4
Shark Sex
Awwww. So cute seeing a baby white.
—Instagram comment about Ocearch researchers’ photo of capturing, tagging, and releasing a baby great white shark
Imagine you’re a bay scallop—a small shellfish living in the shallow Atlantic Ocean waters off the eastern coast of North Carolina. What are you scared of? The cow-nose ray, one of your main predators. You wouldn’t mind if fewer cow-nose rays were around. But actually, more of them share your habitat than ever before. Why? Because the population of smooth hammerhead sharks that prey on cow-nose rays is declining fast.
Atlantic cow-nose rays are recognizable by their notched brow. Females give live birth to one pup per delivery. The gestation period is about one year. Members of the class of vertebrates known as Chondrichthyes, rays are closely related to sharks.
Hammerheads are in trouble. Fishing fleets catch them to sell as food fish, and they sometimes reel them in as bycatch. According to the Census of Marine Life, shark numbers are down so low that cow-nose rays are all over the place. They are eating so many bay scallops that North Carolina has had to close down the scallop fishery that has thrived on its Atlantic coast since the early twentieth century.
All ecosystems have a food chain hierarchy, or ranking of prey and predators. At the top are big apex predators, such as sharks. In the middle are mesopredators, medium-size animals such as cow-nose rays. Mesopredators are both prey for apex animals and predators of smaller animals, such as bay scallops. Balanced populations of each type of animal—neither too many nor too few—keep an ecosystem healthy and strong. But if any of the animals are threatened and their numbers decline—such as those of the hammerhead shark—the ecosystem goes out of balance and gets wobbly and weak.
“As the top predator in the Atlantic, the white shark is like the wolf of the sea, helping to maintain ecological balance by feeding on prey such as seals—especially the weaker and less fit,” says Robert Hueter, a scientist at the Mote Marine Laboratory in Florida. Sharks make the oceans healthier by weeding out weak, sick, and old animals. Without predators in an ecosystem, one species may take over and a cascade of imbalance flows from there. For example, on coral reefs where hammerhead sharks are overfished, hammerhead prey such as snappers are becoming too abundant. The mesopredators are therefore eating more of the smaller herbivorous (plant-eating) fish such as parrotfish that are below them in the food chain. Without parrotfish to feed on the algae that live on the coral, the algae population balloons. The algae then start to take up too much of the oxygen the coral needs to survive.
The Next Generation
For all the trouble that sharks have turned to their advantage over their long evolutionary history, they didn’t factor in humans. And when humans interact with sharks—mostly through fishing—they aren’t factoring in the shark reproduction cycle. Sharks live on average twelve to twenty-seven years. That’s nowhere near as long as a whale, which may live two hundred years. But it’s much longer than the life of the average bony fish. Take swordfish, for example—another top predator that swims in the same waters as sharks. A swordfish reaches sexual maturity, the age when it can reproduce, when it is five or six years old. Thresher sharks aren’t sexually mature until they are nine to thirteen years old. Other late-bloomer sharks can take as long as twenty years to reach sexual maturity. For example, great whites do not mate before they are twenty. They bear very few young, and only some of them survive to adulthood. This is the natural way of keeping apex predator populations in check, because the food chain cannot support too many of them—nor too few. With overfishing, many sharks are caught before they have a chance to reproduce. Others are caught before they have given birth to more than one litter of pups. With fewer pups, the shark population will not stay healthy.
From Eggs to Pups
All female sharks produce eggs, but sharks vary in the ways the eggs develop into baby sharks. Females mate with males, and they fertilize the eggs through sexual intercourse. With many species of sharks, you can tell males from females by the claspers. Males use these extended pelvic fins (along with their teeth) to hold the females in place while they mate and transfer sperm to eggs for fertilization. All shark eggs are fertilized inside the female’s body. What happens after that varies.
Some sharks are oviparous—they lay eggs. The eggs mature and hatch outside the mothers’ bodies. Others are viviparous—the eggs are fertilized and develop inside the female’s body. The female gives birth to live pups, but she doesn’t take care of them after birth. They are on their own.
Many sharks, such as this lemon shark (right), give live birth. The pup comes out tail first, wrapped in the chorion. This membrane protects the developing shark embryo and helps provide it with nutrients and fluids from the mother prior to birth.
A female shark can lay from two to eighty eggs, depending on the species. Once the eggs are fertilized, oviparous shark species, such as zebra sharks and necklace carpetsharks, will lay embryo-filled egg sacs—sometimes called mermaid’s purses—in shallow coastal ocean waters. There, the embryos live on the yolk in the egg sac until it is gone. Then they emerge from the sac to live in the ocean.
But most female sharks are viviparous. They carry pup embryos (developing sharks) in the uterus. Some unborn sharks, such as smooth hound and bull sharks, receive their food through a placenta, a tube that carries nutrients from the mother to the embryos. Other embryos (for example, those of makos and cookiecutter sharks) feed by eating unfertilized eggs produced by the mother’s ovaries. This is called oophagy. And the sand tiger is a unique species that experiences adelphophagy, which literally means “eating your brother.” These embryos feed on one another in the mother’s uterus rather than on nutrients that come from a yolk sac or through the placenta. The majority of pups in these litters are eaten before birth.
Sexual Superpowers
From one mating encounter, a female shark can store sperm for later use. Some shark species live alone, and among these species, males and females meet rarely. So solitary females have adapted their reproductive cycle to store a male’s sperm for months. The world record for sperm storage by a female shark is a brownbanded bamboo shark (in an aquarium) that stored sperm for three and one-half years! When the female is in the fertile part of her reproductive cycle, the sperm release to fertilize the eggs she produces.
Some female sharks, such as blacktip reef sharks, sawfish, and hammerheads, can even create offspring without mating with a male. During parthenogenesis (from the Greek words parthenos, or “virgin,” and genesis, or “creation”), the mother passes on her female deoxyribonucleic acid (DNA), her genetic information. For example, a blacktip reef shark in the Virginia Aquarium & Marine Science Center produced a pup that was a genetic clone (match) of her own mother. Parthenogenesis can lead to inbreeding, an unhealthy mating practice among related animals that can cause deadly disease and physical weaknesses. But it can be useful for breeding sharks that are endangered and facing extinction.
Remoras attach themselves to a lemon shark. Each remora has a specialized dorsal fin covered with a tissue that seals the fish to its shark host.
Where the Pups Are
Female shark gestation (pregnancy), on average nine to twelve months, is among the longest of any vertebrate (animals with backbones). Some species have gestation periods up to twenty-four months. Gestation among basking sharks is almost three years. Most sharks have about 20 pups in a litter, although the blue shark lays as many as 135 pups per litter. Once the embryos have matured, the pups are born through the mother’s cloaca, or birth canal. Pups look like miniature adults and are able to fend for themselves. Then the relationship with their mothers ends.
Pupping nurseries along the coasts of the oceans are home to young sharks that may sometimes live in separate groups according to species, size, or gender. The waters are shallow, protected, and filled with the small fry fish that pups eat. Among the US shark nurseries that scientists know about are these:
- Delaware Bay and Chesapeake Bay on the northeastern Atlantic coast of the United States for sandbar sharks;
- Great South Bay off Long Island, New York, for sand tigers;
- Montauk, Long Island, for great whites;
- Bull’s Bay, South Carolina, for spinner, dusky, Atlantic sharpnose, and many other sharks;
- Dry Tortugas, Florida, for nurse sharks;
- Florida’s Atlantic coast for scalloped hammerheads; and
- Louisiana’s Gulf of Mexico coast for bull sharks.
How do we know where the nurseries are? Sometimes fleets discover them while fishing. Research is the real key to understanding which areas sharks use and how. Take Delaware Bay. Lying between New Jersey and Delaware, the bay is where the Delaware River widens into an estuary (an area where a river meets and mixes with the sea) before flowing into the Atlantic Ocean. Naeem Willett and Dewayne Fox of Delaware State University, along with Brad Wetherbee of the University of Rhode Island, study sharks. They attach transmitters to the backs or fins of sandbar sharks. The transmitters send signals to the scientists’ labs via satellite. The scientists use the data to pinpoint the sharks’ locations and to map their movements in those areas. The maps help scientists understand the boundaries of shark habitats and nurseries and can be used to protect sharks.
A female lemon shark swims with her newborn pup. Lemon shark nurseries are typically in tropical, coastal mangrove areas of Bimini, three western islands in the Bahamas. Researchers have found that female lemon sharks often return to the same place where they were born to give birth to their own young years later.