The Mystery of the Mermaid's Purse - egg case of the hedgehog skate - Brief Article

The egg case of the hedgehog skate provides more than a safe environment.

The ocean washes some pretty strange-looking things ashore from time to time--the dagger tail of a horseshoe crab, the gray aspic of a jellyfish bell, the snapped plunger of a hospital syringe. Among the strangest is a slim leathery pod a few inches long, tipped with four graceful horns. If you don't already know what it is, staring at it for hours may bring you no answer. Mutant chili pepper? Failed art-school project?

It is actually a package for a fish egg. The fish in question is a skate, a relative of rays and sharks. These flattened fish--with their large pectoral "wings" that meet at the front of the head--live in ocean waters worldwide, feeding on bottom-dwelling mollusks, crustaceans, and shrimp. They mate and lay their peculiar egg capsules, which are sometimes called mermaids' (or sailors') purses, on the seafloor. There are well over two hundred species of skate, and in most, the female produces the egg capsules in pairs, with a single embryo developing inside each one. After anywhere from several months to two years, the young skate breaks open a weak seam between two of the horns and swims free of the capsule, which may then eventually wash ashore.

Compared with the spherical jelly-coated eggs laid by most fishes, a skate's horned capsule may seem like pure extravagance. But recent research by Thomas Koob, of the Shriners Hospital in Tampa, Florida; John Long, of Vassar College; and Adam Summers, of the University of Massachusetts, has shown that the capsule--like many beautiful things in nature--is an exquisite piece of engineering, one on which the skate's survival depends.

Koob and his colleagues have been studying the little, or hedgehog, skate (Raja erinacea), a species that lives off the Atlantic coast of North America. A mother little skate creates a capsule inside its body by producing collagenlike fibers from a row of glands inside her reproductive tract. Koob has found that the fibers are arranged into thirty-five distinct layers, all held together by a gluelike substance. All the layers are extruded at the same time. the fibers in one layer oriented at angles different from those in neighboring layers. That the fibers are at different angles makes the capsule tough and resilient in the same way plywood is--able to resist forces in any direction.

The tough capsule helps protect the developing skate from predators but also poses a dilemma: it makes breathing difficult. All fish embryos need a steady supply of oxygen to grow. The gas diffuses easily through the loose molecular structure of the jelly that coats a typical fish egg but hardly any slips through the tight weave of a skate's capsule.

Skates don't suffocate, because their egg capsules are actually little water pumps that take advantage of Bernoulli's principle, which decrees that the faster a fluid flows, the lower its pressure, and the slower it flows, the higher its pressure. Fluids at high pressure naturally move toward areas of low pressure, which, in the case of the skate, means passing through the capsule via slits in the outer edge of each of its horns.

In the turbulent flow of the ocean, water around the four corners of the pod moves at different speeds--and therefore at different pressures. Water at high pressure enters the capsule through one slit and, moving toward a part of the pod surrounded by water at lower pressure, is essentially Sucked back out. This internal current fills the capsule with a new supply of oxygen-rich water about every half hour.

It's a nice trick, but biologists have discovered that horns aren't necessary for pulling it off. Some shark species also lay a leathery egg capsule, but theirs is rectangular and hornless. Slits run along its sides and flush it clean every five minutes. Who needs horns then? Skate embryos do, because, as it turns out, they supplement the passive pump action of their egg capsules with active pumping of their own. The embryo has a string of muscle at the end of its tail, which disappears when it hatches. About a third of the way through its development, an embryo snakes this appendage into one of the capsule's horns and starts beating it furiously--about forty times per minute in one-minute bursts, with only a few moments' rest in between. As the tail undulates, it creates waves that travel down the length of the horn. Water trapped in the curves of the oscillating tail is carried down the waves--as if on a conveyor belt--and pushed out through the slit. The flow of water out one slit draws more water into the pod through the others to replace it. A tail-beating skate embryo can bring about a hundred times more water--and its valuable oxygen--through the capsule than the slits can with their passive pumping.

As the months go by, an embryo squirms around inside its capsule and slips its tail into different horns, beating it all the while. It's a huge effort, raising the skate's metabolism 40 percent above its resting rate. Why work so hard when the shark solution is so easy? Shark embryos may not have to beat their tails, but they can survive only where there is a steady current of water. Shark mothers typically take great care to place their egg capsules up off the seafloor, attaching them to seaweed, coral, and other objects. Skate embryos have to thrash madly inside their eggs, but by making their own flow, they can survive even in slow-moving water. Adult females appear to take advantage of this independence by laying their eggs over huge swaths of the sea: biologists have found the capsules in the shallow waters of estuaries and at midocean depths greater than three thousand feet. Their horns don't simply give skates a certain beauty; they may give them the world.