I’ve never been particularly interested in space. I think the fossil record is to blame. Not because paleontology takes up all the compartments in my brain for wonder or anything like that. It’s more that I don’t really care all that much about distant planets that may or may not harbor life when we have a fantastic record of strange creatures right here on Earth.
Just look at Dollocaris. I hadn’t even heard of the little Jurassic invertebrate until a few months ago, when paleontologist Jean Vannier and colleagues published a new analysis on the critter, but it immediately made me think of a little spacecraft. The small body, big eyes, and grappling appendages make the small arthropod look like one of the little repair ships you might see mending the surface of larger starcraft in the background of a big-budget scifi epic. But this was nothing alien or that we had to conjure through CGI. Dollocaris was real, and lived on our planet around 160 million years ago.
Exactly what Dollocaris was, no one really knows. The most specific paleontologists can get is somewhere in the realm of arthropods. But from there the spineless fossil has been associated with everything from mantis shrimp to remipedes – blind, worm-like crustaceans that live in the darkness of aquatic caves. If nothing else, this shows that having a complete fossil doesn’t always yield all the answers we want. The fossils of Dollocaris Vannier and coauthors present are stunning in their detail, but the little hunter is so odd that it falls into the big folder of Problematica that paleontologists regularly scratch their heads over.
Affinities aside, however, paleontologists have been able to work out a bit about this critter’s biology. For example, the eyes of Dollocaris are huge. Each bulb is about a quarter of the arthropod’s body length, with Vannier and colleagues estimating around 18,000 lenses on each of those eyes. Better still, the eyes of Dollocaris specimens found in southern France are among the best invertebrate eyes ever preserved, their detail intact down to the receptor cells. These show that Dollocaris was a highly-visual predator, Vannier and coauthors write, watching for movement to launch its spiky appendages forward and snag smaller arthropods, some of which were preserved in the gut contents of the precious fossils.
But the eyes of Dollocaris also raise a paradox. Vannier and coauthors report that its visual detection system would have been best near the surface in brighter water. Yet the Jurassic rock in which it was found has often been considered to be a darker place, primarily on the presence of creatures like vampire squid, deep-sea crinoids, and other other invertebrates found in dim waters today. The problem is that this is reconstruction by association, Vannier and colleagues point out, and perhaps these “deep sea” animals lived in shallower habitats in the past, only later rolling downslope into the abyss. Weird as Dollocaris is by itself, the invertebrate may have just made an entire window into the past a little stranger still.
Fossiling does a body good. At least that’s what I told myself as I tied my boots and pulled a slicker over my shoulders to shield myself from the drizzle coming down on the stretch of Charmouth shoreline. The cold I had been fighting since I left London called for bedrest, not soaking myself on England’s southern shore, but I only had two days allotted to try my luck in legendary fossil hunter Mary Anning’s old stomping grounds. There was no way I was staying under the covers. Wrapped up as best as I could manage, I pulled the hood over my baseball cap, sneezed, and started off with my wife Tracey along the shoreline.
I wasn’t expecting to find anything right away. At any fossil site, especially an unfamiliar one, it takes a little while to develop a “search image” – the recognition of telltale characteristics that makes fossils pop out from the background. But even though I was more accustomed to prospecting places that were far less saturated than the beach, I figured that luck would be with me. The stretch of sand Tracey and I walked along had given up fossils like a beautiful skeleton of a 191 million year old Scelidosaurus – one of the earliest armored dinosaurs – as well as reptilian fish mimics called ichthyosaurs and countless invertebrates, such as the coil-shelled ammonites. That’s really all I wanted, to find a little Jurassic coil resting on the sand.
It took about an hour and a half before we started finding anything. But once Tracey and I started to take notice of the sparkly little ammonite shells, shaded in gold thanks to the pyrite that was also slowly destroying them, they seemed to be everywhere. Soon the goal wasn’t to find the Jurassic whorls, but to spot the best ones. And in the process, at the edge of the receding water, I stumbled across a type of fossil I hadn’t held since my childhood. It was about the size and shape of a half-burnt cigar, and if I hadn’t been all stuffed up I might have attempted a Groucho Marx impression. I was given a little bagful of fossils just like these when I was a kid from a family friend I can only dimly remember, but I never forgot what these shells were.
They’re called belemnites. To me that name has always been synonymous with broken, wave-rolled pieces of what used to be part of the cephalopod’s internal skeleton. And, as museum displays and illustrations of ichthyosaurs feasting upon them taught me, their outer appearance was pretty much like that as a common squid that you might find frozen in the seafood section of the supermarket.
But despite my early introduction to the belemnites, I never bothered looking into what was known beyond the bullet-shaped guards I already knew about. My vertebrate bias blinded me. As it turns out, some truly exceptional specimens have not only revealed the form of the belemnites, but how they lived in the oceans of long ago.
Earlier this year, in Biology Letters, fossil cephalopod expert Christian Klug and colleagues described a pair of special belemnitid fossils found in the Late Jurassic limestone of Germany. On a superficial level these specimens of Acanthoteuthis look like smushed banana peels, but, under UV light, they truly shine. Funnel, esophagus, the buccal mass that held the pinching beak, and even that statocysts that helped these invertebrates know where they were in the water are all intact.
The preservation is so delicate that the two specimens can easily be told apart by their fins. One Acanthoteuthis has broad fins jutting from the tip of its streamlined body while the other is more minorly-equipped with itty bitty flappers. And the fact this pair of ten-armed cephalopods have fins at all confirms what paleontologists suspected, but were unable to confirm, for almost a century. Indentations on the internal skeleton suggested the presence of these propulsive structures, but soft-tissue support remained lacking. Now, as Klug and coauthors report, we can be sure that belemnitids sped around the underwater world with the help of fins.
And speed they did. Belemnitids have often been thought of as nektonic animals – that is, ones that lived their entire lives actively swimming in the water column. The fins fit this picture, but so do the statocysts that let the cephalopods orient themselves in the water. “The statocysts of fast-swimming buoyant squids are commonly larger than those of non-buoyant ones,” Klug and colleagues write, and the organs in Acanthotheuthis are more consistent with a life zipping around in midwater, swarming into the great mass of food that allowed the fantastic marine reptiles of the Jurassic to thrive and live as large as they did.
As far as invertebrates go, mantis shrimps are celebrities. They’re so creepy that they have come out the other side to become cool, and their penchant for punching or stabbing their prey with remarkable speed making them a pop science hit every time a new paper about their behavior drops. Not that any of this is brand new in evolutionary terms. Just like every other group of organisms alive today, mantis shrimps have a fossil record, and the latest member of their famous family has shown up in an unexpected place.
Marine biologists have counted 27 living species of mantis shrimp along North America’s Pacific coast. Most of these are scattered through the warmer waters of California and its southern Gulf. But in the past, when sea temperatures were warmer, mantis shrimps had an even wider range, underscored by a new fossil described by Carolin Haug and colleagues.
Thanks to a quartet of fossils, given the name Squilla erini, the paleontologists were able to put a pin in Oregon on the fossil mantis shrimp map. None have been found this far north before. More tepid temperatures probably allowed this species to make a home in a place that’s now inhospitable to the invertebrates, and perhaps offers a glimpse of what’s to come. In a warming world, maybe mantis shrimps will again go on the march.
Meaning:Squilla, Latin for “shrimp”, is the genus that many modern mantis shrimps belong to, while erini honors Erin Kovalchuk, the wife of study author Gregory Kovalchuk.
Age: Around 23 million years old.
Where in the world?: Oregon, U.S.A.
What sort of critter?: One of the mantis shrimps.
Size: About twice the size of today’s small-sized mantis shrimps.
How much of the creature’s body is known?: Most of the body from four partial bodies preserving different aspects of the shrimp.
Ivan the Terrible? He was pretty terrible. Alexander the Great? Pretty great. (At least at conquering.) Their names seem to fit their deeds. But here’s a name that doesn’t: Millipede. I’m talking about those many-footed little guys you sometimes see on the forest floor chewing on leaves, the ones that sometimes curl up into tight little spirals when you disturb them.
Yes, millipedes have feet, lots of them, so I’m good with the last syllable, ped, Latin for foot. But the first syllable, milli, Latin for a thousand? That’s a crazy exaggeration. Millipedes don’t have a thousand feet. Not even close.
The Embarrassing Truth About Millipedes
It turns out the world record holder, the most leggy millipede ever seen by scientists, says zoologist Rowland Shelley, is a little guy found in San Benito County in California in the 1920s. It had 750 feet. That’s a lot, but it ain’t no thousand. “It would have [had] to grow by another one-third … to become a true ‘thousand legger,’” Shelley says.
The sad fact is most millipedes (and there are thousands of varieties) are more “centi” than “milli”; they have fewer than a hundred feet. So as busy and as useful as they are (this time of year they are busy cleaning up our yards munching, munching, munching leaf bits), they aren’t anywhere as footy as they seem, and yet … here’s the surprise: When you think about millipedes too much, as I did today, you discover there’s an early moment in a millipede’s life when the most curious thing about it is not how many, but how few feet they have.
I’m talking about what a millipede looks like on its birthday—when it hatches and joins the world with its original set of baby legs. Say hello to a millipede toddler …
Millipedes are built in segments. Like repeating Lego pieces. Each segment (after the first few) is a fused combination containing two pair of legs, like this …
Put them together, and you have a working animal.
As a millipede grows, it molts, throws off its outer hard cuticle, or skin, and adds segments, so over its life, it adds more and more parts with more and more legs.
But when it’s a hatchling, a baby, it starts with a head (no legs), a helmetlike segment (no legs), and then maybe a preliminary segment (one pair of legs) plus a very few regular segments—which means it starts life with astonishingly few limbs.
Count the feet on this birthday boy (thank you Petra Sierwald, zoologist at the Field Museum in Chicago)—and you will see, Sierwald figures, roughly “six or eight” feet.
That’s not a lot.
Millipedes don’t hop. They can’t slither. They don’t walk like we humans do, one foot in front of another. “They move several legs together,” Sierwald says. “It’s a coordinated movement,” and that requires legs to work in teams. Three legs forward, three legs back—that sort of thing.
So how does a baby with six to eight legs create leg teams? Do they have enough legs to move, or do they spend their first weeks stockstill, staring at the world? “I don’t know,” Sierwald told me on the phone. “We have a bunch of toddlers here at the museum. We’ve never looked to see, but maybe we should.”
I think they should. Sierwald found an old paper by Danish zoologist Henrik Enghoff, who says most millipede babies stay put for their first few weeks, waiting for their next molting. Some though, with eight legs, do venture about, but either way, what we have here is an animal famous for its legginess—too famous, really—beginning life with a leg deficit. This makes me wonder: Do toddler millipedes fall over? Do they bump into roots? Get toppled by buttercups? Do they behave like baby humans and have to learn how to get places?
Just the thought of it makes me smile.
I got thinking about millipedes while reading Sue Hubbell’s Waiting for Aphrodite; Journeys Into the Time Before Bones. She describes herself as a giant “stumbling around in the world of little things,” which include sponges, sea urchins, bees, spiders, and, to my delight, these many-footed millipedes who’ve “been creeping around on this planet” for 400 million years.
The Cambrian was the Age of Weird. True, this is a bias that stems from considering the modern world’s organisms as “normal”, or at least familiar, but it’s difficult to look at some of the world’s first animals without seeing evolutionary oddities that held the potential for an array of unrealized alternate histories for life on Earth.
For example, it took 14 years for paleontologists to realize that they had been interpreting the spiny, 508 million year old Hallucigenia the wrong way up, and it was just last week that researchers finally resolved which end of the invertebrate was the head. Now, hot on the heels of that announcement, Yunnan University paleontologist Jie Yang and colleagues have described a relative of Hallucigenia that took spikiness to an unforeseen extreme.
[A restoration of Hallucigenia in motion.]
Discovered in the exceptional, 515 million year old Cambrian fossil beds of southern China, the new animal has been dubbed Collinsium ciliosum – or, if you like, the hairy Collins’ monster, named in honor of its discoverer Desmond Collins. It was one of the lobopodians – a group of worm-like, stubby-legged invertebrates that include velvet worms and “water bears” today – but this particular animal was exceptionally armed. As Yang and colleagues write, this was a “superarmored invertebrate”.
Other Cambrian lobopodians, like the famous Hallucigenia, had a row of paired spines running down their backs. But, as shown by the delicately-preserved fossils, Collinsium had sets of five variously-sized spikes in each position. And these defenses were so robust that they retained their three-dimensional shape even when the rest of the animal’s body was compressed by the fossilization process.
But the real novelty embodied by Collinsium, Yang and coauthors write, is the diversity of its appendages. A pair of antennae jutted from the animal’s head, followed by six pairs of fringed legs likely used for filter feeding – hence the “hairy” species name – and then nine pairs of clawed legs. This combination of features, the paleontologists speculate, might indicate Collinsium was a bit of homebody, climbing and anchoring itself on prehistoric sponges while it netted tiny particles of food with its fluffy-looking arms.
The closest living relatives of Collinsium, the velvet worms, don’t do anything like this. The living invertebrates are pretty conservative in their lifestyle, scurrying through the tropical forest undergrowth and shooting goo at prey. But back in the Cambrian their forerunners and close cousins thrived in an array of undersea niches. The course of life’s history has winnowed their kind down to a more homogenous set of species, and this is what makes Collinsium so strange. If the hairy monsters had survived to the present, their remains would not seem so wonderful.
[National Geographic News also covered this paper.]
Bones are symbols of death. That makes sense. Before X-rays and CT scans allowed us to look inside ourselves, we’d only see our skeletons after death and decay had stripped everything else away. Not to mention that poison labels, the Jolly Roger, and visages of the Grim Reaper have all reminded us of our eventual fate. But while these associations aren’t wrong, they suffer from being one dimensional. Bone isn’t just about death. For some creatures, bone is a giver of life.
On the surface, the phrase “bone-burrowing worm” sounds like something David Cronenberg thought up. Perhaps, had they been discovered sooner, the body horror director would have used them. The invertebrates weren’t found until the year after The Fly debuted, pocking the skeleton of a whale that had perished off the California coast. In time, these specialized worms – known by the scientific name Osedax – became known as central players in the succession of “deadfall” critters that make their living on bodies that sink to the bottom. The end of one life enriches countless others.
But when did Osedax start drilling their way into the deceased? Pinholes in fossil whale bones, bolstered by estimates of genetic divergence among living worm groups, showed that Osedax – or very similar annelids – have been sinking their roots into undersea skeletons for millions of years. For as long as there have been whales, it seems, there have been worms that could take up residence on their remains.
Whales were not the first creatures to trade life on land for one spent entirely at sea, though. Starting around 245 million years ago, about 190 million years before the earliest whales took their first dip in the water, multiple marine reptile groups slid into the seas. Those disparate forms – the fish-like ichthyosaurs, quad-paddled plesiosaurs, sea turtles, and more – flourished throughout the Mesozoic, the last of these “sea dragons” going extinct about 66 million years ago. Surely these marine reptiles died and sank to the bottom just as whales do today. Is it possible that Osedax evolved to inhabit their bones and only later continued the tradition with whales?
The recent discovery of modern deadfalls sent paleontologists searching for more ancient equivalents. And they found them. A pair of plesiosaurs excavated from the 86 million year old rock of Japan hosted late stage deadfall communities where snails grazed on mats of bacteria, and, last year, Plymouth University paleontologist Silvia Danise and colleagues reported on a 145 million year old ichthyosaur from southern England that documented how the marine reptile hosted a changing array of scavengers. Yet no one was able to find signs of Osedax. The oldest confirmed traces of the worm were 30 million years old, and it was unclear whether the worm hadn’t evolved in the time of the marine reptiles or whether its taphonomic calling cards were absent from the known finds. Now, in a new Biology Letters paper by Danise and Nicholas Higgs, paleontologists have their answer.
Osedax have been taking advantage of deadfalls since the Cretaceous. The evidence, Danise and Higgs report, can be seen on an upper arm bone of a plesiosaur and two sea turtle bone fragments found in the 100-93.9 million year old sediment of England. The Cretaceous bones bear burrows that correspond to those made by modern Osedax species, showing a narrow opening to the surface with a glob-like chamber beneath. Even though the worms themselves didn’t become preserved, the traces of their osteological feast give them away.
But here the fossil record would seem to hit a snag. The last whale-sized marine reptiles died out 66 million years ago, and the first fully-marine whales didn’t evolve until about 20 million years later. This bottleneck in the food supply could mean that worms with an Osedax-like lifestyle evolved more than once – just as multiple forms of bone-burrowing beetles have utilized dinosaur and fossil mammal bones – but, more likely, it speaks to how flexible these worms are.
While some marine reptile lines ended by the close of the Cretaceous, sea turtles survived. Osedax could have kept persisting on their skeletons, at the very least. And, from geologically younger finds as well as experimental studies, marine biologists have found that Osedax are not especially picky about whose bones they’re colonizing. A whole whale is great, but cow, bird, or fish bones will do in a pinch. Magnificent giants that paddle through the surface waters have come and gone, but, for over 93 million years, the bone-eating worms have been waiting for them.
There is nothing wrong with your monitor. Do not attempt to adjust the picture. The creature at the top of this post isn’t an April Fool’s joke, nor is it a Lovecraftian nightmare. Named Yawunik kootenayi, the archaic arthropod is a real animal that undulated through Earth’s seas 508 million years ago.
Following hot on the heels – or is that fins? – of the filter-feeder Aegirocassis – Yawunik is the latest ancient invertebrate to make us ask “What the heck is that thing?” Described by paleontologists Cédric Aria, Jean-Bernard Caron, and Robert Gaines from 42 fossils found in Canada’s Kootenay National Park, the Cambrian critter adds to the wonderful and perplexing spread of body plans that had evolved by this chapter in Earth’s history – jutting out from beneath the invertebrate’s tough exoskeletal hood are paired, pinching appendages arrayed with long wisps. The overall effect is of a lobster tail that’s out for revenge on those who drew butter against it.
At the time that Yawunik swam around delivering deadly pinches to worms and other small prey, though, there weren’t lobsters yet. Aria, Caron, and Gaines propose that Yawunik belonged to a lineage of invertebrates called leanchoiliids – a group so obscure they don’t even have a Wikipedia page summarizing what they are – that fit near the base of the arthropod family tree. This doesn’t mean that Yawunik was an ancestor to today’s insects, crustaceans, and arachnids, but rather that it was part of an evolutionary explosion from which the true arthropod ancestors emerged.
Fossils provide the only views of such startling species. Without the exquisite preservation of the Burgess Shale, we wouldn’t be able to puzzle over Yawunik and its bizarre neighbors. That’s the wonderful thing about paleontology. You can’t make this stuff up.
Paleontologist Jakob Vinther pointed to a rust-colored boulder sitting on the black lab table. “What do you think that is?”, he asked. I hadn’t a clue. I was used to looking at bones – often really big saurian bones – and I couldn’t pick out any endoskeletal signs in the stone. It was mostly the fossil’s size that struck me. Whatever it was, the specimen was almost as big as me. I shrugged and opened my mouth to hazard a guess, hoping that some shot in the dark would get me close to the answer, but Vinther spoke before I did. “That”, he said, “is a giant anomalocaridid.”
There’s no standardized common name for these early animals. The closest is a literal translation of the name – “anomalous shrimp” – that was coined by paleontologist Joseph Frederick Whiteaves over a century ago. But even that title has more to do with history than actual identity. Paleontologists Simon Conway Morris and Harry B. Whittington later discovered that the fossil Whiteaves identified as a crustacean tail was actually a curled feeding appendage of a bug-eyed arthropod that used those spiny arms to stuff small prey into its shutter-like mouth. These animals became known as the anomalocaridids, and they were among the largest, strangest creatures to swim through Earth’s Cambrian oceans.
These anomalous invertebrates were so bizarre that they seemed to be perfect illustrations of Stephen Jay Gould‘s central point in his masterpiece Wonderful Life. Along with other oddballs, the anomalocaridids appeared to die out by the end of the Cambrian, 485 million years ago. They were an early evolutionary success story that was cut off by total extinction, but, had they survived, they would have altered the course of life on Earth. It might have even changed what we find frightening. If the anomalocaridids had survived to the present day, Gould wrote in jest, “Why not a Steven Spielberg film with a crusty seaman sucked into the cylindrical mouth of a sea monster, and slowly crushed to death by multiple layers of teeth lining a circular mouth and extending well down into the gullet?”
But what paleontologists have started to discover is that the anomalocaridids – and other contemporaneous weirdos – were not simultaneously snuffed out at the end of the Cambrian. Some, like a rather cute little anomalocaridid named Schinderhannes, lived about 80 million years after the close of the Cambrian. And while not quite as geologically young, the giant Vinther introduced me to was another survivor. Named Aegirocassis benmoulae by paleontologists Peter Van Roy, Allison Daley, and Derek Briggs, the ancient invertebrate is a sign that the evolutionary aftershocks of the Cambrian Explosion were still felt long after the event’s initial spark.
Aegirocassis – a reference to a mythological Norse giant and the Latin word for shield – swam through seas that covered what is now Morocco between 485 and 443 million years ago. Along with a slew of other creatures, it was part of what paleontologists have dubbed the Fezouata Biota – a combination of more archaic Cambrian creatures that lived alongside more familiar lineages that formed the basis for what we think of as “modern” oceans. In particular, the anatomy of Aegirocassis shows that anomalocaridids weren’t technically arthropods, but the flaps along their bodies represent equivalent parts to arthropod limbs. In other words, anomalocaridids like Aegirocassis might provide a look at parts of the body plan that evolution co-opted and modified in true arthropods.
The newly-named fossil also adds to paleontologists’ understanding of our evolving oceans. At over six feet long, Aegirocassis was one of the largest anomalocaridids. Despite facile headlines calling this creature a “sea monster” and “Frankensquid“, though, Aegirocassis was only terrifying to the tiny. The invertebrate’s great appendages carried rows of fringe-like spikes, reminiscent of a whale’s baleen, that sieved plankton from the water. This adds Aegirocassis to the growing number of filter-feeding anomalocaridids, and, as Van Roy and colleagues write, the invertebrate’s anatomy is a sign of sea changes.
The giant individual Vinther showed me wasn’t a loner. The paleontologists who have sifted through the Ordovician rock have found at least thirteen individuals, and probably more. Some of these might represent shell moults left on the bottom as groups of Aegirocassis sloughed off their old armor, much like some undersea arthropods do today. But, in addition to their size and anatomy, this anomalocaridid abundance is another sign that plankton were flourishing during this time, allowing filter-feeders to become larger and more numerous than ever before. As otherworldly as Aegirocassis may seem, the plankton-sucker helps mark the origin of ocean life as we know it.
A good fossil squid is hard to find. The invertebrates are too squishy to leave much behind, and only in truly exceptional circumstances do paleontologists get to see much more than the chitinous supports the cephalopods kept on the inside. Octopus are even more confounding. Without any remnants of an internal shell, the eight-armed quick-change artists are like prehistoric ghosts. But do not despair. The jaws of ancient coleoids give us reason for hope, and have just revealed a pair of prehistoric cephalopods that may have rivaled today’s ocean giants in size.
Described by Kazushige Tanabe and colleagues in Acta Palaeontologica Polonica, the two lower jaws were found in roughly 80 million year old rock around Hokkaido, Japan’s Haboro-futamata Dam. Both were preserved in three-dimensional detail, providing Tanabe and coworkers with enough anatomical clues to figure out that they were left behind by previously-unknown species.
Octopus and squid beaks can be very informative fossils. That’s because marine biologists have spent a great deal of time studying the chitinous beaks of modern cephalopods. (Not much more than beaks and hooks are left in the guts of squid-eating whales, for example.) So by comparing the shape of the fossil lower jaws with those of fossil and modern cephalopods, Tanabe and coauthors were able to narrow down what sort of creatures the fossil beaks represent.
One of the jaws, assigned to the new species Nanaimoteuthis hikidai, most closely resembled those of today’s vampire squid. Don’t be thrown by the name. The lineage actually falls on the octopus branch of the cephalopod family tree. All the same, based on the relationship between beak size and body length in the modern species, Tanabe and colleagues estimated that their fossil octopus had a mantle length – or, the body minus the arms – of over two feet. That might not be It Came From Beneath the Sea proportions, and it’s assuming that the fossil species was similar to its only living relative, but it’s still pretty big for an octopus.
The other fossil beak sat in the mouth of an even larger cephalopod. Named Haboroteuthis poseidon by the researchers, the creature was a Cretaceous member of the lineage that contains modern squid. And from its jaw size, it was quite an impressive invertebrate.
Measuring a ridge that runs up the front of squid beaks, Tanabe and coworkers found that Haboroteuthis had a “crest length” of about 2.4 inches. A 25-foot-long giant squid caught off New Zealand, by contrast, had a crest length of only 1.8 inches, and a Humbolt squid with a mantle length of almost five feet had a crest length of 1.9 inches. Haboroteuthis was at least comparable to these modern heavyweights. We may never know for sure exactly how largeHaboroteuthis was, but, if its jaw is anything to go by, it was as big as some of today’s undersea giants.
Science often answers questions that I never would have thought to ask. For example, what happens to a pig carcass when you leave it on the ocean bottom?
This isn’t science trivia. While scientists know quite a bit about how bodies decompose on land, forensic researchers Gail Anderson and Lynne Bell point out at the start of their new PLoS One study, most of what we know about undersea carcasses comes from studies of whales, porpoises, and sharks. Very little is known about the waterlogged afterlives of mammals that are more like humans in size and anatomy.
To change that, Anderson looked to pigs – often used as proxies for humans in such studies because of their comparable size and skin. In a previous experiment with N. Hobischak, Anderson documented what happened to a trio of freshly-killed pigs that the researchers anchored in the shallows of British Columbia’s Howe Sound. That study outlined what sort of critters came to dine on the pigs, the effects of depth on decomposition, and other factors of breakdown, but there was a problem. The researchers had to dive to gather their data, meaning that they were missing some of the changes in the porcine breakdown. They lacked a continuous picture of what was happening to the pigs. Picking up where the previous study left off, the new research by Anderson and Bell tells a more detailed tale of what happens to such submerged bodies.
At the rate of one pig each year between 2006 and 2008, the researchers lowered three pig carcasses into British Columbia’s Saanish Inlet at a depth of about 313 feet. The choice of location was critical to what eventually unfolded. For most of the year, Anderson and Bell point out, the inlet water is relatively low in oxygen, only replenished in the fall. And yet, despite these hypoxic conditions, the inlet hosts a rich assemblage of marine critters that would eventually play a part in the breakdown. And as a scientific benefit, the inlet is the base site for the Victoria Underwater Network Under Sea (VENUS), equipped with instruments that allowed Anderson and Bell to gain a constant stream of data on temperature, pressure, dissolved oxygen, and other measurements from the study sites.
Of course, “carcass deployment” and study presented some unique challenges. Each pig had to be weighted down so that they’d stay within range of the underwater cameras needed to photograph them. (Although this didn’t prevent the first carcass from being dragged out of range by crabs after 23 days.) And the researchers had to think carefully about turning on the camera lights. Some of the little scavengers would scatter when the lights flicked on, although the typically returned very quickly. Even so, to prevent the lights from greatly affecting the habits of the dinner guests, the researchers turned on the camera lights sparingly.
I’ll let Anderson and Bell describe what happened once the first two pigs were in position:
Within minutes of placement, large numbers of Munida quadrispina Benedict (squat lobsters, Family Galatheidae) arrived at Carcass 1 and 2 and began to pick at the skin, attracted to the entire carcass, with some preference for the orifices. Scanning the camera around the area showed that very large numbers of M. quadrispina were actively moving towards the carcasses from all areas.
Three-spot shrimp and Dungeness crabs soon appeared, as well, and the scavengers were so quick with their work that, as the researchers report, “On Day 2, a substantial portion of the rump area of Carcass 1 was removed, and a large flap of skin and flesh from the abdominal area was opened.” Crustaceans did most of the disassembly, although a passing blunt-nose sixgill shark also took a sizable chunk from the pig.
The rest of the paper is full of macabre details delivered in a scientific deadpan that could almost pass as gallows humor – “[O]n Day 4 M. magister was seen pulling the tongue out of the mouth and consuming it” – but the scientific details of the breakdown gave Anderson and Bell a serious look at what happens to bodies under shifting sea conditions.
Pig carcasses 1 and 2 followed a similar pattern. Both dropped at times of relatively low dissolved oxygen, the pigs immediately attracted a horde of shrimp, squat lobsters, and crabs that rapidly dismantled the bodies. Pig carcass 3, however, was lowered at a time of even lower dissolved oxygen. Some squat lobsters showed up at the beginning, but their claws weren’t powerful enough to do anything more than graze the surface of the carcass. Without larger crabs with more powerful snipping apparatus, the squat lobsters left and a furry mat of bacteria grew over the body. The pig lay there almost undisturbed until the oxygen in the water was refreshed. Only then did both invertebrate and vertebrate scavengers make short work of the carcass.
The pigs didn’t just rot away. They were consumed down to the bone. While the first carcass was dragged out of view before the end of the experiment, total skeletonization took 38 days for carcass 2, and 135 days for carcass 3 owing to the long period of stasis due to low oxygen.
The overall pattern was quite different from what Anderson observed in the previous study. In the Howe Sound experiment, the pig carcasses went through a familiar pattern of decomposition that included initial floating, sinking, and “bloat and float” – which is exactly what it sounds like – before finally settling on the bottom. In the new experiment, the pigs didn’t fill up with decompositional gases. The water pressure at their depth prevented them from doing the dead pig’s float. The pigs stayed put unless dragged out of place by the scavengers.
One of the lessons from all this, Anderson and Bell found, is that the arthropod community surrounding a cadaver found at sea isn’t a reliable indicator of when that body entered the water. Anderson and Bell didn’t observe any rigid succession of scavengers that could be used to estimate time of death. Instead, varying oxygen levels dictated the degree of scavenging – skeletonization of carcass 3 took three times as long because of low oxygen conditions at the time the pig entered the water. Translating this to search and rescue efforts, a body that came to rest at a depth more than 200 feet in very low-oxygen conditions will be more likely to be in place and intact because the conditions are inhospitable to scavengers.
And at a much more minute level, studying the damage done by the scavengers can be critical to investigating human bodies that wind up in the ocean. “[I]t is not uncommon for disarticulated human appendages to be recovered washed up on beaches,” Anderson and Bell write, “leading to media speculation of dismemberment and foul play.” But, as the researchers found, scavengers can easily dismember and disarticulate a body, leaving tell-tale damage behind. Understanding how scavengers feed, and how quickly they can alter a body, can be critical to accurately reconstructing marine crime scenes. When authorities think they have a maritime murder on their hands, crabs can be critical in court.
We’re fascinated by superlative size. That’s why humungous dinosaurs regularly makeheadlines, and Carboniferous arthropods – dragonflies and millipedes that reached B-movie sizes by dint of higher atmospheric oxygen – are paleo-documentary regulars. And beyond their size, we’re transfixed by why and how such giants could evolve. What’s strange, then, is that we haven’t paid more attention to the giant marine invertebrates of the prehistoric past.
Today’s giant and colossal squids were hardly the first invertebrate giants to inhabit the seas. Squishy and shelly critters with sizes over a foot and a half long have evolved multiple times during the last 500 million years and are well-known among paleontologists who specialize in spineless species. Endoceras giganteum, a 451 million year old cephalopod that lived inside an elonged cone of a shell, could get to be about 15 feet long, and there are rumors of lost specimens 30 feet in length. The 404 million year old sea scorpion Jaekelopterus rhenaniae has been estimated to be over eight feet long, and the 465 million year old trilobite Hungioides stretched nearly three feet long. And that’s just a few of prehistory’s immense invertebrates.
But is there any pattern to the origin of these tentacled and joint-legged giants? That’s the question behind a newly-published study by Universität Zürich paleontologist Christian Klug and colleagues in the journal Lethaia. Within a window of 500 to 300 million years ago, Klug and coauthors looked to see if the occurrence of giant cephalopods and arthropods in space and time corresponded to changes in oxygen levels, temperature, and sea level.
There was no simple connection between the environmental factors – all implicated as possible triggers for gigantism – and large body size among the marine invertebrates. Instead, the superlative species seemed to cluster around two times when marine life flourished.
Within their 200 million year window, Klug and colleagues found, the largest shell-covered cephalopods evolved about 475 million years ago. The largest trilobites weren’t very far behind, at 468-460 million years old, and another group of archaic arthropods – the weird anomalocaridids – counted their largest members in the 488-472 million year range. All three groups independently evolved huge size within a 28 million year window that coincides with the Great Ordovician Biodiversification Event – an evolutionary pulse that not only spun off many new species, but also allowed new, complex marine ecosystems to evolve.
Many of the other species in the study – including large cephalopods, trilobites, and sea scorpions – cluster around another, narrower window. All of these different groups again generated giant species between 400 and 383 million years ago during an event called the Devonian Marine Nekton Revolution. In brief, seafloor and deep water habitats were so saturated with species that competition drove the evolution of more animals capable of living suspended within the water column.
Huge, cone-shelled cephalopods and massive trilobites didn’t evolve because of simple causes such as an abundance of oceanic oxygen or dips in ocean temperature. Giants evolved in cool and warm seas at high and low sea levels and at various oxygen concentrations. Instead, remarkable body size among the marine invertebrates appears to be tied to major diversification events. Whatever allowed the group as a whole to flourish, Klug and colleagues point out, is what gave rise to giants.
But environmental shifts may explain a different part of the evolutionary picture.
Between 500 and 370 million years ago, most of the giants lived at high latitudes, closer to the poles. But starting at 370 million years ago, their occurrences creep down the latitudes towards the equator. And it was during this time that glaciers were forming in the southern hemisphere, the global sea level sank, temperatures dropped, and oxygen levels rose more than 20%, creating cooler, nutrient-rich seas. Perhaps these cooler, more productive seas created new constraints for where the biggest invertebrates could live.
This could be a coincidence. Klug and coauthors point out that sampling bias – where fossils are and can be collected – might influence this pattern, and note that perhaps continental shifts created rich, productive shelf environments that altered where giant species were likely to evolve. But the fact that all the groups investigated in the study – belonging to distinct lineages different in anatomy and physiology – followed this pattern suggests there’s some as-yet-unknown cause for the global shift. Paleontologists can pick out the biggest of the big, but why marine giants emerged where they did requires the continued imagination and investigation of long-lost seas.
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.
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.
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.
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.
The La Brea asphalt seeps are practically synonymous with megafauna. Sculptures of American lions and scrapping Smilodon draw visitors into the on-site Page Museum, well-stocked with Ice Age celebrities that have been reconstituted from the mind-boggling number of bones found beneath Los Angeles. Even more bones are kept in rows upon rows of bins in the collections – perhaps the greatest fossil dataset in the world – but it would be a mistake to think that La Brea is all about sabercats and mammoths. The story of prehistoric California was like, and how the world has changed as the last Ice Age slipped away, is kept by a diversity of meeker creatures, including a pair of unborn leafcutter bees that may be the most intricate fossils ever to be pulled from La Brea.
In 1970, from a La Brea dig called Pit 91, excavators found a tiny little nub of plant material. Anywhere else this pill-shaped fossil might have been treated as an uninteresting bit of ancient scrap, but La Brea had made a policy to collect and catalog every scintilla of fossil material they found, down to beetle wings and plant pieces. The fossil was saved, curated, and was later found to be part of a prehistoric bee nest, but it was otherwise forgotten. The insects of La Brea haven’t received anywhere near the same scientific attention as the site’s large beasts.
That lack of interest from other researchers created an opportunity for entomologist Anna Holden, who wanted to know just what the Page Museum had in their insect stockpile. “I knew that I would find treasures going through the insect collections,” Holden says, and when she opened a small fossil labeled LACMRLP 388E, she immediately knew she had something special. “I got to this snap cap and said ‘Oh my god, these are leafcutters.'”
The fossil itself wasn’t an adult, petrified bee, but rather a leafcutter bee nest. The distinctive way these bees use plant material to make capsule-like enclosures for their young gave them away. No one had found evidence of these bees at La Brea before.
Leafcutter bees aren’t as well known as their honey- and bumble- relatives, but they’re still around. “They’re everywhere and people just don’t know about them,” Holden says. They’re nonsocial pollinators than zip around dusting their bodies with pollen, which the La Brea leafcutters undoubtedly did as mammoths, sloths, and camels trod around southern California around 40,000 to 35,000 years ago.
But the fossil is more than just a shell. Holden took an X-ray of the prehistoric nest to see if anything might be in side. I can’t repeat her exclamation upon seeing the X-ray here – a joyously-delivered expletive common to scientific discovery – but she was elated to see little blobs indicating that there were leafcutter pupae entombed inside their fragile nest. The next step, with the help of paleontologist Justin Hall, was to CT-scan and visualize the Ice Age bees.
“When I saw the CT reconstructions, I would just play them over and over again,” Holden says. “I just couldn’t believe how well-preserved these pupae were.” The two developing bees were so intricately fossilized, in fact, that Holden and her colleagues were able to identify them as Megachile gentilis – a leafcutter bee that still lives in the American northwest and southwestern Canada.
How could something so delicate become preserved in a place that had totally disarticulated and dissembled countless mammal skeletons? Holden pored over diagrams and field notes to retrace where exactly the nest had been found. The possibility that the bee nest had somehow been washed to its location from somewhere else had to be ruled out. The finely-detailed data collecting standard at La Brea became essential. “I’m very grateful that people were very responsible in taking all that information,” Holden says.
The upshot of all those collection details is that leafcutters really did live at La Brea. Rather than tumbling into a tar pit, Holden says, this nest was made in the ground – just as with living leafcutters – and oil seeped into the cells to effectively embalm the nest and pupae inside. And while such a nest might seem fragile, the fact that leafcutters are ground-nesters means that the bees have to make strong surroundings for their offspring. “I have an understanding now that these nests are much sturdier than they appear,” Holden says. That solid construction and a matter of circumstance saved these bees for the fossil record. “If it wasn’t for this fossilization circumstance, Holden says, “we wouldn’t have these specimens.”
Now, with those specimens in hand, Holden and other Ice Age ecologists can get a finer understanding of what the end-Pleistocene world was like. From what’s known of living Megachile gentilis, Holden says, the presence of this leafcutter species at La Brea indicates that Ice Age Los Angeles was a moister environment with woody habitat near streams. The Pacific Northwest habitats where the bee lives now are a rough proxy for Los Angeles 23,000 years ago, giving us another line of evidence for how changing climate has dramatically altered ecology.
And the pupae may yield even more information about the lost world of the Ice Age. As CT scans get better, paleontologists can see and study small fossils in ever-greater detail. Holden and colleagues might be able to scan the nest again to look for pollen that the nesting bee placed in each cell with the pupae. “Bees are often very specific about the kind of pollen they use,” Holden says, so Ice Age pollen would add even more flourishes to the ecological picture these bees are helping to create.
This fossil is “the gift that keeps on giving,” Holden says. “The yield of paleoecological information is so rich. We learn from the leaves, the bees themselves, even where the nest was found, on the ground at Pit 91.” Some people may still prefer mastodons or short-faced bears, but, given the simple beauty of the bee’s nest and all it can teach us, Holden says “This fossil is my favorite one.”
The Cambrian oceans hosted a riot of evolutionary novelty. Over a seabead burrowed by penis worms and tread by living pincushions, multi-eyed invertebrates swung their schnozzles after prey and our closest, archaic relatives squirmed through the water. Largest of all were the anomalocaridids – cousins of arthropods that flapped through the water on segmented wings and were equipped with a pair of “great appendages” hanging below a pineapple-ring mouth. Their size and flexible, spiky arms have made them dead ringers for apex predators in the eyes of paleontologists, but new research has cast at least one of these mind-bending invertebrates as a filter-feeder that was only a threat to plankton.
The pioneering planktivore was Tamisiocaris borealis, a relatively new addition to the anomalocaridid family tree named by paleontologists Allison Daley and John Peel in 2010. That description was based on a sole great appendage found in the 520 million year old rock of North Greenland’s Sirius Passet.
Against the slow grind of paleontology publication, however, discoveries in the field can quickly turn up additional parts of organisms that are already on their way to press. Expeditions in 2009 and again in 2011 uncovered additional appendages of Tamisiocaris in an even better state of preservation.
Paleontologist Jakob Vinther, lead author on the new Nature paper that casts the creature as a suspension-feeder, was immediately excited by the finds. “I remember writing on the package with the fossil, ‘Great Appendage!!!!’,” Vinther says, and the delicate details of the new fossils brought up an intriguing possibility that had only been hinted at before. “The fine bristles, which I could see even in the field immediately,” Vinther says, “made me think that this is a filter feeder and I quickly started thinking about the evolution of baleen whales and whale sharks.”
At a glance, the great appendages of Tamisiocaris look to be tipped with long, nasty, backward-pointing spikes perfect for skewering squishy prey. In detail, however, each of the spines has a network of smaller filaments branching off to create what would have been a flexible net in life. This was not an arm for piercing prey, but an appendage for sifting small organisms from the seas.
Through digital reconstructions done by coauthor Martin Stein, Vinther and colleagues replayed how Tamisiocaris must have snagged little morsels. “We could see that upon contraction,” Vinther says, “the appendage would curl up and form a basket, which would concentrate the food particles and in the process of contraction the basket of goodies would be adjacent to the mouth.”
The question is how Tamisiocaris managed to eat those “goodies.” The filtering appendages weren’t able to directly deposit food into the critter’s mouth, and there’s evidence to suggest that anomalocaridids had additional limbs to help feed themselves. Instead, based on recent work by Allison Daley and Jan Bergström, Vinther hypothesizes that “the anomalocarid pineapple mouth was a suction apparatus, like a goldfish mouth.” Since no one has described the mouth of Tamisiocaris just yet, though, discerning the arm-to-mouth feeding method of Tamisiocaris relies on future fossil finds.
Just how a filter-feeder could have evolved from a line of predators is a little clearer.
Even though there has been some debate about just how rapacious classic forms such as Anomalocaris truly were, there were numerous anomalocaridids with spiky great appendages. Some, such as those of Amplectobelua stephenensis and Stanleycaris hirpex, are the stuff that nightmares are made of. And among this array of anomalocaridids that likely snatched and punctured prey, Tamisiocaris shows how those traits could be tweaked into an entirely different sort of feeding apparatus.
“We see some forms which have slightly longer spines on their appendages,” Vinther says, “perhaps they could have swept up prey in midwater, and by making the spacing of the spines finer, then you would capture things that are smaller.” The netting of Tamisiocaris was a modification of what already existed.
Such transitions have happened multiple times in the history of life. Both filter-feeding sharks and baleen whales evolved from sharp-toothed ancestors, marking alternate routes for planktivores to evolve from predators. And drawing from whales and sharks, artist NocturalSea imagined a speculative version of a filter-feeding anomalocaridid for the All Your Yesterdays art project. “We were all a bit weirded out that someone actually thought up this thing,” Vinther says, as “his reasoning is the same as ours.” A true case of Cambrian convergence.
The partnership between clownfish and sea anemones is one of the most iconic in the animal world. Unlike in Pixar’s film Finding Nemo, clownfish seldom stray far from their anemone. During the day, they dart through the water overhead to catch morsels of food. At night, they snuggle deeply within the stinging tentacles. And the nocturnal half of this routine is still providing us with fresh surprises.
For ages, everyone thought that the two partners were a joint self-preservation society. The anemone’s tentacles provide the clownfish with protection from predators, while the clownfish chase away butterfly fish that would eat the anemone. More recently, Nanette Chadwick from Auburn University in Alabama showed that the fish also fertilise the anemone with their ammonia-rich waste.
Now, Chadwick’s team has found that the fish provide another secret service: They help the anemones breathe at night.
Sea anemones can do very little to control the flow of water across their bodies, and they rely on local currents to bring in oxygen and nutrients. For decades, scientists have suggested that the clownfish could help, but no one has actually tested that idea.
Chadwick and student Joseph Szczebak did so by studying the two-band clownfish and its host, the bubble-tip anemone, both collected from the Red Sea near Aqaba in Jordan. By separating the partners and measuring the oxygen concentrations in the surrounding water, Szczebak and Chadwick showed that, in the dark, they consume around 40 percent more oxygen together than apart. This only happened if they could actually made contact. If they were separated by a mesh barrier, and could see and smell but not touch one another, they used up less oxygen.
To understand why, Szczebak filmed the partners at night with infrared cameras, and saw that the clownfish were far more active than anyone suspected. While other marine biologists had claimed that clownfish stay still through the night, Szczebak saw the opposite—they spent most of their time moving. They would wriggle forcefully to wedge themselves between the anemone’s tentacles, and often made 180 degree turns in the process. And they never behaved like this when anemones were absent.
The fish’s frenetic dance aerates the anemone. Sway though they might, the tentacles aren’t great at moving water back and forth and stagnant zones can build up around them. That limits the movement of prey, nutrients, gases and more, and severely limits the anemone’s ability to grow. But the clownfish actively moves and opens up the tentacles, while encouraging water to flow between them. The fish uses up more oxygen because of the effort it makes and the anemone does so because of its partner’s antics, explaining the duo’s 40 percent hike in oxygen use.
The same dynamics are at work elsewhere on coral reefs. Many fish like gobies and damselfish live in corals at night, and face chokingly low levels of oxygen due to weakly flowing water. Both are known to beat their fins faster than normal to create a refreshing flow.
The fact that clownfishes do the same thing is a new slant to an old story. But there are still chapters waiting to be told. Consider this: Szczebak found that a clownfish is more likely to make its wedging, U-turning movements if it’s downstream of its anemone. Is the host releasing chemicals that spur the fish to start its dance? And what exactly is the clownfish doing? Anemones can expand and contract, and it may be that the fish are just trying to give themselves more room by inflating their hosts. Is aerating the anemone the point, or just a side effect of movements that accomplish some other purpose?