity. So even though the researchers didn’t exactly get what they were hoping for, they picked up something different – the mostly-peaceful and inquisitive ramblings of some of the greatest predators in the sea.


Skomal, G., Hoyos-Padilla, E., Kukulya, A., Stokey. R. 2015. Subsurface observations of white shark Carcharodon carcharias predatory behavior using an autonomous underwater vehicle. Journal of Fish Biology. doi: 10.1111/jfb.12828

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The Rise of the Jaw-Slingers

This is the goblin shark, a deep-sea fish with a terrifying face and the ability to transform into a deep-sea fish with a terrifying face that’s suddenly a lot closer to you. By which I mean: its jaws can shoot forward, turning its already grotesque visage into something truly nightmarish.

The goblin shark isn’t alone. Here’s the aptly named slingjaw wrasse, slinging its jaw.

Here’s a shot of the Red Bay snook doing the same. Here’s a tiger fish not quite matching the snook or the wrasse, but certainly shoving its toothy maw further out than it normally sits. And here’s the most disturbing example of all—the moray eel, which grabs prey that it has already bitten, using a second set of Alien-style jaws that sit inside its throat.

Thousands of fish species can sling their jaws forward, and some can do it up to a quarter of their body length. This ability allows them to snatch prey from inaccessible crevices, to launch ballistic ambushes, and to close those final critical millimetres on targets that are threatening to escape. David Bellwood from James Cook University describes these “protrusible jaws” as the “one of the most important innovations in vertebrate feeding over the last 400 million years”, and he has now charted their evolution over the last 100 million of those.

Fish jaws are complicated machines that comprise many moving bones and parts. But Bellwood found that this complexity can be distilled into one simple variable: the length of one particular jaw bone. By itself, it can accurately predict how far a fish can extend its jaws. And by measuring this bone in fossil fish, Bellwood could reconstruct the rise of extending jaws over time.

During the late Cretaceous period (66 to 100 million years ago), when dinosaurs still dominated the land, the average fish could only extend its jaws by less than 1 percent of its body length. Even the most protrusible protruders could only manage 8 percent. By the Eocene period (34 to 56 million years ago), the average fish could extend their jaws by 2 percent, and the best of them achieved 13 percent. Today, the average extension is 3 percent, and the champion—the slingjaw wrasse—manages a whopping 21 percent.

So, as a group, fish have gradually evolved to sling their jaws out further and further. This trend partly reflects the rise of the acanthomorphs—the massively diverse group that includes many of the fish you’re familiar with. Cod, tuna, mackerel, sole, pike, seahorses, eels, angelfish, and wrasses are all acanthomorphs.

Other fish may have made small evolutionary in-roads into jaw-slinging but the acanthomorphs are the true innovators. Bellwood’s results suggest that they’ve been at it since their origins around 140 million years ago, and different groups have independently evolved the ability between 8 and 15 times. Perhaps jaw-slinging even helps to explain why the acanthomorphs have become so successful, and today account for some 60 percent of all fish species.

And what of their prey? They could have adapted to these new threats in many ways: hiding in sand; developing hard armour; or become faster and more agile. But Bellwood thinks that their most effective tactic would have simple to become smaller, and thus harder to spot.

There’s some evidence that crabs and other crustaceans—among the favoured prey of jaw-slinging fish—started shrinking during the Cretaceous. And certainly, the average crustacean in today’s coral reefs is just 0.4 millimetres long. The fish themselves show signs of these changes: during the Eocene, their mouths became smaller and the eyes bigger, suggesting a shift towards smaller and less conspicuous prey. Protruding jaws closed the gap between fish and their prey by mere millimetres and centimetres, but changed the evolutionary fates of both groups for vast stretches of time.

“False Megamouth” Shark Pioneered the Plankton-Feeding Lifestyle

A hypothetical restoration of Pseudomegachasma. Art by Kenshu Shimada.
A hypothetical restoration of Pseudomegachasma. Art by Kenshu Shimada.
A hypothetical restoration of Pseudomegachasma.

All sharks are carnivores. From the sunny surface waters to the darkest depths, every selachian species lives by feeding on other animals. Of course, the great whites, tigers, and the ones that get lots of basic cable screen time – the macropredators – are the most famous, but the largest sharks of all feed on some of the smallest organisms in the ocean. These sharks are planktivores, and paleontologists have rediscovered two ancient sharks that pioneered a diet based on the very very tiny.

The Cretaceous sharks took a circuitous course to discovery. Back in 2007, Kenshu Shimada – a professor at DePaul University and research associate at Kansas’ Sternberg Museum of Natural History – described the plankton-feeding, megamouth shark Megachasma comanchensis from teeth found in Colorado. Other researchers disagreed with Shimada’s interpretation. The teeth were not those of a megamouth, they countered, but were the damaged and abraded teeth of an already-named, fish-eating shark.

A Pseudomegachasma tooth. Photo by Kenshu Shimada.
A Pseudomegachasma tooth. Photo by Kenshu Shimada.

Shortly after Shimada described another fossil megamouth, however, paleontologist Bruce Welton approached him with a curious shark tooth from the 100 million year old rock of Texas. “That fossil tooth was beautifully preserved with virtually no signs of damage, and yet, it was practically identical to Megachasma comanchensis teeth I described in 2007,” Shimada says. That was enough to send Shimada, Welton, and their coauthors back to have another look at the controversial Cretaceous megamouths.

It turned out that shark teeth from Russia went through a similar back-and-forth. Teeth named Eorhincodon casei and thought to be those of a filter-feeder were later reinterpreted as those of a slice-and-dice sort of shark. Yet, as Shimada and colleagues found, the teeth from Russia were extraordinarily similar to those from the United States and were from about the same geologic age. Despite their geographic range, all the teeth could be attributed to the same genus. Shimada and coauthors have therefore dubbed the shark Pseudomegachasma, the “false megamouth”.

The evolution of plankton-feeding cartilaginous fishes. Illustration by Kenshu Shimada.
The evolution of plankton-feeding cartilaginous fishes. Illustration by Kenshu Shimada.

“Because Pseudomegachasma is based solely on isolated teeth, its exact mode of lifestyle is inferential,” Shimada says, but he notes that ” The overall size and shape of Pseudomegachasma teeth are nearly identical to teeth of Megachasma“, the modern megamouth shark. Since the living megamouth has specialized teeth adapted to straining plankton from the water, it’s likely that Pseudomegachasma fed the same way.

Strangely, though, Shimada and his colleagues found that “today’s megamouth shark has no direct evolutionary link to the Cretaceous Pseudomegachasma.” The fossil sharks were more closely related to the snaggletoothed “sand tigers” than the modern megamouth. The two shark lineages are a new case of convergent evolution – fish-eating sharks gradually being adapted to have teeth better-suited to sieving little invertebrates and other morsels from the water column. And, at about 100 million years old, “Pseudomegachasma represents the oldest known plankton-feeding shark in the fossil record that evolved independent of the four known lineages of modern-day planktivorous cartilaginous fishes: the megamouth sharks, basking sharks, whale sharks, and manta rays,” Shimada says.

So why did all these fish make the switch? “Exactly what triggered the evolution of planktivory in each lineage is still uncertain,” Shimada says, “but the discovery of Pseudomegachasma does tell us that plankton were abundant enough to support the fossil shark in warm shallow oceans during the mid-Cretaceous.” Even if we can’t directly sample Cretaceous seas, the evolution of plankton predators are still indicators of how the oceans were changing. Or, in other words, looking to some of the marine realm’s ancient “gentle giants” may provide critical information about what was going on with some of the smallest. “Together with the recent recognition of some gigantic planktivorous bony fishes that also lived during the Mesozoic,” Shimada says, “I believe we have just barely begun to scratch the surface of the elusive plankton-feeding diet regime that existed in ancient marine ecosystems.”


Shimada, K., Popov, E., Siversson, M., Welton, B., Long, D. 2015. A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States. Journal of Vertebrate Paleontology. doi: 10.1080/02724634.2015.981335

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Meet the Comical Opah, the Only Truly Warm-Blooded Fish

There’s nothing about the opah that says “fast-moving predator”. Tuna, sharks, and swordfish are fast-moving predators and accordingly, their bodies look like streamlined torpedoes. By contrast, the opah looks like a big startled frisbee, with thin red fins stuck on as an afterthought.

It’s pretty (silver body and red fins) and big (up to two metres long), but fast? Nicholas Wegner from the National Oceanic and Atmospheric Administration certainly didn’t think so when he first started studying it. Since then, he has discovered that the opah is an active predator, which has a trait that no other fish possesses.

It is warm-blooded.

Most fish have body temperatures that match the surrounding water. A small number of them can warm specific parts of their bodies. Swordfish, marlins, and sailfish, can temporarily heat their eyes and brains, sharpening their vision when pursuing prey. Tuna and some sharks, including the mako and great white, can do the same with their swimming muscles, going into turbo mode when they need to. But none of these animals can heat their entire bodies. Their hearts and other vital organs stay at ambient temperature, so while they can hunt in deep, cold waters, they must regularly return to the surface to warm their innards.

The opah has no such problem. It can consistently keep its entire body around 5 degrees Celsius warmer than its environment. It doesn’t burn as hot as a bird or mammal, but it certainly outperforms its other relatives.

Nick-Wegner-OpahWegner discovered its ability by accident. His team just happened to catch more opah during one of their research trips, and they used the opportunity to learn more about this little-known species. As they dissected the animals, Wegner immediately noticed that its gills contain a beautiful and intricate tangle of red and blue blood vessels. “That was when we realised what it was capable of,” he says. Wegner had seen blood vessels like those before. They’re called retia mirabilia—Latin for “wonderful nets”—and they’re the secret behind the heating systems of tuna and sharks.

All animal muscles produce heat when they contract, but in most fish, that heat is almost immediately lost to the environment through the skin or the gills. The gills are especially problematic. No matter how much insulation a fish has, the blood that runs through the gills has to make close contact with the seawater. A tuna can produce as much heat as it likes in its swimming muscles, but as soon as the blood from those muscles reaches the gills, as it must do to be reloaded with oxygen, it ought to quickly cool. But it doesn’t, because of the wonderful nets.

In those nets, the veins that carry warm blood away from the hot muscles are interwoven with the arteries that carry cold blood in from the gills. They run so close that the veins offload their heat onto the arteries, before it can reach the gills and disappear. Through these “countercurrent exchangers”, the tuna can retain whatever heat it generates. But since its retia mirabilia are located in its swimming muscles, those are the only body parts that stay warm. That’s why its heart still runs cold.

The opah’s wonderful nets are in its gills, and that makes all the difference. The blood vessels carrying warm blood from heart to gills flows next to those carrying cold blood from the gills to the rest of the body, warming them up. So, while a tuna or shark might isolate its warm muscles from the rest of its cold body, the opah flips this arrangement. It isolates the cold bits—the gills—from everything else.

This allows its huge pectoral muscles, which generate most of its heat, to continuously warm the rest of its body. It also keeps that heat with the help of thick layers of fat, which insulate the heart from the gills, and the pectoral muscles (which produce most of the animal’s heat) from the surrounding water.

Wegner’s team confirmed this by catching opah, implanting them with small thermometers, and then releasing them. The instruments inside the fish recorded consistently higher temperatures than those dropped into the surrounding water. The opah’s brain is warm. Its muscles are warm. And perhaps most importantly, its heart is warm—a first for a fish. Not even a great white shark has a warm heart. “That’s why opah can stay at depth,” says Wegner. “These guys are specialised for living deeper than those other predators.”

So, it’s fast, then? Despite the somewhat comical physique? “That’s what’s really blew my mind about this discovery,” says Wegner. “Just from looking at it, I really thought it was a slow, sluggish, deep-water fish that doesn’t do very much. But all indications are that this is a very fast fish and an active predator. We’ve put some tags on them to show that they migrate thousands of kilometres.”

Reference: Wegner, Snodgrass, Dewar & Hyde. 2015. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science http://dx.doi.org/10.1126/science.aaa8902

Fossil Fish Sliced Prey With Bizarre Jaws

Paleontology collections are wonderful. Shelves and cabinets hold anywhere from thousands to hundreds of millions of years of life’s history, assembling giant ground sloths, Cambrian oddballs, petrified plants, and other fantastic organisms into a fossilized menagerie. And as much as I’ve enjoyed my opportunities to explore prehistoric storage on my own, it’s even better when curators can spare a few minutes to show off their favorite treasures.

The last time I had the pleasure of getting such a tour was in late February. I had made an appointment with paleontologist Ted Daeschler  to chat about our early tetrapod ancestors, and he was kind enough to give me a whirlwind tour of the collections held at the Academy of Natural Sciences. Zipping from one drawer to the next, Daeschler deftly navigated the tight space between the cabinets to show off a variety of beautifully-preserved fossil fish from a variety of times and places.

Many of the fish were preserved whole, as if they had been frozen in mid-swim, but one drawer was lined with the remains of an animal only known from pieces. Light glinting off the enamel-like coating of their teeth, the fossils were curved tooth whorls given the name Edestus heinrichi. In life, the whorls of this ratfish relative were arranged in a vertical pair – what must have looked like evolution’s attempt at pinking shears. There’s nothing on Earth today with jaws like that.

Given that our species is over 306 million years too late to observe Edestus in action, there’s been plenty of debate about how this ancient fish employed its wicked grin. The most obvious answer – that Edestus snipped prey with scissor jaws – doesn’t work. (This has been a recurring problem in paleontology – just because a structure looks like a human-invented tool, it doesn’t mean the structure was used the same way.) The tooth whorls of Edestus curved away from each other along their length, so much so in the early species Edestus newtoni that the fish must have looked perpetually puckered to give a serrated kiss. Only a few teeth at the very back of the row would actually slide past each other to shear through shell or scale.

A hypothetical restoration of Edestus newtoni. Art by G. Raham, from Itano, 2015.
A hypothetical restoration of Edestus newtoni. Art by G. Raham, from Itano, 2015.

With the scissor hypothesis in doubt, paleontologists espoused a variety of speculative functions. University of Colorado paleontologist Wayne Itano ran down the list in a 2014 paper on Edestus. Perhaps Edestus trawled for jellyfish, burrowed in the mud after clams, or scraped shellfish off their moorings. Or maybe Edestus snagged prey with its upper jaw and sawed through it with the lower, working its fishy chin back and forth to cut victims into manageable chunks.

All these ideas are easily-imagined, but they’re also without supporting evidence. A better contender, proposed by John Long and elaborated on by Itano, is that Edestus subdued prey with a bit of prehistoric headbanging.

While there’s nothing quite like Edestus alive today, there are fish that capture and kill prey with some pretty strange anatomy. Both sawfish and billfish whip their snouts back and forth to stun and slice their prey. Edestus, Long  and Itano have suggested, might have done the vertical version of this, throwing their jaws up and down to injure and lacerate fish and cephalopods.

Some Edestus teeth might bear the marks of such throes. On one specimen of Edestus newtoni, Itano observed, the teeth at the furthest reaches of the whorl have intact tips and worn-down serrations. Edestus was slicing into prey rather than puncturing it. This mode of attack was best-suited to prey on the softer side, but Edestus may have occasionally tried tougher fare. One Edestus tooth, Itano notes, has an abraded tip and indicates that its owner probably busted a crown on something hard and had to live with the broken point until the tooth was pushed out.

And while circumstantial, there’s another line of evidence that Edestus were master shredders. The black shales of Indiana, Itano points out, are famous for “amputated fish”. Some are only heads. Others tails. And a few have deep lacerations that left only a fragile band of tissue connecting the severed morsels. Edestus fossils have been found in the same deposits, and, lacking any evidence of a Carboniferous Freddy Krueger, the fierce fish is a top suspect.

[Edestus wasn’t the only ratfish relative with weird jaws. Helicoprion was even stranger.]


Itano, W. 2015. An abraded tooth of Edestus (Chondrichthyes, Eugeneodontiformes): Evidence for a unique mode of predation. Transactions of the Kansas Academy of Science. 118 (1-2): 1-9

Itano, W. 2014. Edestus, the strangest shark? First report from New Mexico, North American Paleobiogeography, and a new hypothesis on its method of predation. The Mountain Geologist. 51 (3): 201-221

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Fish that Walks on Land Swallows With Tongue Made of Water

In the distant past, between 350 and 400 million years ago, a group of our fishy ancestors started crawling up on land. The fins that propelled them through the water gradually evolved into sturdy, weight-bearing limbs. Their hind legs connected directly to their hips, which became bigger. Swimming fish became walking, four-legged tetrapods, such as amphibians, reptiles, and mammals.

Scientists have studied the evolution of tetrapod limbs and skeletons in incredible detail, but other aspects of our invasion of land are less clear. How, for example, did our pioneering ancestors eat?

Many fish feed by sucking. As they open their jaws, a horseshoe-shaped bone called the hyoid pushes down on the floor of the mouth, expanding it, and creating a flow of water that draws prey inside. Even species that take bites and nibbles rely on a similar suction to swallow food once it’s inside their mouths.

This technique works because fish are constantly surrounded by water. It doesn’t work on dry land. Fortunately, tetrapods solve that problem with a muscular tongue, which helps to move food from the mouth to the throat. Once again, the hyoid is involved—it’s the bone that the tongue is attached to. But how did this structure evolve? How did the hyoid go from being a bone that creates suction to one that moves a tongue? How did the first tetrapods swallow?

These questions have been hard to answer because very few fossils of early tetrapods contain decent traces of the hyoid. But Krijn Michel from the University of Antwerp tried a different tactic: he studied a delightful fish called the Atlantic mudskipper. This tiny creature looks like a tiny doorstop with a pair of fins and googly eyes, and it lives throughout the mangrove swamps of eastern Africa, the Indian Ocean, and the western Pacific. It spends a surprising amount of time on land. It hauls itself about on its fins, fighting, mating, and foraging in the open air.

Michel filmed Atlantic mudskippers with high-speed cameras as they sucked up pieces of shrimp that had been placed on dry surfaces. As he reviewed the videos, he noticed something odd. In the moments after a mudskipper leans forward and opens its mouth, a small bubble of water protrudes from its open jaws. The water spreads over the morsel of food, which the mudskipper envelops with its mouth. It then sucks both morsel and water back up.

The water acts like a tongue—a “hydrodynamic tongue”, in Michel’s words. It allows the fish to lap up its food and then swallow it.

Michel showed how important the ‘tongue’ is by placing morsels of shrimp on an absorbent surface and filming the mudskippers with X-ray video cameras. This time, as the mudskippers leant in, their watery tongues were drained away. They could still grab the shrimp in their jaws but they couldn’t swallow. On 70 percent of their strikes, they had to return to water before they could gulp down their mouthfuls.

This explains why mudskippers almost always fill their mouths with water before they come out on land. By keeping that watery tongue, they can swallow several mouthfuls before having to return to the water. By contrast, the eel-catfish, which also ventures onto land but doesn’t use the same trick, must always return to water after it has grabbed its prey.

“These findings suggest that swallowing food in air may have been a substantial problem for the transition from water to land during vertebrate evolution,” says Beth Brainerd from Brown University. “When early tetrapods started feeding on land, they had to evolve a new way to move food to the back of the throat for swallowing.”

Did they use a watery tongue, a la mudskippers? Perhaps, but it’s important to remember that these are modern fish, and not tetrapods-in-the-making. Hundreds of millions of years of evolution separate them from our land-colonising ancestors. At most, they can hint at the kinds of strategies that early tetrapods might have used when they moved onto land.

A watery tongue, for example, could have provided a workable interim solution, allowing the animals to feed successfully while their hyoids changed and they developed muscular tongues. Indeed, when Michel trained his X-ray cameras on a fish and a newt, to watch how their hyoids moved when they ate, he found that the newt’s movements more closely resembled those of the mudskipper. The bone’s making the right sort of movements, even if there’s no muscular organ attached to it.

Reference: Michel, Heiss, Aerts & Van Wassenbergh. 2015. A fish that uses its hydrodynamic tongue to feed on land. Proc Roy Soc B  http://dx.doi.org/10.1098/rspb.2015.0057

PS: Fish do have “tongues” but the term is a loose parallel; unlike our muscular organs, these tongues (usually) can’t stick out of the mouth, and they don’t help with swallowing. They can, however, help with chewing.

The Mediterranean’s Missing Sawfishes

In 1959, off the southern coast of France, a tuna boat hauled up a largetooth sawfish. The catch wasn’t particularly large. The razor-snouted fish only stretched about four feet long; still a baby by the standards of its species. But it was one of the last to be seen in those waters. Within a decade, the largetooth sawfish entirely disappeared from the Mediterranean Sea.

Such accidental catches and sightings in the Mediterranean have often been regarded as signs of largetooth and smalltooth sawfish migrating into the sea from the coast of Africa. When sawfish experts gathered in London in May 2012 under the auspices of the IUCN, they concluded that the Mediterranean gets too cold in winter to have hosted resident populations of the warm-water fish. Therefore all the historic accounts must have referred to “vagrant” animals, and sawfish blades in museums were brought to coastal museums by trade routes that have been in place for centuries. But after trawling through bibliographic records and museum displays, Hopkins Marine Station biologist Francesco Ferretti and colleagues have suggested a different interpretation. The Mediterranean is missing its native sawfishes.

Ferretti and coauthors cast a wide net. They searched everything from records on pre-dynastic Egypt through modern ocean biodiversity databases to find any sign of sawfishes in the Mediterranean. Being that the science of ichthyology didn’t get going until the 16th century, it’s not surprising that the earlier part of their timerange came up empty. But between the 18th and 20th century – when naturalists often kept track of who was landing what at the local docks – the researchers turned up 48 original accounts of sawfish in the Mediterranean, 24 of which could be verified in the literature or in museums. These verified records were split between largetooth and smalltooth sawfish, and the size of many of these fish hint that they were not migrants from the African coast.

Map of Mediterranean sawfish records, with smalltooth in orange and largetooth in green. From Ferretti et al., 2014.
Map of Mediterranean sawfish records, with smalltooth in orange and largetooth in green. From Ferretti et al., 2014.

Largetooth and smalltooth sawfish grow slowly. Largetooth sawfish, in particular, take between eight and ten years to reach maturity, at which time they’re about 10 feet long and start reproducing. But in their youth, sawfish typically stick very close to the place they’re born. Smalltooth sawfish, for example, have a home range of less than a half a mile. Ferretti and coauthors counted 15 sawfishes in their sample that fell within the juvenile category.

If those juvenile sawfishes were swimming from the nearest population centers to the Mediterranean, they must have journeyed more than 2,000 miles – over ten times the distance an adult sawfish has ever been observed to travel. Unless the historic populations of large and smalltooth sawfish undertook truly exceptional journeys, Ferretti and colleagues argue, it’s more likely that they had a resident population in the Mediterranean.

While lacking in as much detail as modern biologists wish for, the 18th and 19th century naturalist accounts of sawfishes also throw some support to the idea that sawfish had a home in the Mediterranean. Some accounts list them as relatively rare, and others as common, but there doesn’t seem to be any hint that it was strange to see sawfishes along Europe’s southern coastlines. It was only in the 20th century – when sawfish populations plummeted and totally disappeared – that sawfishes were regarded as especially rare and the idea of migration started to take hold.

Not all marine biologists are convinced by the historic evidence. In a paper published around the same time as the paper by Ferretti and coauthors, Nicholas Dulvy and colleagues argue that the Mediterranean gets too cold for sawfishes and that the past occurrences really do represent piscine vagrants. More than that, Dulvy and colleagues write, “Whether or not sawfishes were previously extant in the Mediterranean Sea has little bearing on current conservation priorities as any activities benefitting West African sawfishes can only restore migration and improve the likelihood of vagrancy to the Mediterranean Sea once again.”

Ferretti and colleagues disagree on both counts. Regarding temperature, the researchers suggest, past Mediterranean sawfish populations may have been better-adapted to cooler waters than others. If not that, then young sawfishes could have taken refuge in deeper water that maintains warmer, more constant temperatures than those at the surface during winter. There may not be a way to know for sure – the Mediterranean sawfishes are all gone – but temperature alone can’t be used to rule out the previous presence of resident populations.

And this question does have relevance for the future of the largetooth and smalltooth sawfish. If sawfishes previously had a home in the Mediterranean, perhaps they could live there again. Ferretti and colleagues even have a spot in mind – a national park in southern France near where the last recorded sawfishes were seen. What hopes sawfishes might have for survival in such a place are murky, but if restoration attempts are to be considered at all, the Mediterranean may be a place where these awkwardly charismatic fish may find a refuge.

How long the longtooth and smalltooth sawfish will survive is unknown, and their fate largely rests on the decisions we make. And in making those decisions we must be aware of our own history. The only good records of Mediterranean sawfishes we have come from a timespan when these vulnerable fish had already been coping with centuries of human disturbance to their nearshore haunts. Our species only started keeping track of what was “natural” when the sawfishes were already in decline. Marine biologists know this as “shifting baselines“, and it’s the same reason why many don’t feel the absence of ground sloths and mastodons in North America’s forests. The megamammals were already gone by the time naturalists started paying attention to the woods, and we don’t consider how empty the landscape is. We just don’t know what we’re missing.


Dulvy, N., Davidson, L., Kyne, P., Simpfendorfer, C., Harrison, L., Carlson, J., Fordham, S. 2014. Ghosts of the coast: global extinction risk and conservation of sawfishes. Aquatic Conservation. doi: 10.1002/aqc.2525

Feretti, F., Verd, G., Seret, B., Šprem, J., Micheli, F. 2014. Falling through the cracks: the fading history of a large iconic predator. Fish and Fisheries. doi: 10.1111/faf.12108

Guilhaumon, F., Albout, C., Claudet, J., Velez, L., Lasram, F., Tomasini, J., Douzery, E., Meynard, C., Moupuet, N., Troussellier, M., Araújo, M., Mouillot, D. 2014. Representing taxonomic, phylogenetic, and functional diversity: new challenges for Mediterranean marine-protected areas. Diversity and Distributions. doi: 10.1111/ddi.12280

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How Big Are The Biggest Squid, Whales, Sharks, Jellyfish?

A few years ago, Carl Zimmer and I ran a workshop on science writing, where we talked, among other things, about explaining science without talking down to your audience. It apparently left an impression on Craig McClain, a marine biologist and blogger who was in the audience. “I made a comment about how I always wanted to write a post on how giant squid sizes are bullsh*t,” he recalls,” but that those always come off as an arrogant scientist telling the world that it’s wrong. And you said: You should write it, but you just need to find the right tone. That kicked me off.”

Rather than an angry blog post, McClain decided to put together a scientific paper that would accurately answer a simple yet slippery question: How big do the biggest animals in the ocean get?

The oceans are home to giants: blue whales and great white sharks; giant squids and giant clams; elephant seals and Japanese spider crabs. These creatures have no trouble capturing the public imagination, but scientists often have trouble capturing them. Many are rare, elusive, or live in inaccessible parts of the sea. Some are only measured when they wash ashore, after dry land distends or deflates their bodies. Some are so big that they are just plain hard to measure. And so, oceans are also home to exaggeration.

Take the giant squid. Umpteen media report claim that this nigh-mythic animal can grow up to 60 feet (18 metres) in length. That’s absurd, McClain thought. The vast majority are less than half that length. Many individuals are measured when they wash ashore, after decomposition loosens their muscles and eager humans stretch their tentacles. One size estimate even came from someone counting his paces next to a beached squid! Shoddy data had been unleashed upon the kraken.

McClain, together with Meghan Balk from the University of New Mexico, went after better sources. They recruited five keen undergraduate students and large team of colleagues, who divided a list of target species between them. They trawled the scientific literature for measurements. They combed through books, newsletters, and newspapers. They asked colleagues at museums to measure specimens in their collections. They contacted networks that rescue stranded turtles. They reeled in eBay records to find the measurements of giant clams and snail shells. And together, they found the best possible estimates for the maximum sizes of 25 ocean giants.

For some species, widely quoted figures were outrageously wrong. The largest verifiable giant squid was 12 metres long—giant, sure, but a damn sight smaller than 18 metres. The biggest known walrus weighed 1,883 kilograms, a far cry from the 2,500 kilogram titan that a hunter supposedly shot, and clearly embellished.

For some species, estimates were outdated—giant clam sizes all date to the 60s and 70s. For others, like the lion’s mane jellyfish and Japanese spider crab, the team found that accurate data just doesn’t exist.

And “some animals may just not be getting as big as they used to get,” says McClain. In 1885, fishermen in the Aleutian Islands caught a Giant Pacific octopus that was 9.8 metres from one arm tip to the next. “The two octopus experts who were with me on this paper say that they just don’t get that big any more,” says McClain. “It could be pollution or climate change.”

Hype and decline aside, the stats from the paper still tell of oceans that are full of impressive leviathans. There are giant barrel sponges, whose bodies are just two layers of cells sandwiching a jelly filling, but can nonetheless grow to 2.5 metres wide. There are 2-metre-wide Nomura’s jellyfish that can weigh 200 kilograms. In the depths, 3-metre-long giant tube worms thrive near hot, belching, volcanically heated vents, and giant isopods—a kind of undersea woodlouse on steroids—can reach 50 centimetres. Seven treacherous metres exist between the tip of a great white shark’s toothy snout and the end of its powerful tail.

The largest mammal, the blue whale, grows up to 33 metres long, and can swallow half a million calories in one mouthful. The longest fish—the star-backed whale shark—gets to 18.8 metres. The longest bony fish—the bizarre, serpentine oarfish—can reach 8 metres. The heaviest bony fish—the ocean sunfish, which looks like the decapitated head of a much larger fish—grows to just 3.3 metres long but weighs up to 2,300 kilograms. That’s much heavier than the biggest turtle (leatherback, 650 kg), a little heavier than the biggest walrus (1,883 kg), and not a patch on the heaviest seal (Southern elephant seal, 5,000 kg).

These measurements are all as accurate as possible; finding them often involved a labyrinth of references and phone calls. For the Australian trumpet—a beachball-sized monster of a snail—McClain found an issue of Hawaiian Shell News, in which a collector named Don Pisor holds up an enormous and supposedly 90-cm shell. But McClain also found a copy of the Registry of World Record Size Shells, which said that the record-holder was just 72 centimetres long. It was also attributed to Don Pisor. Were these the same specimens? Was this guy a charlatan? Or just phenomenally good at catching escargods? There was only one thing to do: McClain tracked down Pisor and asked him. It was just one specimen, he said, and he donated it to the Houston Museum of Natural Science. McClain called the museum. 72 centimetres, they said. Another data point for the list.

In a few cases, the team discovered new things about the giants. Andrew Thaler, who collected the data on giant isopods suddenly realised that males were, on average, much bigger than females. “That’s not something either of us knew before and both of us know everything there is to know about giant isopods,” says McClain.

And for several animals, the team managed to plot out the distribution of their sizes, rather than just the maximum. Scientifically, that’s more useful. People love to know how big animals can get, but that tells us very little about their typical lives. The biggest known giant squid was 12 metres long, but their average length is 7.3 metres, and most individuals are shorter than 9.2. Its archenemy, the sperm whale, has a recorded maximum size of 24 metres, but 95 percent of these whales are shorter than 15 metres.

As McClain—himself a bear of a man at 6’ 2”—points out, “individuals may reach these extraordinary large sizes through developmental or genetic defects.” The tallest human ever, Robert Wadlow, was 8 feet and 11 inches in height; he also needed leg braces to walk and died at 21 from an infection aggravated by an autoimmune disease. The tallest woman, Zeng Jillian, reached her lofty 8 feet and 1 inch because of a tumour in her pituitary gland; she died at 17. We are fascinated by extremes, but life mostly plays out in the middle.

Reference: McClain, Balk, Benfield, Branch, Chen, Cosgrove, Dove, Helm, Hochberg, Gaskins, Lee, Marshall, McMurray, Schanche, Stone & Thaler. 2015. Sizing ocean giants: patterns of intraspecific size variation in marine megafauna. PeerJ http://dx.doi.org/10.7717/peerj.715

More: The students created a website—Story of Size—where they wrote about their work.

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Electric Eels Can Remotely Control Their Prey’s Muscles

A fish swims in the Amazon, amid murky water and overgrown vegetation. It is concealed, but it’s not safe. Suddenly, two rapid bursts of electricity course through the water, activating the neurons that control the fish’s muscles. It twitches, giving away its position, and dooming itself. Now, it gets zapped by a continuous volley of electric pulses. All its muscles contract and its body stiffens. It can’t escape; it can’t even move. Its attacker—an electric eel—moves in for the kill.

The electric eel can (in)famously create its own electricity. More than four-fifths of its two-metre-long body consists of special battery-like cells, which can collectively deliver a jolt of up to 600 volts. But the way the eel uses that ability is even more shocking. Kenneth Catania from Vanderbilt University has found that this astonishing predator can use its electricity like a remote control, activating its prey’s muscles from afar. It effectively has a button that says “Reveal Yourself” and another that says “Freeze”.

“This is one of the most amazing things I’ve encountered in studying animals, and I’ve seen a lot of unusual things” says Catania. He’s not exaggerating. This is a man who showed that crocodile faces are more sensitive than our fingertips, that tentacled snakes can persuade prey to swim into their mouths, and that star-nosed moles blow bubbles to smell underwater. Guy knows amazing and unusual.

The eel tops them all, not least because its hunting tactics seems unbeatable. It should work on any prey with nerves and muscles, which is most of them.

“I don’t see any defence against it,” says Catania.

Electric eel. Credit: Ken Catania.
Electric eel. Credit: Ken Catania.

Although scientists have studied the electric eel’s anatomy and even sequenced its genome, hardly anyone had looked at how it hunts. “The sense, and I had the same reaction, was that they zap their prey with electricity and eat it; what more is there to know?” says Catania. As it happens: a lot!

Once Catania got his (rubber-gloved) hands on some eels, he realised that they are surprisingly fast. “I thought they might lazily shock their prey and then deal with it afterwards, but they combine the shock with a really rapid strike,” he says. He filmed them with a high-speed camera and noticed something remarkable. When the eels approached their prey, they released an intense volley of high-voltage pulses—around 400 a second. These pulses completely freeze the prey, and that’s when the eel lunges. If the fish isn’t paralysed, the strike would miss. “At that point, I was hooked,” says Catania. “I just had to know more.”

To work out what the pulses were doing, Catania presented eels with zombie prey—lobotomised and anaesthetised fish that were hooked up to a device for measuring forces. An agar barrier prevented the eel from reaching the morsels but allowed its discharges to pass. This macabre set-up confirmed that the eels’ high-voltage pulses force the fish’s muscles to involuntarily contract.

Credit: Catania, 2014. Science
Credit: Catania, 2014. Science

The pulses could either act on the muscles themselves, or on the neurons that control them. To distinguish between these possibilities, Catania injected the fish with curare, a poison that blocks the junctions connecting nerves and muscles. These individuals were immune to the eels’ shocks—a clear sign that the pulses act on the neurons, rather than directly on the muscles.

“That’s pretty much how a taser works,” says Catania. Indeed, getting shot with a taser is probably the closest anyone might come to being on the receiving end of an electric eel. Failing that, “the next most common experience might be to accidentally touch an electric fence at a horse or cattle farm”.

Catania also found that the eels precede their lengthy volleys with a quick pair of pulses. Doublets like these are known to trigger disproportionately strong muscle contractions. The eel, it seems, produces exactly the right kinds of discharge to freeze its prey as efficiently as possible.

But it also produces doublets when there’s no prey in sight. Since the 1970s, scientists have known that hungry eels will prowl around their cages, giving off electric doublets as they explore. And in Catania’s experiments, the eels often released a doublet, and then tried to break through the agar barrier and snatch the fish. That was a big clue: Catania wondered if they might be using the pulses to find their prey in the first place.

“Transport yourself to the Amazon and imagine these nocturnal animals hunting for diverse prey, which are hidden in a complex environment,” he says. So, they release doublets. Any fish or crab that gets hit would twitch and give itself away.

Catania tested this idea by placing the zombie fish in a thin plastic bag to isolate them from any electric pulses. When the eels released their doublets, the fish didn’t react, and the eels never attacked. When Catania deliberately jostled the fish to mimic a twitch, the eels struck. They are extraordinarily sensitive to ripples in the water, and the slightest movement sets them off. And they’re so fast that they can hit the source of the twitch within 20 thousandths of a second. “If you saw this in real-time, you wouldn’t even know that the doublet had elicited a movement in the fish because it would happen so fast,” says Catania.

Many fish can produce weak electric fields, including the elephantfishes of Africa and the knifefish of South America. But only a few can produce strong potentially lethal fields. These include the electric eel (which is actually a knifefish and not an eel at all), the electric catfishes, and the torpedo ray. The eel is the most powerful of these shockers, but Catania suspects that the others might use the same hunting technique. “I wish I had some in the lab,” he says.

Reference: Catania, 2014. The shocking predatory strike of the electric eel. Science http://dx.doi.org/10.1126/science.1260807

PS: Yes, this really is a single-author paper, published in Science in 2014. That’s arguably as shocking as the eel.

More on electric fish:

Snotworms For Dinner

Deep in the sea, on the denuded carcasses of whales, there live humble little scavengers. They’re the snotworms, better known to researchers as various species of the polychaete worm Osedax. They pursue a very dedicated lifestyle. Female snotworm larvae settle onto the exposed bones of dead whales, sending down “roots” to tap into the fatty substances locked in the cetacean’s skeleton. Together with mats of bacteria and other organisms, they’re part of a whale deconstruction crew that can totally break down the world’s largest animals.

And the snotworms have been at this for quite some time. Their distinctive pockmarks have been found on fossil whale bones dating back to about 30 million years ago. And even though they’re most famous for feeding on decaying whales, the snotworms can be food, too. In a new Lethaia paper, University of Otago paleontologists Robert Boessenecker and Ewan Fordyce describe divots and scratches on a 28-23 million year old whale that show fish and sharks gobbled snotworms.

The whale in question – of an as-yet-unnamed prehistoric genus and species – was found on New Zealand’s South Island. There was quite a bit of the mammal in the rock, including the skull, vertebrae down to the torso, ribs, and additional bones left at the site. And upon those prehistoric bones are trace fossils that testify to the presence of other organisms. Among the most numerous are clusters of small circular holes that are concentrated on well-preserved bone – these are the signs of prehistoric Osedax, the first fossil occurrence from the southern hemisphere.

Scrape marks - black arrows - around bone damage caused by Osedax worms. From Boessenecker and Fordyce, 2014.
Fish tooth-scrape marks – black arrows – around bone damage caused by Osedax worms. From Boessenecker and Fordyce, 2014.

But in some spots there are other marks that cut across the snotworm holes. The snotworm burrows on the skull bones are associated with shallow, parallel scrape marks. The most likely culprits, Boessenecker and Fordyce propose, are fish and sharks that dragged their jaws across the whale carcass to snaffle the snotworms, roots and all.

The Osedax feast continues today. Boessenecker and Fordyce point out that ratfish and crabs have been seen eating whale bone covered in snotworms, too. As long as there have been snotworms, other creatures have turned to them for food. It’s another indication that, rather than being silent undersea tombs, whalefalls are sites of vibrant life.


Boessenecker, R., Fordyce, E. 2014. Trace fossil evidence of predation upon bone-eating worms on a baleen whale skeleton from the Oligocene of New Zealand. Lethaia. doi: 10.1111/let.12108

Evolution in the Slow Lane

One late spring weekend a few years back, my wife and I drove out to Delaware to see an amazingly old tradition.

Knowing that both time and tide were critical, we had asked around for the best spot and right hour. Prime Hook National Wildlife Refuge at sunset was the most popular answer, and so, after a day of reading in camp, we pulled up to a beach shaded orange by the evening light. Tracey and I strolled down the beach for a while, watching sanderlings and ghost crabs go about their respective business, but we weren’t greeted by the natural spectacle we had hoped to see. The only sign of the ancient players were dried, gull-pecked husks scattered on the sand.

We were about to give up for the night when a receding wave briefly revealed what we had driven so far to see. There, in the dark water, were two horseshoe crabs, the male clasped onto the back of the larger female. They barely looked alive, more like olive-shaded helmets than animals, yet there they were, doing their part to perpetuate the species. On previous nights the tideline had been covered with similar pairs, but, even though I had missed the peak of horseshoe crab mating season, I was happy to get even a glimpse of nuptials that have been going on much the same way since the Jurassic.

But it would be a mistake to call horseshoe crabs “living fossils.” The term is catchy, and was coined by none other than Charles Darwin himself, but it’s only of those sneaky turns of phrase that quickly breaks down under close examination.

The pop definition of living fossil, as handed down by nature documentaries, is “a species that has gone unchanged for millions of years.” But this doesn’t work for even the most famous examples of supposedly static species.

The evolution of coelacanths (top) versus tetrapods (middle) and ray-finned fish (bottom). From Cavin and Guinot, 2014.
The evolution of coelacanths (top) versus tetrapods (middle) and ray-finned fish (bottom). From Cavin and Guinot, 2014.

Species are always changing genetically, if not anatomically. That’s why classic “living fossil” species aren’t found in the fossil record. Consider the coelacanth. There are two living species of this fleshy-finned fish, Latimeria chalumnae and Latimeria menadoensis, neither of which is found in the fossil record. Granted, there’s a 66 million year span in which the only coelacanth fossils are questionable fish bits, but it’s still worth noting that today’s coelacanths are readily distinguishable from their Cretaceous counterparts. Furthermore, if “existence of a species in the fossil record” is what makes a living fossil, then the fact that paleontologists have found Homo sapiens remains dating back to 200,000 years ago would place us in that category while exempting the coelacanth.

Instead, today’s coelacanths are representatives of a lineage that has evolved relatively slowly and spun off fewer body plan variations than others. That’s why they seem so ancient. In a study of coelacanth evolution published earlier this year, paleontologists Lionel Cavin and Guillaume Guinot compared the rate at which coelacanths evolved new, distinct evolutionary features with those in tetrapods (four-limbed vertebrates that came up on land and proliferated) and ray-finned fish. Coelacanths, Cavin and Guinot estimated, have evolved at a rate six times slower than tetrapods, and three times slower than ray-finned fish. After a brief spurt of wild evolution around 400 million years ago, coelacanths haven’t evolved dramatically-different body types.

So coelacanths aren’t living time capsules, but are part of a 400 million year old lineage with a conserved body plan. Horseshoe crabs tell a similar tale.

Fossils of Limulus darwini. From Kin and Błażejowski, 2014.
Fossils of Limulus darwini. From Kin and Błażejowski, 2014.

Earlier this week, paleontologist Błażej Błażejowski published a new paper coauthored with the late Adrian Kin on a 148 million year old horseshoe crab that belongs to the same genus as the copulating arthropods I saw on the Delaware beach. While the living species is Limulus polyphemus, Błażejowski and Kin elected to name the fossil form, found in the limestone of Poland’s Kcynia Formation, Limulus darwini. The resemblance is striking. While there are a handful of characteristics that distinguish them, juveniles of today’s horseshoe crabs are the spitting image of the Jurassic species. This is the oldest representative of the Limulus lineage yet found, supplanting a Cretaceous horseshoe crab that had been discovered in Colorado.

If we took a condescending view, we could say that horseshoe crabs have been stuck since the Jurassic. While other forms of life flourished and were modified into fantastic new shapes, the horseshoe crabs kept grubbing in the sand for worms and clams. But the truth is that they’ve maintained their shape for so long because they are a great evolutionary success story. Modern horseshoe crabs are generalist feeders capable of living in waters cold and warm, deep and shallow. If the same was true of their prehistoric predecessors, it could explain why horseshoe crabs have gotten along just fine without major anatomical overhauls.

A comparison of modern and fossil Limulus. From Kin and Błażejowski, 2014.
A comparison of modern and fossil Limulus. From Kin and Błażejowski, 2014.

But what should we call such creatures? Living fossil doesn’t work as it obscures the nuances of evolution. And other descriptors coined for slow-evolving lineages, such as George Gaylord Simpson’s concept of “bradytelic” groups, are too technical. Błażejowski and Kin instead suggest a new term – stabilomorphs.

The  concept is a bit more refined than the notion of living fossils. Stabilomorphism, the researchers write, is “relative morphological stability of organisms in time and spatial distribution, the taxonomic status of which does not exceed genus level.” And there’s an additional corollary. Stabilomorphs must have survived at least one major mass extinction. This would mean that today’s Limulus would count as stabilomorphs, but crocodiles, coelacanths, and pearly nautilus would not.

Unfortunately, though, stabilomorph doesn’t have the cultural cachet of living fossil, and I can’t see the term showing up in science headlines anytime soon. If we’re going to sink the term living fossil, we need something that’s a little more accessible. I’m not enough of a wordsmith to coin a new one, but I have always liked Thomas Henry Huxley’s approach to the problem.

While widely known as “Darwin’s Bulldog”, Huxley wasn’t initially enamored with his friend’s formulation of evolution. When he looked into the deep past, Huxley didn’t see transcendent change, but rather minor variations on themes. In Huxley’s estimation, for example, the crocodiles of the Mesozoic looked little different from those of today. He called these examples “persistent types”, and suggested that most of evolution’s great transformations happened during a much earlier, “non-geologic” time, with the products maintaining their general form to the present.

By 1870, though, Huxley had largely given up this view of life. In the “higher Vertebrata”, at least, there were fantastic examples of evolutionary change, among the most spectacular being the evolution of horses from tiny, multi-toed ancestors to their big, single-toed modern forms. And while persistent types still existed, Darwin had squared them with evolution by natural selection. The winnowing edge of natural selection explained change as well as lack of change, meaning that the platypus and the horseshoe crab are just as important to understand as the quickly-diversifying beetles that enchanted Darwin early on.

While Huxley’s term is more qualitative, referring to forms “which have remained with but very little apparent change from their first appearance to the present time”, I think it has a far better chance than stabilomorph at cracking the public consciousness. And while not quite as evocative as living fossil, it still has a tinge of poetry to it. They are forms that truly have persisted, withstanding mass extinctions that have wiped out so many other varieties of life. By seeming to go against the grain, they remind us of the power of natural selection to preserve as well as modify and eliminate. Simply put, persistent types have withstood the test of time.


Cavin, L., Guinot, G. 2014. Coelacanths as “almost living fossils.” Frontiers in Ecology and Evolution. doi: 10.3389/fevo.2014.00049

Kin, A., Błażejowski, B. 2014. The horseshoe crab of the genus Limulus: Living fossil or stabilomorph? PLoS ONE. 9, 10: e108036. doi:10.1371/journal.pone.0108036