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Answer to the Friday Whatsit for April 1, 2011

April 4, 2011

Shark egg cases

Congratulations to Uncle Michael, who guessed his first Whatsit (on Facebook) this week.  These are shark egg cases, sometimes called mermaid’s purses. Certain shark species, as well as their cousins the skates and rays, lay eggs inside these protective cases.  Empty egg cases often wash up on beaches and are found dried out and leathery from the sun.

I snapped this photo during a visit to Chicago’s Shedd Aquarium, where we learned that shark pups can develop in one of three ways, depending on the species.  These egg cases came from a species that was oviparous, or egg-laying (ovi denoting “egg”).  The egg is fertilized inside the mother’s body, and then the embryo and its food source, the yolk, are encapsulated in the egg case in the nidamental gland and then laid.


A shark at the Monterey Bay Aquarium

Other shark species keep their fertilized eggs in the mother’s body until they hatch, giving birth to live young.  These types of sharks are called viviparous sharks (vivi meaning “live”).  These species can be further divided into sharks that use the yolk as the primary food source for the growing embryo and those that make a placenta to provide food directly from mother to young.  But no matter how they develop, as soon as the pups are born they are on their own.  Unlike marine mammals like dolphins and whales, shark mothers provide little care for their young beyond development.

So there you have it, shark egg cases being raised at an aquarium.


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