Honolulu Star-Advertiser © Susan Scott
Friday, October 7, 2008
After my recent column about shark reproduction, several readers found a mistake in it.
I wrote that after a shark couple copulates, the female keeps her eggs inside her body until they hatch and then expels the pups into the ocean.
Most of the world’s shark species do that – but not all. Some lay eggs. And when males are absent, at least two species can produce pups without the assistance of sperm.
The world’s oceans host about 360 species of sharks. Of those 118, about 30 percent, lay eggs. Most egg-layers are classified in the following shark families: horn, carpet, blind, bamboo, zebra, nurse and cat sharks.
About 40 shark species grace Hawaii’s waters, and only one, the spongehead cat shark, lays eggs. If that name doesn’t ring a bell, don’t worry. Only two spongehead cat sharks have been found in the world, one near the Northwest Hawaiian Island of Laysan in 1904, the other in Indonesia in 1913. The Hawaii spongehead, about 20 inches long, is now preserved in the Smithsonian’s Museum of Natural History, along with two of the shark’s egg cases.
We shouldn’t expect to find any shark eggs on island beaches, but it’s worth looking in California’s kelp beds or along that state’s shores. Also, viable ones are sometimes on display at the Monterey Bay Aquarium.
Sharks eggs come tucked inside brown leathery pouches called mermaid’s purses. Some of these cases have corner tendrils for anchoring to rocks and coral. The eggs of horn sharks, a common California species, come in spiral cases.
Most sharks don’t take care of their offspring, but female horn sharks are an exception. These moms carry their egg cases to rock crevices and nudge them inside.
Embryos of egg-laying sharks get nourishment from their eggs’ yolks. When the food is gone, the pouch cracks open and the little sharks swim out.
Sharks giving live birth have evolved several methods of supplying sustenance to their internally-growing pups. One way is similar to the egg-laying sharks, except the females keep the thin-shelled eggs inside them while the embryos eat their egg yolks.
The closely related rays secrete a nutrient-rich uterine fluid that the babies absorb.
And then there’s the cannibal style. In thresher sharks, the first to hatch in each of the mother’s two oviducts eat the eggs that follow. Sand tiger sharks take this a step further, eating their smaller siblings as well as the mother’s eggs.
One advantage of intrauterine cannibalism is that the pups are born big. Thresher sharks are 4 to 5 feet long at birth, and sand tiger sharks are born 3 to 4 feet long. Also, at birth the youngsters are already experienced predators.
Researchers have recently learned that some female sharks can skip the fertilizing part of reproduction. A virgin bonnethead (a type of hammerhead) gave birth to one pup in 2001 in a Nebraska aquarium. In a Detroit facility in 2002, a white-spotted bamboo shark, whose tank mate was another female, laid eggs – and two hatched.
Other species may be able to do this too, but so far it’s only been confirmed in those two captive sharks.
In this column I try hard to get the facts right, and when I don’t, I really want to know about it. To the readers who e-mailed me: Thanks for sharing your knowledge. I’m happy to pass it on.