Demon Fish

by Juliet Eilperin

Carrier and his colleagues have studied their subjects’ intimate dating patterns through a device called Crittercam, whose name sums up its function quite nicely: you attach the camera to the critter, and when you retrieve the footage, you can see what it’s been doing. (Greg Marshall, the National Geographic scientist who invented Crittercam, actually did so after being inspired by watching a shark glide through the water in Belize with a small remora fish attached to its back. Remoras, which are sometimes nicknamed shark suckers, hitch rides on the large predators, and in exchange for getting some of a shark’s leftover food, they pick off some of the parasites that attach themselves to their host. Sharks show no signs that they’re even aware of the remoras’ presence, which is why scientists have modeled their video observation system on the tiny fish.) The Crittercam footage has captured the gang-bang phenomenon nurse sharks engage in, a process Carrier describes as “a cooperative venture, where one male is successful and the other males keep the female from leaving.” One of the most interesting aspects of this act is the fact that an implicit social order seems to determine who gets lucky.

  “We think there’s a social overlay, a social hierarchy, in a way that’s startling,” Carrier says. As in the world of humans, lions, and other animal societies, it’s the dominant male nurse sharks that get the most play.

  In the case of nurse sharks it’s the big, dark ones that reign supreme, according to Carrier and Pratt, which explains why females may shift from shallow to deeper water depending on the suitor. “The ‘refuging’ female is observed to retreat repeatedly from smaller, lighter colored males which are perhaps younger, weaker or inexperienced. The arrival of one of the larger, darker dominant males often elicits a response in the female to remain in the lagoon’s deeper water, or at least not to retreat when approached.”11

  There is a nice inverse of this among hammerhead sharks, which gather in large schools each year to mate. Among that species, the dominant females occupy a prominent place in the center of these schools, pushing more submissive females to the periphery. The UC Davis professor Peter Klimley, who has swum with hammerhead sharks and filmed them while they’ve schooled, describes it as sort of a mix between the old television series Wild Kingdom and a traditional 1950s social mixer where young women are waiting for their male suitors to make the first move. On the one hand, the scars on the upper portion of female hammerheads’ bodies testify to the extent to which they fight each other over which sharks get to stay in the school’s inner sphere. “This is what happens a lot in the animal kingdom,” he says. “It takes one or two ritual combats to establish a hierarchy.” On the other hand, the male hammerheads that manage to get near the school push themselves into its center, casting aside the females on the periphery.

  Once they make it in, moreover, male hammerheads perform elaborate moves to copulate with one of the dominant females. After pushing their way into the inner sanctum, they rotate their claspers so that they’re bent in half at a right angle and then throw their torso forward in what Klimley calls “a very sexual manner.” Other times, the sharks will conduct a corkscrew somersault. This acrobatic maneuver gives off flashes of light, which help attract attention.

  While researchers know what happens when some sharks mate, that’s about where their understanding of shark sex ends. “That part’s not in doubt,” Carrier says of the mating attempts he’s witnessed. “We just don’t know what the hell’s happening afterwards.” After that point Carrier and his colleagues are no longer able to track the nurse sharks for months, until they return the following summer to repeat their courtship pattern once more.

  But new technology now holds the promise of allowing scientists to move beyond what they’ve learned through Crittercam. Accelerometers—also known as acceleration data loggers—are the same motor-sensitive computer chips used in smart phones, iPods, and the Nintendo Wii, and they are providing unprecedented detail about how sharks move beneath the water. Accelerometer tags, which are a bit larger than AA batteries and are designed to fall off a shark’s dorsal fin, can detect every flick of the animal’s tail or tilt of its body. Nick Whitney, a postdoctoral scientist at Mote, worked with a team to fit four female nurse sharks with accelerometers in the Dry Tortugas and was able to document a range of mating behaviors, including thrashing, barrel rolls, and headstands. With any luck, Whitney and his colleagues will be able to attach the tags to more elusive species, such as hammerhead, sandbar, and bull sharks, in order to better grasp these animals’ mating patterns.

  One thing researchers already understand is that the brutal competition that defines mating among sharks does not stop there. Once the eggs are fertilized, shark fetuses sometimes eat each other in utero, acting out the harshest form of sibling rivalry imaginable. To some extent this is merely an extension of the sort of competition for paternity that usually takes place outside the womb: Chapman has determined through genetic testing that while a sand tiger shark is often fertilized by multiple partners, during gestation the offspring of a single father will consume its half siblings in order to emerge as the sole survivor.

  There is another upside to this embryonic cannibalism, or ovophagy, which occurs in both sand tiger and white sharks: it helps prepare young sharks for the difficult conditions they will face once they exit the womb. In the case of sand tiger litters only two babies emerge each time, one from each of the mother’s uteruses, since the surviving offspring has eaten everything from fertilized eggs to embryos inside the womb.

  Chapman, who has examined sand tiger fetuses, describes them as having “big, wicked, nasty teeth” that help them devour their siblings while still in the womb. “They tear them to bits,” he relates, in a dispassionate voice. “A female sand tiger gives birth to a baby that’s already a meter long and an experienced killer.”

  In many ways genetic testing has provided us with the most brutal truths about sharks. It reveals which sharks are being killed by humans and which are being killed by each other. It can be a tool to mete out justice or, in some instances, a reminder that they have evolved over millennia to suit their surroundings. And it hints at one of the most promising aspects of this sort of academic research: when it comes to sharks, the golden age of scientific discovery has just begun.

  6

  SHARK TRACKERS

  Right out there, it’s the Serengeti. We just can’t see it.

  —Barbara Block, Stanford University marine biologist

  It wasn’t until a few decades ago that serious scientists began focusing on sharks at all. The renowned biologist E. O. Wilson became fascinated by them at age nine, just like many other young boys who imagine how they would fare against the wildest beasts on earth. “The great white and I go way back” is how he likes to phrase it. Seven decades later Wilson—who combines the courtliness of a southern gentleman with the patrician New England air of his longtime adopted home, Cambridge, Massachusetts—calls himself an expert in “selachology.” That’s a word he made up himself (though it takes its roots from the Greek word Aristotle favored, selache, or sea fish) and translates roughly into “the study of sharks.” Wilson is a rumpled academic whose hearing gives him trouble at times, but having devoted his life to unlocking the mystery of the natural world, he is extraordinarily skilled in relating his findings to people who are less brainy than he is. Sharks are not just an obsession but a pedagogical tool that helps the professor explain a universe to which most people are oblivious.

  Wilson has taught an introductory biology class to Harvard undergraduates for decades, and when he needs to convey the concept of “adaptive radiation”—how species evolve in different ways depending on the part of the world in which they live—he always uses sharks as his example because they boast such a wide array of physical forms across the globe. Their lifestyles vary too: Some move from deep water to breed in shallow water, lagoons, and estuaries; they can spawn between one and a few hundred offspring at a time. While they traditionally swim in salt water, some survive in freshwater. They live in th
e tropics and at the poles, in both warm and cold water. They began as what researchers at Dalhousie and Florida International universities, led by Francesco Ferretti, called “small coastal consumers” in one journal article, but over time evolutionary forces favored larger species that continued to grow and reached sexual maturity later, so they could “colonize deeper oceanic waters.”1 One group, known as sleeper sharks, live at incredible depths, like the Portuguese shark, which survives at twelve thousand feet below the sea’s surface. These deepwater sharks are some of the longest-living sharks on earth: Greenland sharks live more than an entire century.

  In this way, sharks serve as a lens into an array of worlds that have historically lain beyond our grasp. Researchers are still investigating why hammerhead sharks have such odd, flattened heads, but this form clearly gives them exceptional peripheral vision that helps them hunt. The fact that sharks’ litter sizes vary so enormously reflects the wide range of habitats that the ocean offers: coastal sharks produce larger litters because their offspring enjoy more abundant resources yet at the same time face plenty of predators; open-ocean sharks produce a small number of babies that will have to work harder to find food but are less likely to be eaten. Coastal sharks can grow to be fearsome animals, but at their early stages of development they are prey to an array of species, such as rays and other carnivorous fish.

  The range of shark litter sizes is enormous: the pelagic thresher and bigeye thresher tend to produce two offspring at a time, while the great hammerhead shark can produce several dozen young at once, and a tiger shark can produce more than a hundred.2 Whale sharks are the most fecund sharks on earth, capable of producing three hundred young at a time. These sharks carry eggs at different stages of development, which could be, in the words of the Australian biologist Brad Norman, “a survival strategy” in which the mother births her pups when the external conditions are best, or it could just reflect how much space the shark has in its twin uteruses.

  The variety of tails among shark species also testifies to their adaptability. Threshers boast the longest tail of any shark: its upper lobe alone is as long as its body, ranging between five and eight feet, and serves as a powerful hunting weapon. By contrast, nurse sharks, which stick close to the bottom and swim more slowly, have relatively weak tails. The fast-swimming sharks, including the great white and mako, have tails with upper and lower lobes that measure almost the same length. This particular design, called the thunniform fin, gives it more thrust per stroke. By using a larger surface area to push aside a greater amount of water with each tail movement, a great white or mako can turn forcefully or push itself forward, positioning itself better to attack its prey.3 Tuna and other sharks in the family Lamnidae also boast this feature, allowing them to swim at high speeds and traverse long distances.

  The fact that sharks have so many ways of existing in the world underscores how much they convey about the planet. Sharks have developed unique ways to eat, swim, and reproduce because they have survived such vastly different circumstances. Few other animals provide as clear a lens into the natural world as sharks, yet we are just beginning to crack this code. We would do well to learn from them, for the sake of our own survival.

  Sharks—along with skates, rays, and the nearly extinct chimaeras—are all elasmobranchs. All of these fish have five to seven paired gill openings on the sides of their head. Some rays, like guitarfish and sawfish, look like sharks that have been flattened, which makes them an alluring fishing target because their large fins are ideal for shark’s fin soup. However, they are not actually sharks. The fossil record boasts more than three thousand species of elasmobranchs, a hardened testimony to the diversity of sharks, skates, and rays that once roamed the seas. But this figure has narrowed over time. (Since their cartilaginous skeletons are seldom well preserved, it’s sharks’ spines and teeth, along with impressions of sharks in rocks, that constitute the species’ fossil record.)

  There are roughly five hundred known species of sharks, and this number is constantly inching higher: sometimes in tiny increments, other times in a massive leap. In any given year a scientist might find distinct species either through genetic analysis or by exploring remote waters. But in some instances the number can change on a larger scale, as when CSIRO Marine and Atmospheric Research’s Peter Last, William White, and their team used DNA to identify a total of forty-six new sharks in the course of a year and a half. To put the current global shark count in perspective, it is more than twice as many species as scientists knew about in 1971 when the respected naturalist Peter Matthiessen published his book Blue Meridian: The Search for the Great White Shark. However, this does not mean sharks are doing better than they were several decades ago: it suggests some of these populations are more vulnerable, because they are smaller than previously thought. For example, the CSIRO team identified the northern river shark, which is unique to Australia, grows to be nearly six feet long, and ranks as one of the country’s largest freshwater animals. Until recently, it was confused with another Australian freshwater species in an adjacent region, but now researchers understand it is distinct in its own right.

  Great whites now rank as the world’s most terrifying shark—E. O. Wilson calls them “one of the four or five last great predators of humanity.” But these fish appear to be pikers compared with their ancestors, Carcharodon megalodon, which researchers estimate were double the size of great whites and boasted teeth twice the size of a human hand. Living between 50 and 4.5 million years ago, Carcharodon megalodon may have stretched as long as fifty feet and weighed twenty tons, equal to five elephants piled on top of one another.4 Using three-dimensional computer modeling techniques, a group of researchers from Australia and California have calculated the bite force of both modern and ancient great whites: the largest great whites living today have a force of up to 1.8 metric tons of pressure, compared with the 18.2 tons a Carcharodon megalodon could exert. (For comparison, a large, modern African lion can produce roughly 560 kilograms of bite force and a Tyrannosaurus rex would have boasted a force of 3.1 metric tons—a sixth that of the ancient great white.5) In the history of the earth, virtually nothing has roamed the sea that is as scary as these animals.

  Some living sharks are massive, of course: whale sharks are the largest fish in the world, stretching up to fifty feet long, and basking sharks grow up to forty feet long. But others are tiny—the spined pygmy shark, called Squaliolus laticaudus, is six inches long, and lantern sharks span twelve inches. Three feet represents the median length for sharks, with half of all species measuring less than that. Despite all the hype that surrounds sharks and their killing capacity, most of them aren’t nearly as fearsome as their reputation suggests.

  The sharks that swim in our waters today evolved about 100 million years ago, with a more powerful, mobile jaw that allowed them to target prey more effectively than their ancestors. All sharks have multiple rows of teeth on their upper and lower jaws, and as these teeth break or become worn, spare teeth lying just behind take their place in a sort of conveyor-belt fashion.

  While sharks share a similar jaw structure across different species, modern sharks each take on their own bite sizes—and shapes. A shark’s choppers depend both on the species and the age of the animal, since different types of teeth are effective for various types of prey. Saw-edged teeth, like the kind great whites have, come in handy when biting big chunks out of marine mammals. Baby great whites, by contrast, have pointed teeth that let them grab and swallow smaller prey. Fangs or spearlike teeth are better for gripping squid.

  The cookie-cutter shark, Isistius brasiliensis—a dwarf shark that swims in schools and is bioluminescent, meaning it gives off a greenish glow in the water—extracts a perfectly circular chunk of flesh out of large fish for its meals, and for years researchers were at a loss as to how it managed to attack larger predatory fish such as tuna, swordfish, and even porpoises. In 1998, after making a series of close observations, the Harbor Branch Oceanographic Institute scientist Edit
h A. Widder figured out how they do it. The cookie-cutter shark derives its bioluminescence from thousands of very small photospheres around the edges of its scales. There is one part of the shark’s body that lacks this phosphorescent gleam: a collar around its throat that is darkly pigmented. As the light streams into the water from above, this pattern provides a silhouette that acts as a lure to larger predators, which mistake the cookie-cutter shark for a small fish.

  While many larger sharks travel as loners, these pygmy sharks congregate instead, which gives them an even greater advantage against their opponents. “Schooling may also explain how these very small sharks avoid a counterattack from the very large predators such as swordfish and porpoises on which their crater wounds are commonly found,” Widder writes. “The damage these sharks inflict would make their company as appealing as a swarm of wasps.”6

  Other sharks adopt a more obvious line of attack: the thresher shark wreaks havoc within schools of fish by whacking them with its exceedingly long tail and then returning to consume the ones that have been the most immobilized.7 Flattened sharks such as wobbegongs lie pressed against the ocean floor and then swallow fish as they swim by. A nurse shark has a set of feelers, or barbels, on its nose so it can ferret out small prey in the sand below it.

  Sharks’ jaws don’t just tell us about their evolution: they reveal details about our own. For years scientists have studied vertebrates’ transition from jawless animals such as lampreys to ones with jaws, because it was such a massive step forward in evolution. But they’ve been hampered by the inadequate fossil record dating from the Devonian period, which occurred sometime between 412 and 354 million years ago. For literally a hundred years, researchers focused on a 370-million-year-old shark named Cladoselache, because they had several good fossils to examine. But then a decade ago scientists found a shark relative from Bolivia called Pucapampella that predates Cladoselache by 30 million years.