Monday May 29, 2017 Q&A

What do tiger shark embryos eat?


1 Other embryos in the womb

2 They are fed through their umbilical cord

3 They have direct access to their mother’s stomach

Question related image
Correct answer: 1. They eat other embryos in the womb. The two sections of a female tiger shark’s uterus look like gruesome battlefields when she’s pregnant. When the embryos are about a finger long, they mercilessly attack their siblings until only two are left, one in each section. The others are devoured. This unconventional feeding method, along with the lack of any “competitor” as they develop, enables the embryos to thrive: they can be up to a meter long at birth. That’s already nearly half as long as an adult female. And that means they have an excellent survival rate once born, as few predators dare to attack. Legend has it that this unique feature of tiger sharks was discovered in the 1980s when a US scientist was bitten by an embryo while dissecting the dead mother’s body. More recent research has tried to understand why the embryos developed this behavior. A group of scientists – whose research was published in Biology Letters in 2013 – wondered whether this form of cannibalism plays a role in genetic selection. Could the reason be to exclude all fathers except one? To find out, they analyzed the DNA of female tiger sharks inadvertently caught in fishing nets off the coast of South Africa, as well as the DNA of any embryos they found in the females’ uterus that died before the cannibalism process had finished. The scientists found that 60% of the females had mated with more than one male – and that 60% of the embryos ready to be born had the same father. So it seems that the genes of other fathers can be eliminated after fertilization. What’s more, it turns out that the female tiger sharks may be employing tactics. The researchers discovered that shark mating is a violent affair involving lots of aggressive biting; females would probably rather acquiesce than endure the continued copulation. But that may simply be a case of losing the battle to win the war – they mate first with the male of their choice, meaning that the resulting embryos mature sooner and begin their careers as cannibals before their ill-fated siblings, who are devoured. Although the embryos’ behavior seems cruel to us, in their defense, they appear to be acting merely out of hunger. Fertilized females migrate to warmer waters to help their babies develop. But the warm water makes them so lethargic that they forget to eat. So the embryos, deprived of fresh nutrients from their mother, eat whatever they can get their already sharp jaws on. Read the full article in Biology Letters