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ZOO-ology column: Starfish are not fish

By By Judie Farnsworth
Sept. 3, 2012 at 4:03 a.m.

Sea star showing tube feet.

Unique body shapes make what we call starfish one of our most recognizable forms of sea life.

But they're not fish. There are no gills, fins, scales, brains, blood or even a head.

They're a form of echinoderm sharing many traits of other echinoderms like sand dollars, sea cucumbers, sea urchins and thousands of species in the Phylum Echinodermata. Most of our familiar sea stars are further classified as asteroids (Asteroidea) or true stars.

Sea stars are found in the oceans of the world from intertidal areas to greater depths. They may be less than half an inch to more than four feet across. Body coverings are leathery or prickly and some are brilliantly colored.

Arms are the classic feature of sea stars. Five-armed varieties are most common, but there are some with many more. The sun star may have as many as 40 arms.

Arms grow symmetrically from a central disk, like spokes on a wheel. The mouth is on the underside of the disk. An eye spot at the tip of each arm senses mainly light and dark. Special cells on the skin send signals through nerve networks.

It has a five point radial symmetry. No right side, no left side, just a top and bottom. Sounds like a side-show announcement. There are usually five or a multiple of five body sections evenly situated around the central disk. If cut in half from any point, through the center, there would be two equal halves.

Sea stars have the ability to regenerate lost arms. Each arm contains all the material needed for re-growth. There are even species that can regenerate a whole new sea star from just a portion of a limb and part of the central disk. It may take up to a year, but how cool is that?

Sea stars, like most echinoderms, have a hydraulic vascular system to transport food, waste and allow movement. Sea water is pumped through a tiny sieve plate (madreporite) on top of the central disk. It flows into a vast network of tubes in the body that finally fill long grooves on the underside of the arms. At this point the tubes are closed at the tip and are called tube feet. There may be hundreds and some have suction ability. Muscles force water into the tube feet causing them to push outward. The sea star is now ready to roll. Various muscles control the feet and movement can be quite quick.

You've heard the expression "turn ones stomach," it's all in a day's work for a sea star, and they have two stomachs. They prey on bivalves (mussels, clams) as well as snails and some fish and are able to pry open shells with their tube feet. Even a tiny bit may be enough. A bag-like cardiac stomach slides through their mouth and into the shell where it surrounds and partially purees the prey. Then it slides back into the sea star where a pyloric stomach finishes the job. Very clever. The sea star can enjoy larger prey that wouldn't fit into its mouth.

Marine life is fantastically colorful and full of surprises - just like The Texas Zoo. Be sure to check our website texaszoo.org for upcoming programs and events.

Sources: marinelife.about.com - starfish.ch/reef/echinoderms.html en.wikipedia.org/wiki/Starfish - sheddaquarium.org/seastars.html

Judie Farnsworth is a longtime volunteer at the Texas Zoo specializing in educational programs.

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