AT THE SOUTHERN TIP OF THE UNITED STATES, MORE than 100 miles from the continental mainland, lie the Dry Tortugas, a remarkable island group that few people have heard of and even fewer have laid eyes on. After Ponce de León discovered the islands in 1513, he named them in part after the Spanish word for turtles, tortugas, in recognition of the many sea turtles populating the surrounding waters. “Dry” was later added to the name to let other mariners know that the islands lacked fresh water. Better known today for its treacherous reefs and its nineteenth-century lighthouse, which was built to guide ships through those reefs, the islands are home to a great number of underwater wrecks—sixteenth-century wooden ships, WWII-era German U-boats—that rival the turtle population, littering the seafloor like discarded scrap in quantities large enough to form one of the richest concentrations of shipwrecks in North America. In the middle of the Garden Key Island, the remains of Fort Jefferson still stand, one of the largest forts ever built by the United States, an expensive and once greatly coveted outpost armed with 15-foot-thick walls and bristling guns that never fired a shot.
No one inhabits the Dry Tortugas; the only human presence comes and goes once a day, courtesy of a passenger ferry that escorts day-trippers from Key West who want to gape at the ruined walls of the fort and snorkel among the island’s vibrant coral reefs and kaleidoscopic underwater colors. Beneath the turquoise waters, undulating purple sea fans wave to the passing schools of deep-blue surgeonfish and iridescent parrotfish, while stingrays sweep the sandy bottom for crustaceans. Unique birds, like the rare sooty tern, nest in the quiet of the ocean breezes.
And, of course, there are the sharks.
Because they’ve been left alone, the shark population around the Dry Tortugas has thrived. Undisturbed, sharks patrol the underwater hills and valleys of the coral reefs and skim over the brain and antler corals. At first blush, it’s nothing out of the ordinary. But as I soon discovered, the sharks of the Tortugas were doing something special, something elemental, something the species has been practicing since the beginning of time. The sharks were having sex.
When I began researching shark sex on the internet, two names came up: Jeffrey Carrier, PhD, and Wes Pratt, the world’s foremost experts on the sexual behavior of sharks. These two internationally renowned shark sexperts are based in Key West, less than one and a half miles from the Tortugas mating ground.
In the 1970s, Carrier and Pratt, armed with newly minted degrees in marine biology, launched their careers. Sharing a keen interest in sharks, the two men became fast friends and, over the course of their careers, teamed up to publish some of the most remarkable discoveries about sharks. To meet them, I drove four hours west from Miami along Route 1, a scenic stretch of road that runs past translucent green bays and Atlantic shallows. I met Pratt in his office overlooking Summerland Key. There he told me all about his work and everything I ever needed to know about shark sex.
Pratt earned his bachelor’s degree in marine fisheries biology and applied for work with the National Marine Fisheries Service (NMFS), the federal agency that is responsible for the stewardship of national marine resources and manages fisheries to promote sustainability. During his job interview, a manager asked Pratt about his area of interest. He responded with one word: “Sharks.” Pratt landed the job and, shortly thereafter, shipped off to study large Atlantic sharks. Initially, he focused on the species of sharks with commercial importance: sandbars, makos, and blacktips. He studied their age and growth factors but soon grew fascinated by their reproduction habits. “And who wouldn’t be?” he asked. “They’re fascinating animals. They reproduce more like terrestrial vertebrates than like fishes. And they’ve got all kinds of clever organs and systems and strategies.”
Pratt showed me photographs of various sharks’ anatomies. The male and female sharks are easy to tell apart because the male sharks have two claspers, as Aristotle called them, external organs analogous to a penis. The photo Pratt showed me was of two long, sticklike appendages trailing from the underside of a shark. In the photo of the female, there is no clasper; she has a common vagina, called a “cloaca,” suitably designed to receive one of the two claspers. Internally, above the vagina, are paired uteri.
“Sharks reproduce quite differently than your average codfish, your average bony fish, where the female just makes as many as a million eggs per fish and then . . . ejects them at the right time into the environment at a fishing bank or rocky knoll or something. The male just sends sperm into the mass, and that’s it. Sperm and eggs—it’s all up to them to get together and form these little fragile fish.”
Pratt then held up what looked like a stick. “This organ is the male clasper of a 2,400-pound great white shark that was landed in Montauk years ago. And when it’s inserted in the female, this whole tip rotates back and splays open like an umbrella to lock it into the common vagina. The tip is very sharp, but she has a thick vaginal pad that receives it.”
Sharks, like many animals, are secretive about their mating; they prefer to keep the blinds down, so to speak. During the mating season, sharks stay in schools, but Pratt has shown that, after coital season, male and female sharks separate in something called “sexual segregation.” Nurse sharks are unusual because, unlike most large sharks, they mate in shallow water. This allowed Pratt to study them, up close and personal, in the Dry Tortugas.
A top marine scientist at the Mote Marine Laboratory, Pratt has devoted most of his forty-year career to studying shark reproduction and the last twenty-four years trying to puzzle out the nurse shark and its reproductive history because, as he put it, “they will answer my questions and tell me stories better than other sharks.” For all of his research, though, Pratt had never visited the Tortugas until he befriended Jeffrey Carrier, nor did he know it was a popular breeding ground for nurse sharks.
After earning his doctoral degree at the University of Miami, Carrier, looking to stay in Florida to research shark populations in the South Atlantic, started an educational program for young students called Sea Camp, where curious children could study marine life in the Keys. Carrier came up with the idea of the program when he noticed a group of small islands 70 miles west of Key West: the Dry Tortugas.
Carrier hopped into his skiff and headed into the Tortugan shallows, where he saw plenty of sharks. They were definitely up to something, Carrier said, but he wasn’t quite certain what. Through his binoculars, he saw a bubbling mix of white foam, blue water, and what appeared to be shark tails at the surface. He gunned the engine to get closer, but by the time he arrived, the commotion had come to an end. This pattern occurred, time and time again, and Carrier feared that he was never going to figure out what the sharks were up to.
One time, though, he saw a shark struggling nearby, churning up the water. Carrier assumed the shark had been struck by another boat. He peered down into the water. As he tried to identify the shark, he realized there were actually two sharks, one on top of the other. Anything but injured, the sharks were having sex.
To the best of Carrier’s knowledge, humans had never before witnessed such an intimate moment between sharks up close, in the wild. Of course, scientists had witnessed mating in aquariums, which gave them a basic sense of sharks’ reproductive behaviors, but some of the most basic questions remained unanswered. “Most of the studies in those days inferred reproductive behavior,” Pratt said. “This was back in the late seventies, and we didn’t really do anything with it at that point.”
Carrier invited Pratt to the mating area around the Dry Tortugas. Their inaugural trip was a bust, but Carrier invited Pratt back the following year. This time their luck turned; they witnessed fifty mating events. Sharks twisting and turning against each other, sending up white water in the light green shallows. “The first time we jumped in the water, we absolutely had no idea what to expect,” Carrier said. “We were in about three or four feet deep, and when we got in the water and swam through this cloud of silt that had been stirred up, we found that we weren’t face-to-face with two sharks. Rather, we were face-to-face with about eight sharks.”
Outnumbered four to one, the odds were not in the young scientists’ favor. But they quickly realized, given the act the sharks were engaged in, that the sharks were a lot more interested in each other than in the two of them.
Studying sharks in the act, Pratt observed that the males have developed cooperative behavior. Together, male sharks work to get a female out into deeper water. To latch onto a female during sex, a male shark uses its teeth for leverage. Pratt showed me a picture of a female blue shark he snapped during a cage dive. The shark is fairly small, probably about 5 or 6 feet long, not the most impressive of species. What is remarkable about the shark, though, are numerous bite marks clearly visible on its dorsal fin. “So this is the tough love of the blue shark,” he told me, “but one of my findings also was that the female has skin that’s up to three times thicker than the skin of the male to accommodate this behavior that they’ve evolved over the years.”
Pratt then talked me through a video of nurse sharks mating that he took on a recent trip to the Tortugas. “The sperm will go into these claspers and be thrust in by hydraulic action by the siphon sacs of the male, more interesting anatomy for those who like that sort of thing, which I do,” he said. “And [the sperm] are propelled up through the uteri, and in some species, stored in special modifications of a shell gland up there for fertilization. Not all sharks do this, but many do. The female can then store the sperm and have young when it suits her. When she’s ready, and her liver has been regenerated, like a strong battery, she can self-fertilize.”
If only human females were as fortunate.
As a species, sharks bear pups three different ways. Sharks that lay eggs are oviparous, a strategy many fish species follow. Birth is straightforward for oviparous sharks. A black, purse-shaped case that contains the pup embryo comes out of the cloaca. The case has hooks that attach to something in the seabed, like kelp or seaweed. After a few weeks, the shark pup hatches out of the case. Beachgoers find the discarded cases on beaches around the world. Other sharks like tigers and great whites are ovoviviparous, which means the eggs hatch inside the female shark, but the pups are born alive. Because these baby sharks are nourished from the nutrients in the egg, they are born without an umbilical cord. The third way sharks give birth is to bear live pups (viviparity) like humans with placentas and umbilical cords, which lets the mother nourish the young through her bloodstream. Large sharks like hammerhead and bull species give birth this way.
Pratt then held up a picture of a yolk sac and yolk stalk on the ova of a shark. “This is a shark embryo. It’s seven or eight days old.” He pointed out the various parts of the embryo, “These are gills. This is probably the beginning of an eye there. And then the tail. It is remarkably similar to a human embryo.”
I had to agree. They were remarkably similar. I left Pratt’s office with a tremendous amount of new information. But as hours turned into days, I couldn’t get that shark embryo out of my mind. One thing Pratt told me continued to resonate: “Ontogeny recapitulates phylogeny.” All animals, in utero, go through the same stages of development. If you go back far enough, we all have the same common ancestor as the shark. One could argue that here is the foundation of the Buddhist belief that all animals are interconnected and come from the same universal source. After speaking with Pratt, I thought back to my high school biology classes, when my teacher showed pictures of human embryos in various stages of development. Human embryos have fish tails early in development, an uncanny similarity to sharks at the same stage. Pratt told me that we all come from the same basic plan; we only differentiate as species later.
I wanted to see nurse and other sharks in action for myself, especially since the mating grounds were less than a hundred miles away. I drove to see Pratt’s friend and research collaborator, Jeffrey Carrier. Carrier works in Key West, and it was a short car ride over to his office, where he and I talked about his experiences tracking the sex lives of sharks before I was to head to the Tortugas on my own.
I helped Carrier untie his 17-foot skiff, and we headed out into Key West’s light-green bay, which was so clear that I felt as if we were skimming over liquid glass. We kept an eye out for shadows moving along the bottom. While at the helm of the skiff, Carrier explained the key relationship between mangroves, one of Florida’s true native plants, and marine life. Florida is blessed with an estimated 469,000 acres of mangrove forests, which contribute to the overall economic and ecological health of the state’s southern coastal zone. Anchored in mud by multiple thick roots, mangroves are one of the few plants to thrive in salty environments. They obtain fresh water from salt water by secreting excess salt through their leaves; some mangroves block the absorption of salt altogether at their roots. Above the roots and water are thickets of branches that can extend 100 feet above the waterline. They have densely packed small green leaves and form rookeries where coastal birds often nest.
In many countries, the mangroves are being uprooted and cleared to make way for fish ponds. In the Philippines, for example, as much as 50 percent of mangrove areas have been converted, primarily to grow shrimp for the worldwide commercial market. I asked Carrier what would happen if the mangroves here in Florida were cut down for fish ponds. He said, “Without healthy mangrove forests, Florida’s recreational and commercial fisheries would drastically decline.”
The most immediate benefit of a healthy mangrove ecosystem is the breeding, nursing, and nesting grounds for the local fauna and avian populations, in and out of the water, in either mangrove roots or branches. But the mangroves provide benefits beyond that. Mangroves also help recycle nutrients in coastal waters. As the trees shed their leaves, they drop in the water and are broken down by bacteria and fungi. The detritus of minerals and key elements is then available to the food chain, from plankton to aquatic animals. The mangrove roots not only act as physical traps but also provide attachment surfaces for various marine organisms. Many of these attached organisms filter water through their bodies and, in turn, trap and cycle the nutrients back into the system so the trees and vegetation can grow and start the cycle all over again. Moreover, the nearby ecosystems—terrestrial wetlands, salt marshes, and seagrass beds—also benefit. As the tides come in and out, nutrients from the mangroves are swept into the surrounding areas. This means that there is plenty of food available to a multitude of marine life, including large fish such as tarpon, jack, and red drum.
As Carrier and I were boating, we saw many birds such as brown pelicans and roseate spoonbills, which flapped their wings loudly overhead. Carrier pulled back the throttle, letting our boat drift into a small inlet of white and red mangroves. White herons caused their branches to sag. The water was so clear I could see the mangroves’ smooth brown roots buried in the muck. Barnacles clung to the bark. Hidden in the dense, intertwined roots, Carrier told me, were oysters, crabs, sponges, anemones, and many other species.
Because the depth was only 5 or 6 feet, I was able to see the ridges snaking across the water’s sandy bottom. As the boat glided to a stop, Carrier dropped a small anchor. Without the motor running, the only sound we heard was the water lapping against the skiff’s hull. A gentle breeze softened the intensity of the afternoon sun. In only a couple of minutes, we saw something. A baby scalloped hammerhead slipped by, its head moving side to side, hunting among the mangroves. A perfectly miniaturized version of an adult hammerhead, the shark was, at most, 18 inches long. Its sand-colored skin mirrored the sand below, which made viewing it difficult, but I could still make out the shark’s unique head, with scallops carved along the front. A few minutes later, a 2-foot-long baby lemon shark swam into the inlet. Startled to see us, it darted back into the mangroves. We were in the center of a shark nursery. Regardless of how sharks are born, shark pups from the moment of birth must learn hunting skills, or they will perish. In the mangroves, baby sharks are safe during their first years of life before they swim off to deeper, more dangerous waters. Without the mangrove forests, sharks, in particular the nurse shark, would be vulnerable. As nurse sharks grow into teenagers, they stay in the mangrove forest, which acts as their playground. “We don’t see the big adults in here, the mature adults, seven and eight footers,” Carrier told me. “We’ll see nurse sharks only up to about six feet.”
Animals often display a marked tendency to return to previously visited locations. Natal philopatry, where animals return to their birthplace to breed, is the most common form of such site fidelity. Salmon are well known for their ability to return to the river or creek where they spawned. To date, Carrier has tagged more than a thousand nurse sharks, mostly in the surrounding mangroves. Through his tagging studies, he demonstrated that nurse sharks return to the same area to mate year after year, in much the same way that salmon return to the same river or creek to spawn.
Carrier and Pratt’s research has been corroborated by other studies. Studies of lemon sharks in Bimini, Bahamas, also identified site fidelity among the sharks there. For thirty-five years, scientists witnessed female lemon sharks return to the same spot where they were born to drop their pups.
On the way home, we anchored the boat in a shady spot along a copse of mangroves. Wiping his brow, Carrier summed up what he has learned after conducting shark research for over forty years. He has achieved a remarkable recapture rate of 25 percent of the one thousand sharks he’s tagged, which makes him comfortable describing nurse sharks as homebodies. These sharks, Carrier said, don’t exhibit any aggressive behavior. In fact, he has never heard of an unprovoked attack by a nurse shark in this area.
After my boat excursion with Carrier in the mangroves of Key West, my journey continued to the Dry Tortugas island by ferry. The turquoise water is remarkably clear and warm, and it’s the perfect spot to snorkel to see the biodiversity of the island. I can understand why the nurse sharks flourish here, seventy miles from land and undisturbed by humans. Around these waters, they are born, reproduce, and live out their lives like Adam and Eve in the Garden of Eden, as they have for eons. My visit finally came to an end when the ferry had to go back to Key West.
Because I was still curious about how often sharks mate, I wanted to talk with another shark sexpert. Another researcher with similar interests is Jack Musik, who lives in California. A professor emeritus at the Virginia Institute of Marine Science at the College of William and Mary, Musik has studied sharks for more than fifty years. He began his career in 1961 at Sandy Hook Marine Lab in New Jersey. I was able to talk with him by phone about his research. Curious about how often sharks mated and their gestation period, he maintained meticulous records over the years, which have led to some remarkable discoveries. While humans try—sometimes unsuccessfully—to mate as often as possible, sharks prefer to take their time. “Most males will mate every year, but females less so,” Musik explained. “Some females mate every year, but in some species, the females might mate every two or three years.”
From conception, a shark’s gestation period varies, depending on the biology of individual species and, in general, the size of the shark. The bigger the shark, for instance, the longer the gestation period. Some species like great whites have an eleven-month gestation period. Other species have a relatively short gestation period of four or five months. Despite this difference, the gestation period of sharks is more similar to that of primates than that of fish.
The number of pups also varies by species. Tiger sharks rank near the top of the list and typically produce thirty or forty pups at a time; sometimes they rear as many as eighty pups. Tiger sharks are exceptional, though. On average, most other sharks give birth to about a dozen live young. When it comes to sex and breeding, scientists are still uncovering the reproductive habits of sharks like great whites. They rank at the bottom of the list with only four to eight pups at a time, and no one has ever witnessed the birth of a great white.
Wes Pratt has been working with other species of sharks to find out when they reach sexual maturity. “It takes between five and fifteen years for a shark to mature,” Pratt said. They mature like humans, in slow stages. A four- to five-year-old juvenile shark weighs only 13 to 15 pounds, growing at the glacial rate of 4 to 5 inches a year. During their youth, nurse sharks attend school in areas like mangroves, where they learn how to hunt. Pratt added, “In the case of nurse sharks, we don’t know exactly when they reach sexual maturity. We suspect that they probably are teenagers at fifteen to eighteen, and maybe start reproducing at eighteen to twenty.” Larger sharks like tigers and great whites, however, need twenty to twenty-five years to reach sexual maturity. As far as life expectancy goes, the nurse sharks live about as long as humans do. When the nurse shark dies, its body is recycled back into the marine ecosystem.
Sharks need many elements to reach the pinnacle of their development, but one component is crucial, and that is time: it takes decades for an individual like a great white to be able to reproduce. Fish species have followed two paths when it comes to producing the next generation. Some species at the bottom of the food pyramid produce a prodigious number of young. A female codfish, for example, can lay a million eggs. Sharks, like humans, take another path, which is to produce a limited number of offspring with each one having a long developmental period. Typical shark litters range from as few as five to as many as twenty pups, a far cry from the millions of eggs that other fish produce. “In the case of sharks,” Pratt told me, “these animals will replace themselves very slowly.”
Considering these factors, it became clear to me that sharks can’t survive at the current rate. They literally can’t produce fast enough to replace the 100 million sharks that are killed annually. Humankind has taken the ocean’s apex predator and upended its position, treating the shark as if it had the fecundity of a codfish.
THE HISTORY OF THE COD IS A SOBERING REMINDER OF WHAT can happen when governments ignore the status of fish populations. What happened to codfish is a cautionary tale of what can happen to sharks and other fish species.
In the sixteenth century, Europeans had discovered the Grand Banks, an underwater plateau just to the southwest of Newfoundland, Canada, one of the most productive fishing grounds in the world. The Grand Banks soon became the world’s largest source of fish, particularly cod. England, Portugal, Netherlands, France, and Spain (mainly the Basques) sent their fishing fleets to North America to catch them. The English thought of the huge schools of cod as “British gold.”
As one of the most sought-after fish in the North Atlantic, cod became the economic linchpin of what was called the “triangle trade.” The traders in the colonies dried, salted, and shipped the cod to Europe in exchange for European products to sell. Traders then headed down to the Caribbean, where they sold a lower-quality cod, called West India cure, to slave owners in that market.
Historically one of the most productive fishing grounds in the world, the Grand Banks once yielded what was thought to be an inexhaustible supply of cod. It turns out that this perception was incorrect. The supply of cod began to decline after World War II, when Newfoundland dropped its independence and joined Canada. Jurisdiction over Newfoundland’s fishery fell under Canada’s Department of Fisheries and Oceans in 1949. To promote jobs in the 1960s, the department allowed an increase in the number of fishing trawlers working the Grand Banks. As the trawlers increased, the cod catch soared. In the 1960s, 2 billion pounds of cod were harvested annually from the Grand Banks.
Over time, local fishermen started to notice dwindling catches. They informed local government officials. In 1986, scientists determined that, in order to conserve cod fishing, the catch rate had to be slashed in half. However, even with these new statistics brought to light, allotments remained unchanged. In fact, the Canadian government continued to predict, without any scientific proof, that the population of the species would rebound from its low point in 1975. Overly optimistic, the Fisheries Department set new quotas in the 1980s based on a drastic overestimation of the total stock. That decision was a disaster. The cod population kept plummeting.
By the time the Canadian government finally acted, it was too late. The number of cod had diminished past the point of recovery. In 1992, the government finally closed down cod fishing. Initially, the 1992 moratorium was scheduled to last two years, during which time it was hoped that the northern cod population would recover, and along with it the fishery. The damage done to Newfoundland’s coastal ecosystem proved irreversible, however, even after almost thirty years. To this day, the northern cod population has not rebounded, and the cod fishery remains closed. Even though surviving cod continue to produce millions of eggs, ocean predators pick off so many that only a very few cod make it to maturity.
The closure was devastating economically: 35,000 fishermen and fish plant workers lost their jobs. Today, the once-great schools of cod are only 1 percent of their historic spawning biomass, and the 500-year history of cod fishing has come to a bitter end. Diners may see cod on a menu now and then, but the cod served in restaurants is sometimes pollock or another white fish instead of actual Atlantic cod.
If the cod could be decimated, as massive as their biomass once was, any fish species can be wiped out. When it comes to shark populations, they can be just as vulnerable as codfish. Pratt told me that because most sharks give birth to an average of only twelve to eighteen pups per female, they are vulnerable to sustained fishing operations.
Pratt’s warning made me worry that the sad plight of codfish could happen to sharks—or any fish species, for that matter. I wanted to find out just what will happen to the marine ecosystem if sharks go the way of cod, so I headed to Australia, where I soon learned about a world, in the middle of the Pacific Ocean, that I never knew existed.