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.

Reference:

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

“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.”

Reference:

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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Why Do Glowing Sharks Glow?

There are about 550 species of shark in the oceans. Around twelve percent of them glow.

These luminous fish belong to two groups: the kitefin sharks and the lanternsharks. They are little creatures, no bigger than 50 centimeters long, and they feed on small fish, squid, and crustaceans. They also live in the deep ocean, which means that we know very little about how they live. But whatever they’re doing, they’re clearly doing it well. “They’re some of the most successful groups of sharks,” says Julien Claes from the Catholic University of Louvain. One genus alone—the Etmopterus lanternsharks—includes 38 distinct species. “And we discover new ones every couple of years.”

Claes has been studying these elusive fish for a decade, and has been slowly piecing together the purpose of their glow. The light comes from many small organs called photophores, which dot their bellies and sometimes their flanks. No one knows how these structures produce light, but Claes and his team are getting close to knowing why they do so.

Glow from a smalleye pygmy shark. Credit: Jerome Mallefet
Glow from a smalleye pygmy shark. (Photograph by Jerome Mallefet)

First, they showed that the sharks use their light for camouflage. Even though they live at extremely dark depths, any predators watching them from below would still be able to make out their silhouette against the faint remnants of sunlight welling down from above. But the glow from their bellies perfectly matches this downwelling light, and cancels out their outlines. In technical terms, the sharks use counter-illumination. In simple terms, they cast no shadows.

Next, the team showed that at least one species, the velvet belly lanternshark, uses its light as a warning. It has two wicked spines behind its back fins, which are illuminated by a row of unusually placed photophores—Claes called them light sabers. Together with vision expert Dan-Eric Nilsson, he showed the floodlit spines are visible at a distance of 3-4 meters—a perfect distance to ward off an approaching predator, without also advertising the shark’s presence.

Finally, the team thinks that some lanternsharks also talk to one another using light. Most species only have photophores on their bellies, and are probably using those solely to camouflage themselves. But the Etmopterus lanternsharks also have photophores on their flanks, and the patterns vary from one species to another. Perhaps they act as badges of identity, helping the sharks to find others of their own kind. (That is certainly a valuable skill, since several of these species are often found in the same area.)

Claes knew this idea had legs after watching captive velvet belly lanternsharks swimming in a tank. Suddenly, he noticed that the glow from their flanks seemed to turn on and off. “We thought, Oh my god, these sharks can flash!” he says. “Then we realised that it’s an optical illusion.” The photophores produce a very narrow beam that’s only visible from certain angles. And as these sharks swim, they rotate their bodies from left to right, turning what’s actually a steady stream of light into what looks like a strobe. “You really get the feeling that it’s flashing like a firefly,” says Claes. And that, he thought, looked like communication.

“Testing communication in deep-sea animals like glowing sharks is extremely challenging and almost impossible,” he admits. So, his team had to resort to other methods. Working with Nilsson again, they confirmed that the velvet bellies can clearly see each other’s flank markings from a reasonable distance.

Next, they showed that the Etmopterus lanternsharks, with their flank photophores, have diversified into new species far more quickly than those which only have belly photophores. That’s exactly what you’d predict if the glow was indeed acting as a badge of identity. By allowing the sharks to recognise their own species, these markings ensure that they don’t cross-breed. And without cross-breeding, newly diverging species will become even more firmly separated, quickly giving rise to a lush family tree full of new shining branches.

“The very first glowing sharks probably only used the light organs only for counter-illumination,” says Claes. “Then, some genetic change allowed some glowing organs to move a bit up to the side, and that allowed the owners to see members of their own species more easily. When you’re living in the permanent darkness of the deep sea, it’s a big advantage to be able to signal your presence to [others of your kind].”

Reference: Claes, Nilsson, Mallefet & Straube. 2015. The presence of lateral photophores correlates with increased speciation in deep-sea bioluminescent sharks. Royal Society Open Science http://dx.doi.org/10.1098/rsos.150219

More: The Barnacle That Eats Glowing Sharks

Bizarre, Prehistoric Ratfish Chomped Prey with Buzzsaw Jaws

Helicoprion had saws for jaws. That’s really all there was to the 270 million year old ratfish’s dental cutlery. No upper teeth or anything else to slice against – just an ever-growing whorl of spiky teeth anchored to the lower jaw.

This new, definitive image of Helicoprion debuted last year thanks to the efforts of artist Ray Troll and a team of researchers led by Idaho State University paleontologist Leif Tapanila. A very special fossil – IMNH 37899 – preserved both the upper and lower jaws in a closed position, finally solving the mystery of what the ratfish’s head actually looked like. But determining the exact placement of that vexing spiral was just an initial step.

Paleontologists and artists had often supposed that Helicoprion had upper teeth to pierce slippery cephalopods and squirming fish, but the fossils Tapanila and colleagues examined showed that Helicoprion only had a buzzsaw embedded in the lower jaw. How did this long-lived and prolific genus of Permian fish eat with a saw for a jaw?

Part of the original Helicoprion project involved creating a virtual model of the fish’s skull from CT scans of IMNH 37899. Now, in a Journal of Morphology paper, University of Rhode Island biologist Jason Ramsay and the rest of the team from last year’s Helicoprion study have gone back to those models to outline how the freaky ratfish fed.

Helicoprion was a biter. The ratfish’s jaws were too narrow to suction feed, Ramsay and colleagues point out, and so old buzzsaw jaw had to actively chomp prey. And chomp it could. The researchers calculated that when IMNH 37899 opened wide and bit down hard, the points of highest bite force along the ratfish’s tooth row ranged between 267 and 538 pounds. Not the greatest bite force of all time, but not too shabby for a 20-foot-long carnivorous fish.

[A model of how Helicoprion bit into prey, on display in the Whorl Tooth Sharks of Idaho traveling exhibit.]

Despite such a powerful bite, though, Helicoprion probably didn’t eat highly-armored prey. The ratfish’s jaws were so slender that trying to balance hard-shelled brachiopods or bivalves in its mouth would have been an exercise in futility, and broken Helicoprion teeth are so rare that it seems the predaceous fish preferred soft fare.

Prehistoric cephalopods – the kin of today’s squid and octopus – were probably the main Helicoprion menu items. Some would have looked squid-like, albeit supported by robust hard parts inside, and been easy enough to snaffle up. Others, however, jetted around in coiled shells and would have presented Helicoprion with the challenge of drawing out the squishier parts from their protective casing. The unique bite of Helicoprion was able to do just that.

The Helicoprion buzzsaw didn’t only slice. It also conveyed food back into the mouth during the bite. The overall effect, Ramsay and colleagues conclude, was like a biological miter saw.

The process went something like this. Teeth at the front snagged the prey and, as the jaws closed, moved the flesh backward. Here, the middle teeth speared the food, securing it in the mouth, before the back teeth bit in and sent the morsel down the hatch.

Not a happy cephalopod. From Ramsay et al., 2014.
Not a happy cephalopod. From Ramsay et al., 2014.

If Helicoprion bit one of the coil-shelled ammonoids or nautiloids just right, this same process would have automatically shelled the cephalopod. In the case of a head-on bite, Ramsay and coauthors write, the back teeth of Helicoprion could have gripped the cephalopod’s body while the front teeth pushed the shell out of the mouth. The overall effect, Ramsay and colleagues propose, “may have formed a novel mollusk shucking system.” Helicoprion – it slices, it dices, and makes Julienne ammonoids.

Reference:

Ramsay, J. Wilga, C., Tapanila, L., Pruitt, J., Pradel, A., Schlader, R., Didier, D. 2014. Eating with a saw for a jaw: Functional morphology of the jaws and tooth-whorl in Helicoprion davisii. Journal of Morphology. doi: 10.1002/jmor.20319

Megalodon: The Monster Shark’s Dead

Megalodon is dead. This shouldn’t come as a shock. The fossil record is clear that after about 14 million years of feasting on marine mammals, the 50-foot-long, “mega-toothed” shark exited the evolutionary stage by two and a half million years ago.

But the monstrous shark is too good to let go. If a great white shark is scary, the supersized version is even more thrilling, and despite our ancient fear of sleek, hungry shapes slicing through the water, people really want Carcharocles megalodon to still be alive somewhere in the deep. Peter Benchley toyed with the idea that the great shark might still be out there in JAWS, cryptozoological lore has often spoken of massive sharks, and nature documentaries traditionally state that the prehistoric leviathan is extinct… maybe. Unlike the shark itself, the legend of living megalodon just won’t die.

Discovery gave audiences what they wanted. Megalodon had taken up recurring roles in many of their Shark Week programs – including a life-sized “Sharkzilla” solely employed smash stuff – and last year the channel dipped deep into their burbling chum bucket to dredge up Megalodon: The Monster Shark Lives. The program was a shark version of Forrest Gump, with fabricated evidence showing the shark popping up through human history, and had just enough polish to convince many viewers that megalodon is still chomping whales and snaffling up the occasional boat.

The fauxumentary was popular enough that Discovery hastily threw together a sequel for this year’s Shark Week carrying the unimaginative title Megalodon: The New Evidence. To quote Marty McFly, “The shark still looks fake.” But I don’t want to simply despair over Discovery’s penchant for trading in on their reputation to peddle attention-grabbing sludge. Bit by bit, we’re getting a closer look at megalodon. Paleontologists are continuing to investigate the life of prehistory’s most famous shark, including some new, real evidence about when the celebrity selachian slipped into extinction.

C. megalodon teeth from a "nursery" found in Panama. From Pimiento et al., 2010.
C. megalodon teeth from a “nursery” found in Panama. From Pimiento et al., 2010.

University of Alabama paleontologist Dana Ehret is one of the researchers who has been poring over the remains of megalodon. He’s helped figure out that the huge carnivore was actually a relatively distant cousin of today’s great white shark that should be properly called Carcharocles megalodon, and he’s also worked on a trove of teeth that preserve a nearshore nursery for the infants of this imposing species. He’s among the scientists who are showing that megalodon doesn’t need the reality TV treatment to inspire awe.

Most often, C. megalodon is portrayed as a pumped-up great white. That’s because of shared habits. Megalodon were “large macropredatory sharks that ate marine mammals,” Ehret says, “and white sharks are a good model for that.” The teeth of the extinct shark are distinct from those of any living shark, and the fact that the megatooth lineage split from the great white lineage over 66 million years ago indicates that the megatooth sharks probably had other anatomical differences, but, at least in terms of lifestyle, a huge white shark “is really the closest thing we can imagine megalodon to be,” Ehret says.

That megalodon preferred meals of whale and seal fat comes from more than analogy. “We find lots of broken or partial whale ribs that have nice scrape marks or drag marks across them,” Ehret says, adding “I’ve actually taken meg teeth [to compare to the bite marks] and the serrations match up perfectly.” Megalodon may not have been above cannibalism, either. “I’ve seen a tooth with the same scrapes running down the surface of the tooth,” Ehret says, although he cautions that this could be a sign of the tooth sliding past another while being shed rather than one shark biting another in the mouth.

Even if they weren’t regularly eaten by their own kind, though, megalodon had to cope with competition. Their fossils are found all around the world, Ehret says, in the remnants of coastal environments patrolled by sand tiger sharks, lemon sharks, and other species, including ancient great whites that grew to be 30 feet long. Even at six feet long – as estimate Ehret calculated from a well-preserved vertebra – a newborn megalodon had to cope with similarly-sized neighbors who were chasing after the same food sources.

A reconstruction of C. megalodon jaws, based on those of modern great whites. Credit: Spotty11222
A reconstruction of C. megalodon jaws, based on those of modern great whites. Credit: Spotty11222

Striving to find meals may explain a key difference between C. megalodon and modern great whites. From the same vertebra he used to calculate the shark’s birth size, Ehret calculated that megalodon grew comparatively faster than great whites. “They just wanted to get  a big as they could as fast as they could, and they were a big shark to begin with,” Ehret says. Packing on the pounds would have allowed young megalodon to start taking larger prey, and would have prevented them from ending up in the stomachs of their sharp-toothed cousins. “They were trying to get big and get out into more open waters to not have as much competition for resources,” Ehret says.

Even with an amped-up growth rate, though, megalodon were never truly able to fully escape early life rivalry. This may have been part of their ultimate demise.

Researchers have traditionally pointed to a cooling global climate as the principal C. megalodon killer, the temperature dip spoiling the shark’s warm, nearshore haunts as the water chilled and whales started to migrate towards polar ice. But Ehret suspects that megalodon was able to maintain a high body temperature, much like modern great whites, and would not have been as restricted in range as scientists previously supposed. “I have a bit of a problem saying meg couldn’t follow resources to colder waters,” Ehret says. Instead, megalodon might have suffered a one-two punch related to growth and food supply.

The evolution and extinction of megalodon tracks the proliferation of prehistoric whales. “You see a peak in whale diversity in the mid-Miocene when megalodon shows up in the fossil record and this decline in diversity in the early-middle Pliocene when meg goes extinct,” Ehret says. Without a rich supply of fatty, medium-sized whales, Ehret says, “Meg might’ve gotten too big for its own good and the food resources weren’t there anymore.” On top of that, huge great white sharks were still around and vying for the same remaining food sources as juvenile megs. Rather than a victim of temperature, megalodon might have been a victim of “diversity, abundance, and competition,” Ehret says.

So when did the last megatoothed shark go extinct? Some sources say that megalodon persisted into the Ice Age, or even into the last 10,000 years. That would make the idea of modern megs seem plausible, but, Ehret says, these late dates are based on faulty evidence. “These teeth get moved, they get tumbled, they get washed down, washed around,” Ehret says, and so it’s not surprising that dredgers sometimes pluck up megalodon teeth alongside mammoth or mastodon bones. When paleontologists look at in situ megalodon teeth, still in the sediment they were originally buried in, they come up with much older dates.

Robert Boessenecker, a paleontologist at the University of Otago, has led a new study with Ehret and other authorities that pins down the disappearance of megalodon along the California coast. (The research has not yet been published, but soon will be.) The research started with a lucky find.

In late 2007, on the day before Christmas Eve, Boessenecker found a big, bluish green C. megalodon tooth in the Purisima Formation along California’s coast. “I got lucky”, Boessenecker says, “but I also wondered why we don’t find C. megalodon in younger deposits, and why it’s so damn rare in latest Miocene/earliest Pliocene deposits.” While amateurs and collectors pick up hundreds of megalodon teeth from East Coast sites every year, Boessenecker notes, only about 150 teeth have ever been found in California and the Baja Peninsula.

Comparing the occurrence of the shark along the California coast to records of teeth found elsewhere, Boessenecker and colleagues found that the Carcharocles megalodon did not survive past the end of the Pliocene, about 2.5 million years ago. “No credible records of Pleistocene (or Holocene) C. megalodon exist anywhere,” Boessenecker says, “and if we cannot even prove that a giant shark survived past 2-3 million years ago, the case for C. megalodon survival is hopelessly poor.” Much of the ocean has yet to be explored, it’s true, but it’d be really difficult to miss a 50-foot-long, nearshore shark with a taste for whales. The shark is long gone.


[This clip from Shark Attack 3 is just as educational as Discovery’s Megalodon programs.]

Yet the ghost of the shark lives on. Ehret often fields questions directly inspired by the Discovery fiction. “Not only do I see a surge in people asking about megalodon’s survival,” Ehret says, “it seems that a lot of people that ask really want it to be true.” The science Discovery claims to purvey in their fictional programs is getting totally lost. “I really think it does diminish the message [for science] they’re putting out there,” Ehret says, adding “I think it’s all about audience, and it’s a little bit like selling out.”

Boessenecker has even harsher words for Discovery. The channel still holds a high reputation for factual programming amongst the public, Boessenecker says, which makes “docufiction” like Mermaids and Megalodon especially loathsome. Says Boessenecker:

I find the willful distortion of science by Discovery in favor of entertainment and ratings reprehensible. Discovery may have a short disclaimer in front of the documentary, but it’s just lip service; they either know full well that they’re being intentionally misleading, or being hopelessly naive in thinking that the public will be able to separate fact from fiction while watching a show advertised as a documentary on a network with a reputation for putting out informative nature documentaries.

For Boessenecker, Discovery’s chicanery cuts even deeper because he grew up watching the channel’s nature programs and was partly inspired to become a scientist because of them. “And now,” Boessenecker says, “that same channel which was so great and educational only two decades ago has dismissed over a century of research in my field with the casual wave of an arm, and for nothing more than ratings.”

A Forgotten Fossil Megamouth Gets a Name

Sharks are paleontological paradoxes. They have an extensive fossil record going back 409 million years, yet, except in cases of exceptional preservation, little more than their teeth remain. They are everywhere yet are nearly invisible, their identity and appearance often contingent upon what we know about their living relatives.

But what do you do when you’ve got a shark tooth that doesn’t resemble the dentition of any species known to swim the modern seas? That’s the puzzle the late paleontologist Shelton Applegate faced while studying the different fossil shark teeth of Pyramid Hill. Among the teeth found at this 23 million year old site in the San Joaquin Valley were small, hooked teeth with little nubs of side cusps sitting on an A-shaped root. Teeth like this had never been found before, and, in a 1973 report, Applegate surmised that they belonged to a new species.

Despite their novelty, Applegate didn’t formally name the curious teeth. They entered the rank and file of known fossils without a name, even as they began to turn up at other sites of similar age in California and Oregon. Then a chance discovery of a mysterious modern shark started to bring the fossil fish into focus.

On November 15, 1976 the U.S. Navy ship AFB-14 hauled up a large shark that had been accidentally ensnared by the vessel’s sea anchor. This was a totally new species of blunt-headed, filter-feeding shark – officially named Megachasma pelagios in 1983. Fossil shark experts immediately took notice. The newly-found shark had teeth very much like those Applegate reported from Pyramid Hill a decade earlier. There was little doubt that those teeth, and similar specimens excavated elsewhere, represented an ancient megamouth shark.

Even then, though, the shark languished in obscurity. No one gave the fossil shark an official name. The shark was all but forgotten. Now that has finally changed.

A tooth of Megachasma applegatei. Photo by Kenshu Shimada.
A tooth of Megachasma applegatei. Photo by Kenshu Shimada.

In the latest issue of the Journal of Vertebrate Paleontology, DePaul University paleoichthyologist Kenshu Shimada and colleagues have named the shark Megachasma applegatei to honor its initial discoverer. The prehistoric shark’s teeth differ in some key ways from the modern species – such as shorter crowns and having a pair of side cusplets – but the dental remains are similar enough that the 23 million year old shark currently sits as the sister species to the living one.

Exactly what Megachasma applegatei looked like is difficult to tell beyond teeth. Modern megamouths have about 85 teeth in their upper jaws and 125 in their lower jaws, but whether the fossil species had a similar count is impossible to know on the basis of isolated teeth alone. The same problem obscures how large the shark actually was, but, using the relationship between tooth and body size in the modern species as a template, Shimada and colleagues estimate that most Megachasma applegatei would have been about 12 feet long, with the largest reaching an impressive 27 feet in length – about ten feet longer than the longest known specimen of the extant shark.

A restoration of the fossil megamouth by Kenshu Shimada.
A restoration of the fossil megamouth by Kenshu Shimada.

How the fossil shark made a living is just as enigmatic. While most similar to those in the maw of the modern megamouth, the prehistoric teeth also retain some characters reminiscent of sand tiger sharks. Instead of being a dedicated filter-feeder, Shimada and coauthors tentatively suggest, maybe Megachasma applegatei was snaffling up small fish as well as zooplankton. And even though the geological context of the teeth suggest that the shark slurped small prey at a variety of depths, it’s unclear whether the fossil megamouth was at home in disparate habitats or was making daily journeys through the water column like its modern relative. The present was the key to identifying this extinct shark, but the past still holds many selachian secrets.

Reference:

Shimada, K., Welton, B., Long, D. 2014. A new fossil megamouth shark (Lamniformes, Megachasmidae) from the Oligocene-Miocene of the Western United States. Journal of Vertebrate Paleontology. 34, 2: 281-290. doi: 10.1080/02724634.2013.803975

When Sharks Ate Dinosaurs

Once upon a time, roundabout 86 million years ago, a dead dinosaur drifted out to sea. The shovel-beaked hadrosaur expired somewhere inland, and, despite the herbivore’s bulk, the gases from decomposition buoyed up the carcass just enough to float the animal out into the warm waters where hungry sharks tucked into the dinosaur’s flesh. The scant details of the feast are recorded in bone.

In 2005, in the Smoky Hill Chalk of western Kansas, amateur fossil hunter Keith Ewell discovered a set of nine dinosaur tail bones. The ancient setting in which the bones were deposited made them remarkable. During the time the sediment of the Smoky Hill Chalk was being laid down, Cretaceous Kansas was blanketed by the Western Interior Seaway. Dinosaurs did not live in this marine environment, but their bodies were sometimes transported by local floods or other water-bound means to the jaws of seagoing carnivores. The quad-paddled plesiosaurs and Komodo dragon-like mosasaurs were the top reptilian predators, and prehistoric sharks took their fare share, too. In the case of the unfortunate hadrosaur that Ewell stumbled upon, we know this because of distinctive tooth marks on the tail bones.

Sternberg Museum of Natural History paleontologist Michael Everhart described the tail with Ewell in a 2006 Transactions of the Kansas Academy of Science paper. Aside being a rare occurrence of a dinosaur washed out to sea – only six had previously been found in the Smoky Hill Chalk – at least four of the hadrosaur’s vertebrae bore tooth marks that could have only been created by a scavenging shark.

But which shark? One serrated, A-shaped tooth of the shark Squalicorax was discovered right beneath the tail bones, yet the proximity does not automatically mean that the tooth was shed by a shark feeding on the dinosaur. Association does not always imply interaction.  In fact, Everhart and Ewell observed, the set of bite marks were apparently made by a shark with non-serrated teeth. The large, approximately 21 foot long shark Cretoxyrhina was the best match, especially because tooth fragments of this shark have been found embedded inside other bones showing similar bite marks. At least one Cretoxyrhina had sliced into the hadrosaur.

While remarkable, Ewell’s shark-bitten dinosaur was not unique. According to Everhart, all but two of the partial dinosaurs found in the Smoky Hill Chalk have bite marks on them. In the case of a heavily-armored Niobrarasaurus, for example, bite marks indicate that the lower portion of one of the dinosaur’s forelimbs was sliced off by a scavenging Cretoxyrhina.

A Squalicorax tooth associated with ankylosaur Aletopelta, hinting that sharks might have fed on the carcass after the dinosaur was washed out to sea. Photo by Brian Switek.
A Squalicorax tooth associated with ankylosaur Aletopelta, hinting that sharks might have fed on the carcass after the dinosaur was washed out to sea. Photo by Brian Switek.

Shark-bitten dinosaurs have been found in strata formed in other places and times, too. While the relatively small shark Squalicorax didn’t create the damage Everhart and Ewell saw on their hadrosaur, in 1997 paleontologist David Schwimmer and coauthors mentioned a hadrosaur foot bone with an embedded Squalicorax tooth that had been found in the 80 million year old rock of Alabama. And at last year’s Society of Vertebrate Paleontology meeting in Raleigh, North Carolina, New Jersey State Museum paleontologist Jason Schein presented a poster on the upper arm bone of a small hadrosaur found in the state’s 66 million year old marl that was badly sliced by scavenging sharks. Without a doubt, sharks ate dinosaurs.

But none of these cases record a shark bite on a living dinosaur. All are cases of scavenging. Despite a recent Discovery News piece that suggested Squalicorax and other sharks might have killed dinosaurs, there’s no reason to think this was true.

There is no solid evidence that any non-avian dinosaurs were adept at swimming in the seas. They were terrestrial animals, and the sites found in the Smoky Hill Chalk, New Jersey’s Hornerstown Formation, and others are marine environments that dinosaurs would have had to swim considerable distances to reach. From what we know about dinosaur biology and the geologic context of known shark-bitten bones, seagoing sharks only dined on dinosaurs when the carcasses of hadrosaurs, ankylosaurs, and their ilk occasionally drifted out to sea.

This doesn’t mean that living dinosaurs and sharks never encountered one another. For one thing, some forms of the extinct hybodontid sharks were swimming through rivers and lakes long before dinosaurs evolved and persisted through the mass extinction that closed the Cretaceous. So far, there’s no indication that these sharks ever attacked or fed upon dinosaurs, but such interactions aren’t outside the realm of possibility. The only way we’ll know, though, is if such long lost sharks left their toothy calling cards in dinosaur bone.
References:

Everhart, M., Hamm, S. 2005. A new nodosaur specimen (Dinosauria: Nodosauridae) from the Smoky Hill Chalk (Upper Cretaceous) of western Kansas. Transactions of the Kansas Academy of Science. 108, 1/2: 15-21

Everhart, M., Ewell, K. 2006. Shark-bitten dinosaur (Hadrosauridae) caudal vertebrae from the Niobrara Chalk (Upper Coniacian) of western Kansas. Transactions of the Kansas Academy of Science. 109, 1/2: 27-35

Schwimmer, D., Stewart, J., Williams, G. 1997. Scavenging by sharks of the genus Squalicorax in the Late Cretaceous of North America. PALAIOS. 12: 71-83

Healed Bone Gives Away Prehistoric Shark Bite

Of all the jaws to have evolved in their roughly 440 million year old history, few were as formidable as those of the enormous Carcharocles megalodon. The shark’s razor-toothed maw is what has made the extinct fish a regular star of pulp novels, b horror movies, and even basic cable hoaxes. Science backs up the shark’s reputation. In 2008, based on estimates from the study of a great white shark’s biting abilities, biomechanics expert Stephen Wroe and colleagues calculated that C. megalodon really did have a terrible bite – a crushing chomp between 24,000 and 41,000 pounds, depending on the position of prey in the 50-foot shark’s mouth.

But since C. megalodon forever disappeared from the world’s oceans as long ago as 3 to 4 million years before the present, we’ll never get to see those jaws in action. Images brought to life by science, special effects, and the darker parts of our imaginations are the closest we’ll get. Still, despite our temporal distance from the shark, there are damaged fossils that may testify to the predatory prowess of C. megalodon and help us envision the destruction this superpredator was capable of.

Credit: Spotty11222
Credit: Spotty11222

In the collections of Maryland’s Calvert Marine Museum, there  is a curious piece of whalebone. A damaged rib, found in the spoil pile of a North Carolina phosphate mine, the fossil bears a trio of ugly pathological bumps that look like osteological zits. Together, the lesions outline a 3-4 million year old bite.

Calvert Marine Museum paleontologist Robert Kallal and his coauthors described the specimen in a 2012 International Journal of Osteoarchaeology study.  Even though the rib fragment wasn’t found in place, the researchers pointed out that the identity of the fossil as part of a baleen whale narrows down the age of the specimen. At the locality where the shard was picked up, baleen whale remains are only found in the 3-4 million year old Pliocene rock of the Yorktown Formation. From the anatomy of the bone and the site’s fossil roster, the best candidate for the owner of the rib might be an ancient humpback whale.

That the rib was damaged by a predator, and not disease or some other cause, is clear from the spacing of the lesions. “Evenly spaced bony protrusions such as those preserved in [the whale rib fragment] are not known to occur naturally on mammalian ribs,” Kallal and colleagues observed, but the lesions are consistent with the bite of a carnivorous creature. And at this time in Earth history, there were several marine predators capable of inflicting the damage seen on the whale rib.

Whoever the predator was, they were not successful. The whale’s rib does not show sharp tooth slashes or fresh crushes, but healed bone. The whale survived the encounter. Still, the shape and arrangement of the wounds give away the presence of a large predator that attacked the whale’s flank. The space between each lesion is about 2.3 inches, which best matches a very large great white shark – bigger than has been found at the site before – or a young C. megalodon, fossils of which have been found at the locality. (Kallal and collaborators note that some kind of sperm whale could have created the damage, although they doubt this because the lesions are laid out in a crescent pattern that better matches the mouth of a shark.)

So who bit the whale? We may never know for sure. Healed bite wounds will only take us so far. But the rib fragment could bear the marks of a young C. megalodon with unrealistic predatory ambitions. The piece hardly stands alone. Other damaged whale bones support the idea that C. megalodon regularly fed upon – if not hunted – the large whales of their day, the chief confounding factor being that the tooth marks left in the aftermath of a successful hunt will look no different than those of a scavenger who shows up late to a carcass.

Even though distinguishing between hunting and scavenging in any single case of a tooth-sliced bone is often impossible, the fossils of marine mammals found in the same deposits as C. megalodon often bear awful bite damage. It’d be unreasonable to assume that all these cases represent either hunting or scavenging alone. The great prehistoric shark certainly did both, equipped with frightening jaws adapted for the dirty work of cutting through skin, blubber, muscle, and bone.

References:

Kallal, R., Godfrey, S., Ortner, D. 2012. Bone reactions on a Pliocene cetacean rib indicate short-term survival of predation event. International Journal of Osteoarchaeology. 22: 253-260

Wroe, S., Huber, D., Lowry, M., McHenry, C., Moreno, K., Clausen, P., Ferrara, T., Cunningham, E., Dean, M., Summers, A. 2008. Three-dimensional computer analysis of white shark jaw mechanics: how hard can a great white bite? Journal of Zoology. 276, 4: 336-342

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Like Shark Week, But With Actual Facts

It’s Shark Week, and the Discovery Channel have already jumped the shark with a fake documentary, asking if a giant prehistoric shark Megalodon is actually still alive. It’s not, and the show was filled with lies, fabrications and actors playing scientists.

Here’s a radical alternative idea: I thought I might celebrate Shark Week by telling you some actual facts about sharks. This, therefore, is a collection of my earlier Not Exactly Rocket Science posts on sharks (and a couple of rays, for good measure).

Cookie-cutter shark, by the NOAA Observer Project
Cookie-cutter shark, by the NOAA Observer Project

What Bit This Great White Shark? A Cookie-Cutter

On 25 August 2010, diver Gerardo del Villar, saw a great white shark off Guadalupe Island with two odd wounds on its head. One was a crescent-shaped scar. The other was a round crater, still open and bloody. Both were just behind the corner of the young male’s fearsome mouth. Del Villar took photos of the animal and sent them to a team of scientists, including Yannis Papastamatiou from the Florida Museum of Natural History.

He had seen wounds like these before. “A wound from a hook should leave more of a hole and would not be as smooth,” he says.  Instead, Papastamatiou thinks that they were the bite-marks of another shark, just a sixth of the size—a cookie-cutter. “I dont know of any other animal that leaves a bite like that.”

Thresher-shark

Thresher Sharks Hunt with Huge Weaponised Tails

For most sharks, the front end is the dangerous bit. Thresher sharks are the exception. They’re deadly at both ends, because they’ve managed to weaponise their tails. The top halves of their scythe-like tail fins are so huge that they can be as long as the rest of the shark. For around a century, people have been saying that the threshers lash out at their prey with these distended fins—hence the name. But no one had ever seen them do so in the wild.

In 2010, one team showed that they can lash out at tethered bait under controlled conditions. But Simon Oliver has done better. His team spent the summer of 2010 in the Philippines, watching and filming wild pelagic thresher sharks—the smallest of the three species—hunting large shoals of sardines. The videos are spectacular and unambiguous: threshers really do hunt with their tails.

Sand-tiger-shark
Credit: Jeff Kubina from Columbia, Maryland
Sand tiger shark, by Jeff Kubina

Shark Dads Lose Babies to Unborn Cannibal Siblings

Inside its mother’s womb, an unborn sand tiger shark is busy devouring its brothers and sisters. It’s just 10 centimetres long but it already has well-developed eyes and a set of sharp teeth, which it turns against its smaller siblings. By the time the pregnant female gives birth, it only has two babies left—one from each of its two wombs. These survivors have already eaten all the others. They’re the bloody victors of a pre-birth battle.

Credit: Joshua Drew, Columbia University.
Credit: Joshua Drew, Columbia University.

Badass Shark Teeth Weapons Hint at Shadow Diversity

When life gives you lemons, make lemonade. When life sticks you on an isolated island surrounded by shark-infested waters, make utterly badass weapons out of shark teeth.

This is what the people of the Pacific Gilbert Islands have been doing for centuries. Sharks are a central part of their lives. Many social customs and taboos revolve around the finer points of shark-hunting. Young boys go through initiation rites where they kneel on a beach, looking towards a rising sun and slice their hairlines open with shark teeth, letting the blood run into their eyes until sunset. And with no metal around, they used shark teeth to adorn their weapons.

Credit: Diliff
Credit: Diliff

How the sawfish wields its saw… like a swordsman

If you ever saw a sawfish, you might wonder if someone had taped a chainsaw to the body of a shark. The seven species of sawfish are some of the wackier results of evolution. They all wield a distinctive saw or ‘rostrum’, lined with two rows of sharp, outward-pointing ‘teeth’. But what’s the saw for?

Barbara Wueringer has an answer: the saws are both trackers and weapons. They’re studded with small pores that allow the sawfish to sense the minute electrical fields produced by living things. Even in murky water, their prey cannot hide. Once the sawfish has found its target, it uses the ‘saw’ like a swordsman. It slashes at its victim with fast sideways swipes, either stunning it or impaling it upon the teeth.

Credit: Terry Goss
Credit: Terry Goss

Sharks gone walkabout – how Australian great whites ended up in the Mediterranean

In the 18th century, Europe started sending boatloads of white settlers to Australia. But unbeknownst to these colonists, Australia had sent its own white contingent to set up colonies in Europe, around 450,000 years earlier. These migrants were sharks – great white sharks.

When Chrysoula Gubili from the University of Aberdeen compared the DNA of white sharks from around the world, she found a big surprise. The great white is the most genetically diverse shark studied so far but the Mediterranean fish are only distantly related to nearby populations in the North-West Atlantic, or even in South Africa. Their closest kin actually live half a world away in the Indo-Pacific waters of Australia and New Zealand.

Credit: Barry Peters
Credit: Barry Peters

Widely set eyes give hammerhead sharks exceptional binocular vision

The hammerhead shark’s head is one of the strangest in the animal world. The flattened hammer, known as a ‘cephalofoil’, looks plain bizarre on the face of an otherwise streamlined fish, and its purpose is still the subject of debate. Is it an organic metal detector that allows the shark to sweep large swathes of ocean floor with its electricity-detecting ability? Is it a spoiler that provides the shark with extra lift as it swims? All of these theories hypotheses might be true , but Michelle McComb from Florida Atlantic University has confirmed at least one other -the hammer gives the shark excellent binocular vision.

Credit: Spotty11222
Credit: Spotty11222

Prehistoric great white shark had strongest bite in history

Thanks to Hollywood, the jaws of the great white shark may be the most famous in the animal kingdom. But despite its presence in film posters, the great white’s toothy mouth has received very little experimental attention. Now, Stephen Wroe from the University of New South Wales has put the great white’s skull through a digital crash-test, to work out just how powerful its bite was.

A medium-sized great white, 2.5m in length and weighing in at 240kg, could bite with a force of 0.3 tonnes. But the largest individuals can exert a massive 1.8 tonnes with their jaws, giving them one of the most powerful bites of any living animal.

Credit: Mark Conlin, SWFSC Large Pelagics Program
Credit: Mark Conlin, SWFSC Large Pelagics Program

Male and female mako sharks separated by invisible line in the sea

In the middle of the Pacific Ocean, Gonzalo Mucientes has discovered an invisible line in the sea that separates male mako sharks from females. The line runs from north to south with the Pitcairn Islands to its west and Easter Island to its east. On the western side, a fisherman that snags a mako will most probably have caught a male. Travel 10 degrees of longitude east and odds are they’d catch a female. This is a shark that takes segregation of the sexes to new heights.

Juvenile bamboo shark by Steve Childs
Juvenile bamboo shark by Steve Childs

Shark embryos use electric sense to avoid danger by freezing

Sharks can sense their prey’s minute electric fields, such as those produced when muscles twitch or nerve cells fire. This super-sense is part of what makes sharks such formidable hunters—you can’t hide from them if the very act of living can give you away.  But the tables are turned when sharks are young. As they begin life, they’re as vulnerable as any other fish, and their electric sense helps them to hide instead of hunt.

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Thresher Sharks Hunt with Huge Weaponised Tails

For most sharks, the front end is the dangerous bit. Thresher sharks are the exception. They’re deadly at both ends, because they’ve managed to weaponise their tails.

The top halves of their scythe-like tail fins are so huge that they can be as long as the rest of the shark. For around a century, people have been saying that the threshers lash out at their prey with these distended fins—hence the name. But no one had ever seen them do so in the wild.

In 2010, one team showed that they can lash out at tethered bait under controlled conditions. But Simon Oliver has done better. His team spent the summer of 2010 in the Philippines, watching and filming wild pelagic thresher sharks—the smallest of the three species—hunting large shoals of sardines. The videos are spectacular and unambiguous: threshers really do hunt with their tails.

“It was absolutely extraordinary,” says Oliver, who is founder of the Thresher Shark Research and Conservation Project and based at the University of Liverpool. “We always expected this but there’s never been any solid documented evidence. This is the first time the behaviour has been observed in the sharks’ natural environment, and we observed a lot of it.”

When I first read about thresher sharks as a kid, I imaged that they would swim towards its prey, bank sharply, and lash out sideways with their tails. Oliver’s team showed that the sharks do use sideways slaps, but rarely.

Instead, here’s what usually happens. The thresher accelerates towards a ball of fish and brakes sharply by twisting its large pectoral fins. It lowers its snout, pitches its whole body forward, and flexes the base of its tail. This slings the tail tip over its head like a trebuchet, with an average speed of 30 miles per hour. (The fastest shark managed to whip its tail at an astonishing top speed of 80 miles per hour.)

“It’s fast, aggressive and violent,” says Oliver. When the tail hits sardines, the results aren’t pretty. “We saw everything from swim bladder ruptures to broken spines to parts afloat.” The sharks then swim round and swallow the pieces at their leisure.

Best scientific figure ever? From Oliver et al, 2013. PLOS
Best scientific figure ever? From Oliver et al, 2013. PLOS

The threshers are only successful on a third of their strikes but during these victories, they always kill several sardines at once. That’s far more efficient than chasing after agile individuals in a confusing shoal, and it suggests that the sharks aren’t just relying on direct hits.

During three of the hunts that Oliver filmed, he saw plumes of bubbles at the tip of the shark’s tail. That’s probably because it moves so quickly that it lowers the pressure in front of it, causing the water to boil. Small bubbles are released, and collapse again when the water pressure equalises. This process is called cavitation, and it releases huge amounts of energy. Another sea creature—the mantis shrimp—uses cavitation to attack its prey, and Oliver suspects that thresher sharks may do the same. “I think the shark’s causing a shockwave that’s strong enough to debilitate small prey,” he says. (However, he cautions that he’d need to use some physical models to prove that this is actually happening.)

Stills from a thresher shark attack video. From Oliver et al, 2013. PLOS
Stills from a thresher shark attack video. From Oliver et al, 2013. PLOS

“It’s extraordinarily rare in the animal kingdom to see animals hunt with their tails,” says Oliver. Killer whales and other dolphins sometimes do so, but the strategy is unique among sharks.

Oliver suspects that no one has witnessed this behaviour before because thresher sharks hunt in the open ocean, and usually at night. “The ocean’s a big place and studying sharks is very difficult,” he says. “You need a lot of luck. We got very lucky.” One of his team heard about a large shoal of sardines that were staying off Pescador Island in the Philippines, and the team set up a research station there. The sardines stayed around for several months, and the threshers stayed with them.

Since then, the shoals have dispersed and the sharks have also disappeared. Oliver hopes they’ll come back, but he’s also worried. “These habitats where prey can aggregate are fewer and further between,” he says. “These sharks normally hunt at night and all of our observations were during the day. It’s counter-intuitive to their normal strategy.” It’s a reminder that these astonishing animals—all three of which are classified as vulnerable—need support and protection.

Reference: Oliver, Turner, Gann, Silvosa & Jackson. 2013. Thresher Sharks Use Tail-Slaps as a Hunting Strategy. PLOS ONE. http://dx.doi.org/10.1371/journal.pone.0067380

More on sharks:                                                                      

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Shark Dads Lose Babies to Unborn Cannibal Siblings

Inside its mother’s womb, an unborn sand tiger shark is busy devouring its brothers and sisters. It’s just 10 centimetres long but it already has well-developed eyes and a set of sharp teeth, which it turns against its smaller siblings. By the time the pregnant female gives birth, it only has two babies left—one from each of its two wombs. These survivors have already eaten all the others. They’re the bloody victors of a pre-birth battle.

The arched back, upturned snout and protruding teeth of a sand tiger shark give it a particularly brutish look. Its reproductive habits don’t help. After sex, any fertilised eggs settle in one of the female’s two uteri. Like a quarter of shark species, the sand tiger never lays these eggs. Instead, they hatch inside her once they reach a certain size.

Timing matters. The first embryo to emerge in each uterus—the ‘hatchling’—always cannibalises its younger siblings. It’s so voracious that at least one scientist has been bitten by a sand tiger pup while unwisely sticking a finger in a pregnant female’s uterus.

The cannibal not only nourishes itself on its siblings’ bodies, but also gains sole access to the nutritious supply of unfertilised eggs that its mother provides. On this rich diet of yolk and flesh, the hatchling grows at a tremendous pace. When it is eventually born, it’s already a metre in length—that’s big enough to protect it from many predators. “Only really big sharks eat baby sand tigers,” says Demian Chapman from Stony Brook University in New York.

Chapman studies the mating behaviour of sharks, and was fascinated by what the sand tiger’s bizarre practices mean for males. If a female mates with many males, her litter could initially include pups from many fathers. But it’s entirely possible that the pups of some males are cannibalised by the pups of others, before they’re even born! So if you gave sand tiger pups a paternity test, what would you find?

Chapman studied the bodies of 15 pregnant sand tiger sharks that died near South Africa’s beaches after getting caught in protective nets. (The nets were put up after a series of shark attacks in the 1960s.) He took tissue samples from mothers and embryos, and analysed microsatellites—short, repetitive pieces of DNA that are used in family tests—at 10 places in their genomes.

A sand tiger shark hatchling next to an embryo (the smaller one on the top left). Credit: D.ABERCROMBIE.
A sand tiger shark hatchling next to an embryo (the smaller one on the top left). Credit: D.ABERCROMBIE.

The results showed that females regularly mate with at least two partners. In cases where the hatchlings hadn’t finished devouring their siblings, the embryos were fathered by two or more different males. And when only the hatchlings remained, they were often half-siblings rather than full ones.

Chapman’s results showed that, yes, some males manage to mate with females but never actually contribute to the next generation of sharks. Their young are devoured in the womb.

This is yet another reminder that sex is just one step towards actual reproduction. In many animals, females exert a surprising amount of choice over who fathers their young. Even after sex, females can store the sperm of different partners in separate compartments and determine which ones get to fertilise her eggs.

For males, this means that sexual competition continues after sex. It’s not just about finding mates, but about ensuring that your sperm fertilises her eggs. This leads to fierce “sperm competitions” and bizarre adaptations, where males scrape away the sperm of past mates, guard or plug females so they can no longer accept partners, “traumatically inseminate” her through her back, or even poison partners with toxic sperm to limit future sexual encounters.

The sand tiger shark’s “embryonic cannibalism” takes these competitions to a whole new level. Even if a male successfully fertilises a female’s eggs, there’s still no guarantee that his offspring will actually be born. This may explain why, in captivity at least, male sand tigers often guard the females they mate with. You’d also expect natural selection to also favour males whose offspring grow particularly quickly, so they become the cannibal hatchlings rather than the devoured runts. This may explain why sand tiger embryos have evolved to have such unusually well-developed eyes and teeth.

Females, meanwhile, may not need to bother with carefully choosing their mates. By having sex with several males, and just letting their offspring duke it out inside her, she ensures that the she gives birth to the offspring of the fittest partners.

Reference: Chapman, Wintner, Abercrombie, Ashe, Bernard, Shivji & Feldheim. 2013 The behavioural and genetic mating system of the sand tiger shark, Carcharias taurus, an intrauterine cannibal. Biol Lett http://dx.doi.org/10.1098/rsbl.2013.0003

More on animal sex:

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Badass Shark Teeth Weapons Hint at Shadow Diversity

When life gives you lemons, make lemonade.

When life sticks you on an isolated island surrounded by shark-infested waters, make utterly badass weapons out of shark teeth.

This is what the people of the Pacific Gilbert Islands have been doing for centuries. Sharks are a central part of their lives. Many social customs and taboos revolve around the finer points of shark-hunting. Young boys go through initiation rites where they kneel on a beach, looking towards a rising sun and slice their hairlines open with shark teeth, letting the blood run into their eyes until sunset. And with no metal around, they used shark teeth to adorn their weapons.

A shark is a fast, electric-sensing torpedo, whose business end holds two conveyor belts of regenerating steak knives. To further weaponise its weapons is practically the definition of being badass. Here’s how to do it: You drill a tiny hole in each tooth, and bind them in long rows to a piece of wood, using braided coconut fibres and human hair. Depending on the shape of the wood, you can make a sword. Or a dagger. Or a trident. Or a four-metre-long lance. And then, presumably, you hit people really hard with them.

Shark-tooth sword (left) and a puffer-fish helmet (right). Because, obviously, how else would you defend yourself against someone running at you with a shark-tooth sword? Joshua Drew, Columbia University.
Shark-tooth sword (left) and a puffer-fish helmet (right). Because, obviously, how else would you defend yourself against someone running at you with a shark-tooth sword? Joshua Drew, Columbia University.

No one knows when the Gilbertese first fashioned these arms, but they were already doing so by the time the first Western sailors arrived on the islands in the late 18th century. Many of them ended up in museums and Chicago’s Field Museum of Natural History has a particularly rich collection. It includes 124 specimens, including swords, tridents and a lance that Joshua Drew from Columbia University describes as “2.5 interns tall”.

Drew saw this collection was more than just an amazing armoury. It was also a time capsule. Since every item was carefully tagged with date and place of collection, and since different shark species have distinctively shaped teeth, Drew could use the weapons to identify the sharks that swam round the Gilbert Islands in centuries past.

The teeth came from 8 different species. Tiger sharks feature heavily—their thick, cleaver-like teeth, which can punch through turtle shells, make for good cutting edges. Most of the weapons featured teeth from just one shark species, but several have a rare blue shark tooth in the penultimate position—possibly the signature of an artisan.

But the biggest surprise was that some of the teeth belonged to two species—the dusky and spottail sharks—which no longer exist near the Gilberts!

Back then, they were common enough that their teeth were among the most popular choices for weaponsmiths. Today, no one has seen them within several thousand kilometres of the islands. Even before scientists knew that they were there… they weren’t any more.

(Drew has also found teeth from a third missing species—the bignose shark—on a weapon held at the American Museum of Natural History.)

Shark tooth trident, by Joshua Drew, Columbia University.
Shark tooth trident, by Joshua Drew, Columbia University.

Could these teeth have been imported from neighbouring people? No, says Drew. The Gilbertese had a strong culture of shark-fishing and if they were already heavily catching sharks, why would they trade for teeth? Besides, there is no historical, linguistic or archaeological evidence that these people communicated with those who live in the areas where those missing sharks are now found.

Could it be that the three species still live near the Gilberts but that no one has seen them? Again, it’s unlikely. All three are quite common in the areas where they actually live, so it’s doubtful that biologists have simply missed them.

It’s not clear why the sharks disappeared. Humans may well have been responsible—people were hacking off shark fins in the Gilbert Islands as far back as 1910 and by the 1950s, around 3,000 kilograms of fins were being shipped from the islands ever year. For sharks, many of which grow and reproduce slowly, it doesn’t take long for finning operations to drive a population locally extinct.

Whatever the reason, the teeth are signs of what Drew describes as “shadow diversity”—fleeting ghosts of the vivid splendour that once existed in the same waters. “Today’s Gilbertese live in a fundamentally duller environment than their forefathers,” he says.

That’s an important reminder for conservationists. Both coral reefs and shark populations are under severe threat and scientists are working on ways of restoring them. But what state are we going to restore them to? “Nested within this story is a cautionary tale of… how what we see today is not necessary indicative of the past,” Drew writes. We must not succumb to a “cultural amnesia, where people forget how vibrant reefs really were. “

Shark tooth weapon. By Joshua Drew, Columbia University.
Shark tooth weapon close-up. By Joshua Drew, Columbia University.

Note: I originally covered this research for Nature News when it was presented at the 2012 Ecological Society of America Annual Meeting in August, 2012. Here is the original piece.

Reference: Drew J, Philipp C, Westneat MW (2013) Shark Tooth Weapons from the 19th Century Reflect Shifting Baselines in Central Pacific Predator Assemblies. PLoS ONE 8(4): e59855. http://dx.doi/org/10.1371/journal.pone.0059855

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Weapons made from shark teeth are completely badass, and hint at lost shark diversity

I was at the Ecological Society of America’s Annual Meeting when I saw this tweet:

As you might imagine, I did check out that talk.

For those of you who are wondering how you weaponise shark teeth, which are already regenerating, serrated meat knives at the business end of a streamlined, electric-sensing torpedo, here’s how. You drill a tiny hole in them, and then bind them in long rows to a piece of wood to make a sword. Or a trident. Or a four-metre-long lance. And then, presumably, you hit people really hard with them.

That’s what the people of the Gilbert Islands have been doing for centuries. Sharks are an ingrained part of their culture and their teeth have been an ingrained part of their weapons. Tiger sharks feature heavily – they have thick, cleaver-like teeth that can slice through turtle shells so they make a good cutting edge. But the weapons also include the teeth from spottail, dusky and bignose sharks (you can identify species from their teeth), and none of these actually live around the Gilbert Islands today.

Drew, who studied 124 of these weapons, says that their teeth reveal a “shadow diversity” – traces of sharks that disappeared from the surrounding waters before we even knew they were there. I wrote about this story for Nature News – head over there for the full details.

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How the sawfish wields its saw… like a swordsman

If you ever saw a sawfish, you might wonder if someone had taped a chainsaw to the body of a shark. The seven species of sawfish are some of the wackier results of evolution. They all wield a distinctive saw or ‘rostrum’, lined with two rows of sharp, outward-pointing ‘teeth’. But what’s the saw for?

Barbara Wueringer has an answer: the saws are both trackers and weapons. They’re studded with small pores that allow the sawfish to sense the minute electrical fields produced by living things. Even in murky water, their prey cannot hide. Once the sawfish has found its target, it uses the ‘saw’ like a swordsman. It slashes at its victim with fast sideways swipes, either stunning it or impaling it upon the teeth. Sometimes, the slashes are powerful enough to cut a fish in half. Even less dramatic blows can knock a fish to the sea floor, and the sawfish pins it in place with its saw.

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Sharks gone walkabout – how Australian great whites ended up in the Mediterranean

Great_white_shark

In the 18th century, Europe started sending boatloads of white settlers to Australia. But unbeknownst to these colonists, Australia had sent its own white contingent to set up colonies in Europe, around 450,000 years earlier. These migrants were sharks – great white sharks.

When Chrysoula Gubili from the University of Aberdeen compared the DNA of white sharks from around the world, she found a big surprise. The great white is the most genetically diverse shark studied so far but the Mediterranean fish are only distantly related to nearby populations in the North-West Atlantic, or even in South Africa. Their closest kin actually live half a world away in the Indo-Pacific waters of Australia and New Zealand.

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