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MERMAID'S PURSE |
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Open an oyster and, some times, you may find a pearl. Look inside a seashell, and you could find a hermit crab. But you won’t find keys, a wallet, or even a cell phone if you open up a mermaid’s purse!
These packages contain a very important item – a developing embryo. Produced by most species of sharks, skates, and rays, these egg cases surround and protect fertilized eggs. This method of reproduction, called oviparity, allows embryonic development to occur outside the female’s body, after internal fertilization. The egg case forms around the embryo inside the reproductive tract. A specialized gland secretes collagen and meshes together layers of proteins, forming a strong, leathery casing. The shape, size, and color of the purses vary between the species. The largest recorded egg case in the world was produce by a whale shark, and measured 12 inches by 3.5 inches by 5.5 inches! Just as each individual species is unique from the others, so are the egg cases produced by them.
The biggest difference between shark egg cases and those belonging to skates and rays is whether they have “horns” or “tendrils”. Horns can be seen on the purses of skates and rays, and differ in length and width. Four tendrils can be found in each of the corners of a shark’s purse, which help to keep the purse anchored in place. This is important to a developing shark embryo. In order for the embryo to survive, oxygen must be able to enter the egg case. In shark species, this is controlled by the differences in water pressure, in and out of the egg case. Therefore, female sharks need to be picky about where they place their developing eggs. Skate and ray embryos, on the other hand, are able to physically pump water in and out of the egg case. By wiggling their tails, clean, oxygen-rich water can be pumped into the egg case through holes in the horns. In this same way, carbon dioxide and other wastes can be expelled from the egg case.
Gestation is anytime between nine months and two years, usually longer for eggs laid in deep, cold water. When the young shark, ray, or skate is ready, it will propel itself out of one end of the egg case headfirst. The empty egg cases often are found washed up on shore or caught in fishing nets. By collecting these empty purses, researchers are able to determine which species it was produced by. It also helps to identify new species.
The behavior and distribution of different species can be studied as well by tracking where the cases were gathered, and in what type of habitat the females chose to deposit their eggs. So the next time you walk along the beach, keep your eye out for one of these unique treasures from the ocean. Use your knowledge of mermaid’s purses to identify the owner of these packages, because you won’t find an ID when you look inside!
Researched and written by Autumn Rajczak, Aquarist
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