Demon Fish (25 page)

John Maisey, a curator in the American Museum of Natural History’s Division of Paleontology, determined that
Pucapampella
had a jaw that was attached to its braincase in a way that was more like a bony fish, or osteichthyan, than a chondrichthyan. The finding Maisey made along with Philippe Janvier at the National Museum of Natural History, Paris, has been bolstered by the unearthing of another early Devonian chondrichthyan fossil,
Doliodus problematicus
, discovered in New Brunswick, Canada.
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The nearly 409-million-year-old
Doliodus
has sharklike jaws and rows of sharklike teeth, whereas
Pucapampella
has what Maisey calls “a very odd single row of teeth.” Scientists are still puzzling over the fact that while the two specimens are very early chondrichthyans and date from roughly the same time, they are anatomically very different. They are also exploring additional clues—including osteichthyan fossils from the end of the preceding Silurian period, between 423 and 416 million years ago, and sharklike skin denticles from Silurian-period rocks—which indicate jawed vertebrates might have evolved earlier than previously thought.
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This research is significant because it suggests—for the first time—that modern shark jaws are a more advanced characteristic than the jaws of bony fish. It also indicates that an essential part of the human anatomy originated in fish. As Maisey said, “The psychology of evolution is interesting. People don’t mind being called a primate or a mammal, but they don’t like being called a fish.”
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In his excellent book,
Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body
, Neil Shubin details the many evolutionary debts we owe to sharks. This includes not only the bones in our inner ear but also the lever system we use to bite. (The muscles and cranial nerves that enable us to swallow and talk are the same ones that move the gills of sharks.)
¹¹
Not every aspect of our shark inheritance is a plus, at least if you’re a man. Sharks’ gonads are nestled near the heart. But in human males, gonads are positioned within the scrotum in order to keep sperm at the proper temperature. This has created a weak spot in the body wall, which in turn accounts for why men experience hernias. As Shubin writes, “Men’s tendency to develop hernias is a trade-off between our fish ancestry and our mammal present.”
¹²

Our common ancestry with sharks extends to the genetic level as well. In December 2006 researchers revealed that the genome of the elephant shark, which is native to waters off New Zealand and southern Australia, features a large number of ancient DNA fragments held in the human genome. These fragments regulate genes that produce proteins integral to human development and physiology. The team from A*STAR’s Institute of Molecular and Cell Biology in Singapore and the U.S.-based J. Craig Venter Institute described the discovery as a major development that could help scientists understand how our genes are regulated, and unlock the origin of several human diseases.
¹³

Other aspects of shark biology, such as their electroreception, are totally alien to humans. Many shark species have a row of small holes that run from head to tail, which picks up weak vibrations. This network, along with tiny, fluid-filled sacs in their snouts and chins known as ampullae of Lorenzini, helps sharks find fish buried in the sand because they can detect the electromagnetic fields generated by a fish’s beating heart or gills. Other fish have a lateral line to sense movement, but they do not have the gelatinous material that serves as a conductor for electric vibrations, radiating these signals out to a shark’s nervous system.

This sensing ability, a critical asset in the wild, can prove a liability in captivity because the electrical signals emitted by an aquarium’s lights, pumps, and metal can confuse the animals. But scientists across the United States are exploring whether they can capitalize on sharks’ unique voltage-charged gel for more practical purposes. University of San Francisco physics professor Brandon R. Brown has extracted the material from dead sharks to gauge its thermal sensitivity, while Case Western Reserve University nanoengineering professor Alexis Abramson is leading an effort to develop a synthetic gel with similar thermoelectric properties that could be used to convert waste heat, from devices such as car engines, into usable electricity.

Sharks’ extraordinary ability to hear low-pitched frequencies also helps them identify weakened fish: in the early 1960s, the University of Miami researcher Don Nelson theorized the spasms of a dying fish produce water movement sharks can detect through their lateral line and a staccato sound they can pick up through their inner ear.
¹⁴

Sharks’ ability to pick up vibrations also helps them migrate across ocean basins, because they can orient themselves within the earth’s magnetic field. These magnetic particles, which became embedded in the basalt in the aftermath of volcanic eruptions, provide a path for sharks to follow. The UC Davis marine biologist Peter Klimley describes it as a series of underwater highways as elaborate as anything surrounding a major American city. “You end up having these magnetic roads,” Klimley says. The seafloor boasts a pin-striped pattern of strong and weak magnetic fields, which only sharks know how to navigate. Periodically, the earth’s magnetic field reverses, a shift that sharks can detect without a problem.

Klimley views sharks’ multiple senses as their greatest asset. Having studied hammerheads swimming around Pacific seamounts, he’s documented that each night as they forage for food, the sharks swim more than twelve miles in one direction and then return along the precise same path, in the dead of night.

When you add in sharks’ sense of smell, which can detect a scent from miles away or help them find a mate, the species boasts a total of six senses, outpacing humans. Their acute sense of smell stems from a series of nasal flaps that lie in front of a shark’s mouth. When seawater passes over these flaps, they guide it to delicate membranes that can detect small substances in the ocean. As they glide through the ocean, sharks can determine where the scent is coming from by comparing what they perceive from one nostril with the other. This question of timing is crucial: sensing small differences in when the scent hits each nostril helps sharks steer the most efficient path toward their prey, as they follow an odor plume to its source.
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Some scientists estimate that more than a fourth of a shark’s brain is dedicated to its sense of smell. Their eyesight isn’t shabby either: while it’s not as keen as humans’, sharks have a reflective layer over their eyes, called tapetum lucidum, which makes it easier to see in the dark.
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They can also see in color and have several mechanisms to protect their eyes when they attack prey: many species have a nictitating membrane that closes when they strike, and some roll their eyes back in their heads so their victims cannot scratch or poke their eyes out.
¹⁷
Every sense they possess works to their predatory advantage.

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But it is the way sharks resemble humans—and differ from most fish—that helps account for why they are now in trouble. While most people think of fish as spawning millions of eggs, only a fraction of which will survive, sharks generally take the opposite approach. They take years to mature sexually—sometimes more than a decade—and only then do they produce a small number of young, which stand a good chance of making it to adulthood. This is why sharks are so vulnerable to human predation: while they are adept at devising different ways to produce their offspring, they simply don’t generate enough young on a regular basis to withstand a sustained assault from fishing.

There’s no single way sharks produce their offspring. Some species give birth to live pups; others lay eggs encased in yolk in leathery cases, which can sustain the embryo through its gestation. Others disperse eggs at the bottom of the sea. The number of young that sharks produce varies just as widely. While some sharks can carry dozens or even hundreds of eggs at a time, a spiny dogfish takes nearly two years to produce a single pup. The spiny dogfish’s pregnancy ranks as one of the longest gestation periods recorded in the animal kingdom, rivaling that of the elephant, which leaves it particularly vulnerable to overfishing.

Of the roughly 40 percent of shark species that reproduce by laying eggs, known as oviparity, many do this shortly after fertilization. The mothers anchor these eggs, which have a tough protective layer covering them, in the seabed so they will hatch in the sharks’ nursery grounds. Horn sharks produce a spiral-shaped egg case that can be wedged into rocks, while cat sharks anchor their egg cases to whatever is growing on the seabed. Over the course of their gestation these shark fetuses will receive sustenance by absorbing the yolk contained in the egg. Some species, however, choose to lay their eggs only a matter of weeks before they hatch in order to ensure that other ocean predators don’t have a chance to eat their young.

Even the sharks that engage in live births do it in very different forms. Both whale and spiny dogfish sharks keep their eggs internally, allowing the young to consume the egg yolk and hatch inside of their mothers, before releasing them into the wild. Other species don’t rely exclusively on the yolk sac, and instead produce infertile eggs they feed to their fetuses to keep them growing. This hybrid form of reproduction, called ovoviviparity, takes places in about a quarter of shark species.

Still other sharks engage in the more sophisticated form of reproduction, placental viviparity, in which they produce a placenta and engage in live births. This form of birth, which takes place in 10 percent of sharks, most closely resembles humans’: the fertilized egg develops into a placenta, which is connected by a cord to the uterine wall so the mother can feed her pup. In the case of hammerheads, which can produce a few dozen young in a single litter, each pup is connected to its mother through an umbilical cord. The similarity between humans and sharks ends after labor, however, because once a shark gives birth, her kids are on their own. Right after birth lemon sharks will rest for a short period, still attached by an umbilical cord, before breaking it by swimming away. At that point the babies must depend on mangroves and other natural fortifications, rather than their parents, to protect them.
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In many ways, the mother shark is doing her offspring a favor: in the immediate aftermath of birthing, female sharks experience a rush of hormones that minimizes their natural instinct to attack their children. But after a period of time those hormones dissipate and the mother takes off, leaving her children—which are born fully independent—to fend for themselves.

I got to witness this phenomenon myself when I waded through Bimini’s mangroves with Sonny Gruber and two other shark researchers, Ellen Pikitch and Elizabeth Babcock. After we trekked to one of the island’s most reliable lemon shark nursery grounds and threw a bit of fish into the water, a parade of juveniles came flocking over. The whole experience amused me: Gruber has developed plenty of shark tricks over his years in the field, and one of them is that he can hypnotize a shark by flipping it over. It’s an impressive yet comical feat: while the shark first struggles and flaps about as Gruber repositions it, within moments it lies harmless and still, in a trance. Soon I was holding my own small lemon shark, suspended upside down. I felt the rough, scratchy surface of its skin, and then, having put it right side up, I released it back into the wild to join its companions.

By this point the water was teeming with teenage sharks—not exactly in a school, but rather a disorganized pack, roaming together for a finite period before each one broke off to pursue its own target. There was something a little sad about it, this group that had no real bond. There are instances when sharks travel in schools out of self-interest, such as when female hammerheads are seeking mates or when smaller sharks need a form of defense against larger predators. Whitetip reef sharks gather together for a very specific purpose: to seek out and consume their prime targets, bony fish. These fish are out and about on the reef during the day, but they hide in the cracks and crannies of the reef once night falls. That’s when the whitetips emerge, traveling in groups in order to more effectively sniff out the fish they plan to tear to pieces.

“They are the most vicious predators; they hunt in packs, like wolves,” relates Elliott Norse, who heads the Marine Conservation Biology Institute in Bellevue, Washington. “They put their faces in every crack of the reef—they are social predators. I don’t know of any animal who has that strategy in the ocean.”

Most large shark species, like tiger sharks and the lemon sharks that circled before me, have little need to travel with their kind once they become adults, because other sharks represent potential competitors for food, rather than allies. As I looked at them, I reminded myself that sharks aren’t about bonding or establishing elaborate social structures. They’re about surviving and dominating everything that comes in their path.

Pikitch, who directs the Institute for Ocean Conservation Science at Stony Brook University, has devoted a significant portion of her career to tracking sharks in locales such as Glover’s Reef, an atoll by Belize in the western Caribbean. I journeyed with her there to get a sense of the mechanics of how such tracking works, since no matter how high-tech the devices scientists use, attaching them to sharks can involve backbreaking work. Most of the time researchers employ traditional fishing methods, which may involve a baited line, spear, or lasso. Hovering over the water in Glover’s Reef, I learn how addictive lassoing sharks can be.