Demon Fish

by Juliet Eilperin

  “Right now there isn’t any money in it. Is anyone going to pay me to do this? I don’t think so,” he acknowledges, comfortably ensconced in his sunny office, a stone’s throw away from the Monterey Bay Aquarium. “At this point the case for doing this is not quite obvious to everyone.”

  Sitting in front of his office computer, he plugs a specific genetic sequence into it: sure enough, 1,146 bases begin to stream across the screen. The computer program he’s using takes sixty-three taxa and scans their family trees, to see where the shark in question fits into the evolutionary path. This sort of letter crunching can provide a lens into all sorts of groundbreaking scientific findings: what shark populations looked like before humans started hunting them in earnest, for example, and whether they’re poised to adapt quickly to the earth’s changing environment.

  Palumbi is fairly confident of the answer: they’re not, because sharks evolve so slowly. While they’ve survived for millions of years, he explains, they’ve done so by sticking to a similar genetic formula.

  “They have relatively little ability to adapt evolutionarily,” Palumbi says. Unlike some marine organisms that can change quickly in the face of intense environmental pressures, sharks have historically adapted “on 40- to 60-, or 100-million-year timescales.”

  The genetic codes make it clear: just because sharks have survived for hundreds of millions of years doesn’t mean they’re well positioned to weather the most recent challenge to their existence.

  One group of scientists, who are drawing on both Shivji’s and Palumbi’s work, have a grand vision of DNA mapping that will ultimately capture the most significant species on earth: they call it the Consortium for the Barcode of Life. A few years ago, only a handful of researchers were trying to assemble the genetic sequences of animals large and small. Now hundreds of them work on the project: they have already collected at least 300,000 records for 30,000 different species.

  David Schindel, the consortium’s executive secretary, describes it as “a sort of telephone directory of well-identified specimens” that will allow scientists to quickly determine what species they’re examining. To ensure that it’s a universal genetic marker, they are constructing these codes from the same mitochondrial gene regardless of the species: cytochrome oxidase 1, also known as CO-1. Using this specific marker, which is located on the gene that serves as the powerhouse of the cell, allows researchers to distinguish among even closely related sharks with a high degree of precision: Robert D. Ward, a scientist at CSIRO Marine and Atmospheric Research in Hobart, Tasmania, along with two collaborators, examined 945 specimens from 210 Australian shark species and distinguished them with 99 percent accuracy.8

  To assemble this barcode directory, researchers are—in some cases—lifting samples from plant and animal relics that have been lingering on dusty natural history museum shelves for decades. “It’s pretty remarkable how durable the DNA in specimens is proving to be,” Schindel says. “We’re using insect legs that are fifty, sometimes a hundred years old.”

  Robert Hanner, a biology professor at Canada’s University of Guelph, coordinates the Fish Barcode of Life campaign, a subset of the overall barcode campaign. While the prospect of cataloging all the fish in the sea seems daunting—there are thirty thousand known fish species, and the number continues to rise—Hanner appears confident he’s up to the task. (And, as Schindel points out, they try to keep the project in perspective: “We’re not trying to do all species in all places.”) In order to do their work, Hanner asks for five specimens per species, across a shark’s geographic range. When it comes to elasmobranchs, which include skates and rays as well as sharks, the group has analyzed 6,074 specimens from 573 species, which means they’ve created barcodes for a little more than half of all known elasmobranchs. Many sharks are particularly easy to differentiate genetically because they developed so long ago, allowing them plenty of time to accumulate genetic mutations that distinguish one species from another.

  “These are the early days,” Hanner cautions, adding that once it’s complete, it will represent “the biggest communal database for the molecular diagnostics of fish in the world.”

  Assembling a global barcode for sharks and other fish, he suggests, will give researchers a quick fix on whether they’ve stumbled on something new. Ultimately, these scientists hope, researchers can venture out with a handheld DNA sequencer to assess what they’re encountering in the wild. “The value of barcoding is it can tell you very clearly, is this a match with something in your database, or is it an unknown?” Hanner says.

  No such DNA sequencer exists as yet: Hanner believes private companies will be happy to develop it once they’re confident enough barcodes exist to make it financially worth their while. “It’s not until we develop the Yellow Pages so every species has a lookup number that these big companies are going to get interested enough to throw investment dollars at it,” he argues. “People will say, ‘This is a market opportunity that needs to be looked at.’ ”

  In the meantime, however, Hanner and his colleagues have a long stretch of letter crunching ahead of them.

  DNA analysis isn’t used just to solve criminal and taxonomic mysteries, however. Shivji and Demian Chapman—his talented former student—have employed it to explain one of the most puzzling shark appearances in years. How sharks mate and give birth is one of the biggest remaining mysteries about them; genetics provides one of the few solid clues to understanding shark reproduction.

  On the afternoon of December 14, 2001, aquarium employees at Henry Doorly Zoo in Omaha, Nebraska, found themselves confronted with an inexplicable sight: a baby shark had suddenly materialized in their tank overnight. The day before the zoo had three bonnethead sharks: all of them were four-year-old females that had spent all but the first six months in captivity. These sharks had not reached sexual maturity, and there wasn’t any male for them to mate with in the tank. So the zookeepers puzzled over how one of them could have produced an offspring. Was it a prank, even though there was no sign of entry? Could one of the sharks have stored sperm for more than three years, which would have been an unprecedented act since this sperm has traditionally lasted for just six months inside a shark’s womb? Within twenty-four hours a stingray living in the tank had killed the baby female, severely rupturing its liver, so aquarium officials took the shark’s body out to preserve it, and wrote off the incident.

  Over the years, however, other zoos experienced mysterious births similar to Henry Doorly’s. In Detroit’s Belle Isle Aquarium, a baby shark had appeared out of nowhere, and researchers reported similar findings from other institutions. These scattered reports of virgin births intrigued Chapman, who at the time was pursuing his Ph.D. at Nova Southeastern University under Shivji’s tutelage. Chapman wondered whether these unexplained appearances meant that sharks, like some birds and reptiles, were capable of parthenogenesis, or asexual reproduction. In 2006, Chapman called officials at Henry Doorly and asked them to send him small samples from the baby shark’s fin and from the three female adults. Then, working with scientists at Queen’s University in Belfast, where he was putting in a short stint, he conducted a blind test of the four sharks’ genetic makeup. Chapman, an ebullient New Zealander by birth, engaged in a bit of good-natured wagering with his colleagues on the final outcome of his hypothesis. “We were bidding pints of Guinness on what it was going to be,” he recalls now.

  Within a matter of weeks the doctoral candidate had his answer: the baby shark (nickname: Jesus, despite being female) had exactly half as much genetic variation as one of the captive female sharks, meaning the shark had inherited an exact replica of its mother’s genes rather than getting half its genes from one parent and the other half from its father. The female shark had produced the baby on her own. After making the discovery, Chapman literally ran across the room in his lab to look at Irish scientific textbooks that detailed some of the instances where this has occurred in other species, such as rattlesnakes and the Komodo dragon. When an egg is
formed in one of these creatures, the animal also creates three polar bodies, a form of waste material that is genetically identical to the egg. Most of the time, these polar bodies are eventually discarded. But in the case of parthenogenesis, one of the polar bodies fuses with the egg, forming an embryo with half the genetic variability of its mother.

  Chapman, who is now an assistant professor at Stony Brook University working at the school’s Institute for Ocean Conservation Science, collaborated with Shivji and a Henry Doorly scientist to confirm the bonnethead’s virgin birth, and subsequently identified another such birth at a second aquarium with an entirely different species of shark. He sees this capacity for parthenogenesis as both an asset and a liability. On the one hand, it highlights how resilient sharks can be in the most unforgiving of environments, whether they are isolated in captivity or in the wild, if fishing has decimated their numbers to the point where few mating partners remain. “It just goes to show, life will find a way,” says Chapman, who published his findings along with Shivji and another colleague in the British journal Biology Letters. On the other hand, genetic variability is essential to a species’ survival, so sharks produced through a virgin birth don’t add as much to a population as normal offspring; a shark with half the genetic diversity of its parent will be less prepared to compete.

  “Biodiversity is based on genetic diversity,” Chapman explains. “Genetic diversity is like lottery tickets: you don’t know which one will win out in the future. The more diversity they have, the more chances they have to win, and the chance to survive for the next thousands and millions of years.”

  After Chapman and Shivji’s paper came out, a rush of aquariums started reporting suspected virgin births in a wide range of shark species. The Belle Isle Aquarium in Detroit examined the babies that a white spotted bamboo shark appeared to produce on its own, and determined the two daughters were born to a mother that had never engaged in intercourse. European aquariums have reported that a zebra shark and a whitetip reef shark may have produced offspring single-handedly. In one case, a shark held in a tank within a Texas classroom seems to have engaged in parthenogenesis.

  Chapman and other researchers have been able to confirm that this phenomenon has occurred in at least one other instance besides in Omaha and Detroit: a nine-year-old female blacktip shark that had lived in Virginia Aquarium’s Norfolk Canyon Aquarium died during a physical exam while under sedation, and it turned out the shark had been carrying an embryo despite having been separated from male blacktip sharks for at least eight years. Chapman, Shivji, and the Virginia Aquarium and Marine Science Center scientist Beth Firchau found that the shark, named Tidbit, had just reached sexual maturity and was carrying an embryo with no paternal DNA at the time of its death.9 This finding also challenged the scientific assumption that smaller sharks might be more inclined to engage in parthenogenesis because these species live in more isolated habitats and therefore may have trouble encountering males: the blacktip shark is a large species that migrates across the ocean.

  At this point, Chapman thinks researchers should start out with the assumption that anytime a female shark produces offspring in the absence of a male, it’s likely to be parthenogenesis: “This form of reproduction is more common and widespread than anybody realized.” And it may not be as disturbing a discovery as Chapman first feared: the two Belle Island Aquarium virgin births were alive and thriving more than five years later, showing no signs that they were any less healthy than sharks produced by two parents.

  The revelation also underscores another, broader point about sharks: nearly everything about their reproductive and parenting life is weird.

  Male and female sharks don’t intermingle frequently, according to scientific surveys. And researchers are beginning to learn that the nitty-gritty details surrounding when they do spend time with each other—to have sex—are harsh. These revelations highlight a central fact about sharks: they cannot be anthropomorphized the way some other creatures have been. They are vastly different from humans in how they behave, and won’t ever warm the hearts of the public the way penguins can.

  For centuries humans have recounted only the most fleeting observations of interactions between male and female sharks. While Aristotle might have composed the first written record of shark sex in the Western world, a fur seal observer with the New Zealand Department of Conservation evoked a similar theme thousands of years later. After witnessing an incident in 1991, A. Strachan wrote, “I have unwittingly been fortunate to witness a mating [between two white sharks]. I had thought at the beginning they were fighting as one animal appeared to be attempting to grasp the other with its great mouth, making great gouges in its side.”10

  Many scientists don’t like to talk about shark sex, because they worry it will only reinforce the popular perception that these creatures are brutish and unrelenting. But one day I coax Chapman to give me a lecture on the subject, despite his reluctance. We are sitting in an idyllic setting—out on a dock in Belize looking at the Caribbean—and there are dozens of other things he’d obviously rather discuss. But I’m after the facts, and he obliges me.

  Shark sex is, as Chapman puts it politely, “very rough.” Some of this reflects simple mechanics: male sharks have a pair of reproductive organs called claspers, which they insert into a female shark’s reproductive opening, or cloaca. (No matter how sharks gestate their young, they need to engage in internal fertilization in order to produce their offspring.) These claspers, which harden as a male becomes sexually mature, have tiny hooks inside them, which allow them to hold the female alongside as they’re mating. On top of that, during courting among larger sharks the male is usually biting the female to keep her around. This stems from the fact that, with a few exceptions, the female is almost always resisting the male’s advances. Marine biologists have an easy time determining if a female has been mating in the recent past because her skin will be raw and possibly bleeding. Female sharks build up defenses, to the extent they can, to cope with such a brutal coupling. The skin of most mature female sharks is measurably thicker than that of their male counterparts, and the fact that females tend to be larger also helps them withstand the beating they take during sex. Smaller shark species often mate by intertwining their bodies rather than the male dominating the coupling, a slightly less violent form of courtship.

  When mating season rolls around, female sharks—at least those that have been observed mating, a rare event in itself—tend to stay in shallow water. This is one of the few ways they can exercise any form of mate choice, since female sharks can resist being pinned down in shallow water. “If they stay in deep water, what’s waiting for them is a roaming band of males,” Chapman explains. “If she’s in shallow water, it’s difficult to roll her over; she will press her cloaca against the bottom. The key is to get her into deep water.”

  Once a female is cornered by a group of males in deeper water, they will take turns inserting their claspers in her. Often, a male shark will bite a female in order to grip her during mating. The end result? When a nurse shark gives birth to a litter of fifty pups, Chapman says, “what you’ll see is there’s anywhere from two to seven fathers.” Lemon sharks exhibit the same phenomenon: a litter of twenty pups often boasts several male parents.

  Of course, Chapman knows this, for the most part, from DNA analysis rather than from firsthand observation. Researchers rarely get to witness this mating—though nurse sharks are better observed than other species when it comes to intercourse—but advances in genetic testing have expanded our understanding of this ancient ritual. As Chapman notes, “You can actually use genetics to know what mating patterns they have.”

  One scientist has, most likely, viewed more shark sex than any other researcher in the world: Jeffrey Carrier, a marine biologist at Albion College in Michigan who conducts his research in the Dry Tortugas, a protected marine reserve that lies seventy-eight miles away from Key West. The animals his team has observed over the past two decades have demonstrated a
n amazing degree of fidelity to this site: the females return every two years, while the males come each year, during June. “We pretty much know who’s dating whom,” he explains. He’s acquired this level of detail by watching more than a thousand shark mating attempts, a formidable record.

  In 2000, Carrier co-authored a paper with the Mote Marine Laboratory scientist Harold L. “Wes” Pratt Jr. in the journal Environmental Biology of Fishes with the deceptively bland title “A Review of Elasmobranch Reproductive Behavior with a Case Study on the Nurse Shark, Ginglymostoma cirratum.” It is the most definitive account of shark sex ever published, and it makes for fascinating reading.

  Here are some of the facts Pratt and Carrier have to offer: among male sharks (as well as skates and rays) “biting or holding by some means appears to be universal.” Female blue sharks—which produce young at a much higher rate than other sharks—seem to get the worst of it, since they “receive so much precopulatory biting that they often appear to be severely wounded while on the seasonal mating grounds.”

  One of the best sections in Pratt and Carrier’s article is the chart they’ve constructed detailing the fish’s “courtship and mating behaviors.” When a nurse shark wants to demonstrate “acceptance,” for example, “female arches body towards male, ‘cups’ pelvic fins.” If it’s “avoidance” she’s after, then the shark opts for “ ‘lying on back’ the female rests motionless and rigid.” The sand tiger shark even engages in “stalking,” according to the scientists, but at least it’s not aimed at females. In this case, the sharks target other species within a captive environment, just to ensure that no other animal interferes with their chances of hooking up.

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