The amazing starfish

StoryImage( ‘/Images/Story//Auto-img-11339787557577.jpg’, ”, ‘A starfish begins the laborious task of opening the shells of an oyster.’);

Our tendency to give inappropriate names to various things has been extended into the Animal Kingdom. This accounts for the name starfish for an animal that is only distantly related to true fishes. The starfish (perhaps it is better to call it a seastar) is the most common and best-known of that great group of animals known to zoologists as the echinoderms, or spiny-skinned animals. Included in this group are such creatures as the sea urchins, sand dollars, sea cucumbers, sea lilies and others.

In some parts of the world, these spiny-skinned animals—such as the sea urchins and sea cucumbers—are relished at the table, but I know of no human society that routinely dines on starfish. I recall years ago snacking on sea urchins at a bistro in the Les Halles market in Paris, and I recall the taste was not unpleasant but unique.

During the day, starfish rest quietly on the bottom (all echinoderms are marine), but when night falls, they become active, moving about in search of food. Its athletic powers may be demonstrated easily by turning one over on its backside. It will bend its five arms so as to turn sort of a handspring, and will eventually right itself. One biologist claims to have taught a starfish to use one particular arm in righting itself, but I am inclined to believe this is just another “starfish story.”

Although many animals, including man, have devised ways to open the shells of oysters, clams, and scallops, few, if any, will attempt to do so by pulling the two shells apart. But a starfish does just that. Using the hundreds of tube feet on the underside that are part of what is called the water vascular system and is unique to all echinoderms, the starfish attacks the bivalve and begins the process of getting to the meat inside. The tube feet end in suction pads, and they keep up a steady and relentless pressure on the shells. By alternating groups of tube feet, a constant pressure can be maintained on the prey. Finally, the oyster or clam is worn down, and the fatigued muscles relax and the protective shells open.

The starfish then performs one of the neatest tricks in the natural world. It pushes its massive stomach out through its mouth, turning it wrongside out, and wraps it around the soft parts of the shellfish. The starfish remains in this awkward position until digestion is partially complete before it retrieves its stomach back into its body and moves on to the next victim. I have wondered what would happen if the bivalve was able to re-energize its muscles and clamp down on the intruder’s stomach. I have not read of this occurring, but I imagine it has.

One recent report, however, suggests that the starfish does not depend entirely on the suction of its tube feet to open the shells. There is evidence that it secretes a substance that partially paralyzes the shellfish, thereby facilitating the opening of the shells of a stubborn oyster or clam.

However the starfish manages to open the shells, there is no doubt that in some areas they are serious predators of oysters and clams. Oystermen and clammers learned long ago that starfish could be caught in large numbers by dragging string mops over an oyster or clam bed. The starfish catch onto a strand with pincher-like organs they have around their gills, and can be hauled out of the water. Here they would be set upon by the fishermen, and chopped into pieces and cast back into the water. It was learned that the joke was on the watermen and not on the starfish. These animals have tremendous powers of regeneration, and any piece of the body readily regenerates the missing parts and rapidly grows into a complete animal. Knowledgeable watermen today now dump the captured starfish into a caustic solution to kill them before tossing the carcasses overboard.

Though starfish and other echinderms may appear to be primitive animals, they are more evolutionarily advanced than any other of the invertebrate types. They possess a true body cavity (coelome) that few other invertebrates have. And, in their early embryology, a structure called the blastopore develops into the anal opening rather than the mouth as it does in other invertebrates.

As all animals with backbones (chordates) have a coelome and similar early development, most scientists believe the echinoderms are the ancestors of the first primitive animals with backbones.

From the Dec. 7-13, 2005, issue

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