Why Shark Embryos Eat Each Other Up in Utero
Sept. 5, 2012
-- Five rays and sharks are receiving priority attention at this week's IUCN World Conservation Congress held in Jeju, Republic of Korea. The congress is the world's largest conservation event. One of the five sharks is the porbeagle, shown here. Porbeagles are vulnerable globally, and are classified as critically endangered in the Northeast Atlantic. Their low reproductive capacity combined with over-fishing has led to severe population declines over several parts of its range. "Sharks and rays have traveled the Earth for more than 400 million years," said Dr. Cristián Samper, president and CEO of the Wildlife Conservation Society and keynote speaker at the Jeju congress. "Yet, in only recent decades, many of these species have become threatened from overfishing and, in some instances, have disappeared entirely from major portions of their range." He added, "The potential loss of one of only two groups of the world's living fishes is a crisis the world community must take decisive action to address. We are calling for governments around the world to vigorously support CITES international trade regulations and strengthen fisheries management and protection measures for shark and ray species. We cannot continue to allow the destruction of these wonders of evolution."
Manta Rays Fate Worse Than Sharks
The oceanic whitetip shark is critically endangered in the Northwest and Central Atlantic Ocean. It's frequently sought after for its fins, used to make shark fin soup. Usually the rest of the dead or dying shark is then tossed back into the sea. Andrew Brierley, a University of St. Andrews marine ecologist, told Discovery News that when sharks like this die off, the deaths can cause a domino effect of other ecosystem losses. Shellfish, for example, may go into decline because they are consumed more by predators that the sharks would normally prey upon. "The trophic cascade brought about by the increasing demand for shark fin soup has not only left once economically valuable bivalve fisheries in crisis, but has precipitated an ecological and culinary bankruptcy," explained Brierley.
Scalloped hammerhead sharks are endangered all around the globe. This tropical species forms large migratory schools at certain times of the year. The aggregating behavior, distinct from that of other sharks, makes the shark particularly vulnerable to fishing. "We estimate that many millions of sharks are killed annually through both legal and illegal, unregulated and unreported fishing for the trade in fins, the prime ingredient in shark fin soup," said Dr. Rachel Graham, director of WCS's Gulf and Caribbean Sharks and Rays Program. "The high price for fins has caused the global shark fishery to expand far beyond what is sustainable. The need for international regulation and enforcement has never been greater."
Shark Paradise Found
Many rays, such as these, are also in peril. The Wildlife Conservation Society and over 35 government agencies and NGO partners at the congress additionally highlighted population loss problems associated with so-called "devil rays," which feed on planktonic crustaceans. Small schooling fishes become trapped on their specialized gill rakers. Rays are frequently captured in target fisheries and as bycatch across much of their range. One reason is that the gill rakers are dried and exported for the Asian medicinal market.
Dr. Dwayne Meadows, NOAA/NMFS/OPR
The reef manta ray, shown here, is among the world's largest fishes. Both it and the giant manta ray can grow several feet across. They are slow growing, however, typically giving birth to only one pup every two to three years. They are migratory and occur in small, highly fragmented populations that are sparsely distributed across the world’s tropics. Manta rays are captured in targeted fisheries and incidentally as bycatch. In addition, manta rays are used for human consumption, shark bait, and -- as for devil rays -- are increasingly sought for their gill rakers. "The international trade in shark and ray products, including fins, meat, and other body parts, is driving shark and ray fisheries around the world, and most of these are unmanaged or only minimally managed," said Dr. John Robinson, WCS's executive vice president for Conservation and Science. "Lack of controls on fisheries and international trade puts species at risk, but also jeopardizes sustainable fisheries, ecosystems, and food security." The Convention on International Trade in Endangered Species of Wild Fauna and Flora will meet in Thailand in March 2013.
PHOTOS:Shark Teeth Weapons Reveal Surprises
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 today (April 30) 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.
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.)
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.
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.
Image Gallery: Great White Sharks
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On the Brink: A Gallery of Wild Sharks
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