AP file

A sand tiger shark swims in the "Shark ocean" of the "Sea Center" in Burg auf Fehmarn, an island in the Baltic Sea, northern Germany. Cannibalism seen in sand tiger shark embryos is a competitive strategy by which males try to ensure their paternity, researchers say.

By Tia Ghose, LiveScience

Shark embryos cannibalize their littermates in the womb, with the largest embryo eating all but one of its siblings.

Now, researchers know why: It's part of a struggle for paternity in utero, where babies of different fathers compete to be born.

The researchers, who detailed their findings Tuesday in the journal Biology Letters, analyzed shark embryos found in sand tiger sharks (Carcharias taurus) at various stages of gestation and found that the later in pregnancy, the more likely the remaining shark embryos had just one father. [In Photos: Baby Sharks Show Off Amazing Ability]

That finding suggests the cannibalism seen in these embryos is a competitive strategy by which males try to ensure their paternity.

"In some species, the struggle for paternity continues beyond the point where the female [mates with] the male," said study co-author Demian Chapman, a marine biologist at Stony Brook University of New York.

Mini-cannibals
Full-grown sand tiger sharks are approximately 8.2 feet (2.5 meters) long, and mothers typically give birth to two baby sharks, each about 3.3 feet (1 m) long.

Since the 1980s, when detailed autopsies of sand tiger sharks revealed embryos in the stomachs of other embryos, researchers had known that the shark fetuses cannibalized each other in utero about five months into their nearly yearlong gestation. Legend has it that a shark embryo actually bit a researcher's hand during a dissection when the researcher reached into the uterus of the shark's mother, Chapman said.

While 12 littermates may start out the journey, all but one is devoured by the biggest in the pack. That strategy allows sand tiger sharks to have much larger babies at birth than other shark species, making the little ones relatively safe from other predators, Chapman said.

But scientists didn't know why the sharks were cannibalizing each other. One possibility is that females were mating with multiple partners and that the cannibalization helped only one father's genes remain dominant.

To find out, Chapman and his colleagues studied genetic samples from 15 pregnant female sharks that had died in nets off the coast of South Africa. (The nets were put in place to protect swimmers from deadly bites from great white sharks and bull sharks, but the nets occasionally snare and kill sand tiger sharks.)

Paternity struggle
Of those 15 female sharks, 10 of the sharks carried just two embryos, while the remaining five were in an earlier stage of gestation and had five to seven embryos in utero.

The team then used DNA analysis to determine paternity.

"It's exactly the same sort of DNA testing that you might see on Maury Povich to figure out how many dads there are," Chapman told LiveScience.

Those litters with five to seven embryos had at least two fathers (embryos from other fathers may have already disappeared), while the litters with just two sharks more often had just one father.

That suggested one embryo — possibly the one that grew biggest first — tended to devour embryos from other fathers over its full siblings.

"Basically, that loser father ultimately provided food for a rival male," Chapman said.

Sexual selection
It's still a mystery exactly what makes one father successful over another, said James J. Gelsleichter, a marine biologist at the University of North Florida who was not involved in the study.

"Sexual selection is very much like an evolutionary arms race, and the males and females are basically one-upping each other," Gelsleichter told LiveScience.

A possibility is that embryos from the first male to fertilize the female simply get biggest first, devouring their littermates.

The strategy could also help females select good mates. Shark mating involves violent biting, so intrauterine cannibalism may allow females to avoid resisting and avoid being "too choosy" about mating, while still ensuring that a high-quality male sires her offspring, Gelsleicther said.

Follow Tia Ghose on Twitter @tiaghose. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.com.

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Neil Hammerschlag

A great white shark cruises underwater in search of prey.

By Douglas Main, OurAmazingPlanet

For the first time, migrating great white sharks have been tagged and their movements around the oceans tracked for years, as opposed to the few months they have previously been tracked, according to a researcher.

Scientists used special satellite tags that tracked several sharks from a specific great white population for up to three years off the coast of Mexico. The study found that adult female sharks complete a two-year breeding cycle and avoid male sharks whenever possible, said study author Michael Domeier, a researcher and the president of the Marine Conservation Science Institute.

Published recently in the journal Animal Biotelemetry, the study followed four female great white sharks from their mating grounds off Mexico's Guadalupe Island until they returned 24 months later, Domeier said. During the first 18 months, the females followed an ambling path through the open ocean, he said.

 They then arrived in off Baja California to give birth to shark pups, putting themselves at risk of running into shipping traffic on their voyage along the shore, the study found.

"During the time the females are giving birth along the Baja Peninsula they are exposed to an array of commercial fishing activities that put them at risk," Domeier told OurAmazingPlanet in an email. "Of course, the baby white sharks are at even more risk since they spend the first years of their life in coastal waters and their small size makes them even more susceptible to capture." [Image Gallery: Great White Sharks]

Once the young sharks are born, the females return to Guadalupe Island to mate again.

The study found a high prevalence of bite marks on the sharks. Male sharks "bite the head, flank or pectoral fin of females during the mating ritual, but certainly these sharks are biting each other out of aggression as well," Domeier added. "Males may be battling it out for access to females or preferred hunting grounds."

While the females return to mate every two years, the males only return every other year. When they're not mating, both males and females may range as far afield as the waters off Hawaii, Domeier said.

The researchers tag the great white sharks by affixing the device to the tip of the animal's dorsal fin, during which time they are very close to the predators.

"It's surreal and humbling," Domeier said. "It is also stressful since the shark's life is in our hands during the short time it takes us to capture and tag each individual."

Email Douglas Main or follow him @Douglas_Main. Follow us @OAPlanet, Facebook or  Google+. Original article on LiveScience's OurAmazingPlanet.

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USGS

Populations of the American eel have massively declined in recent years.

By Douglas Main
LiveScience

Nobody knows exactly how American eels make it to the Sargasso Sea, a mysterious expanse of flotsam-ridden waters in the middle of the Atlantic Ocean where the animals breed.

But a recent study to chart the migration of these enigmatic eels only deepened the mystery, when six of the eight eels tracked with satellite tags were eaten by sharks.

Soon after the eels were tagged in Gulf of St. Lawrence, all eight devices were found floating on the surface of the water suggesting the animals had met an untimely end. The tags, which record depth and temperature, revealed that before surfacing the devices had suddenly entered an environment much warmer than the gulf's frigid waters.

Further analysis found that these conditions could be encountered only one way: inside the body of a porbeagle shark, according to a release from Dalhousie University in Halifax, Nova Scotia, Canada, where some of the study authors work.

The study, detailed in the online journal PLOS ONE, suggests that efforts to conserve eels, whose populations have massively declined in recent decades, could be confounded by predation by porbeagle sharks. These sharks were one of the species that were themselves voted to be protected under the Convention on International Trade in Endangered Species (CITES) earlier this week.  

"Both species are in trouble, and measures to conserve one may well be at odds with efforts to protect the other," said Julian Dodson, a researcher a Laval University in Quebec City and study author, in the statement. "What we really need now are studies to quantify just how important eels are in the diets of sharks and just what impact shark predation has on eel abundance."

Eels breed in the Sargasso Sea but return to freshwater streams as adults, making them vulnerable to pollution, urban development and the construction of dams.

"We could hope that there will be increased pressure to protect eels in fresh water, particularly during downstream migration through power dams," said study author Mélanie Beguer-Pon, also a Laval researcher, in the statement. "We can't do anything about shark predation, but we can limit mortality in turbines."

An additional 113 adult eelswere fitted with simpler acoustic tags, which can be detected by receivers moored in the ocean. The study found that only four of these eels made it out of the Gulf of St. Lawrence into the Atlantic Ocean, according to the release. Many of these eels were also likely eaten by porbeagle sharks, the statement said.

Email Douglas Main or follow him @Douglas_Main. Follow us @OAPlanet, Facebook or  Google+. Original article on LiveScience's OurAmazingPlanet.

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Ryan Kempster

Baby brown-banded bamboo sharks (Chiloscyllium punctatum) still developing within leathery egg cases can sense the electric fields of predators and freeze in place to avoid detection, researchers report online Wednesday in the journal PLoS ONE.

By Charles Choi
LiveScience

Baby sharks still developing within leathery egg cases can sense the electric fields of predators and freeze in place to avoid detection, researchers say.

These findings could help in developing more effective shark repellents, scientists said.

A number of species of sharks deposit embryos in rectangular capsules once whimsically called mermaid's purses or devil's purses. These egg cases often possess long tendrils at each corner that help anchor them to surfaces.

The shark parents of these embryos use highly sensitive receptors known as the ampullae of Lorenzini to detect the electric fields generated by the muscle contractions of potential prey. Now scientists find their embryos can similarly detect the electric fields of potential predators to help them escape being eaten.

The researchers focused on the brown-banded bamboo shark (Chiloscyllium punctatum). Their embryos spend up to five months inside egg cases, where they are vulnerable to attack from fish, marine mammals and even large mollusks.

The investigators discovered that even within their egg cases, the embryos could apparently detect electric fields in the lab created to mimic those of predators such as fish. Video recordings showed the developing shark babies responded by ceasing all movements of their gills and keeping their bodies perfectly still. [See Video and Images of Developing Bamboo Sharks]

Learning more about such behaviors may help researchers develop effective shark repellents, ones that generate electric fields that sharks keep away from, said researcher Ryan Kempster, a marine neuroecologist at the University of Western Australia.

"There are a variety of commercially available, nonlethal electric shark repellents, but the scientific data supporting their effectiveness is limited," Kempster told LiveScience. This line of research helps analyze what reactions different species of sharks have toward predatorlike electric fields and how these responses might or might not change over time.

Although shark attacks attract attention worldwide, humans are far more dangerous to sharks than sharks are to humans.

"As founder of the shark conservation group Support Our Sharks, a driving force behind my work is not only in producing a repellent to protect ocean users from potential attack, but also to protect sharks from being killed," Kempster said.

"In an attempt to make the ocean a safer place, governments in western Australia, Hawaii and France's Reunion Island have previously implemented pre-emptive killing of sharks. Given the crucial role that sharks play in ocean ecosystems, I believe it is much more appropriate to take advantage of nonlethal shark-control measures instead."

Shark numbers are decliningrapidly worldwide, mostly as a result of accidental catches in fishing nets. An electric shark repellant may also help reduce such catches "by keeping sharks away from fishing gear, to decrease the number of sharks unnecessarily killed each year," Kempster said.

The scientists detailed their findings online Wednesday in the journal PLOS ONE.

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