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Monday, Dec. 18, 2000

NATURAL SELECTIONS

Sea cucumbers: radially different


Sea cucumbers.

Earthworms of the sea, delicacies of the kitchen, these sedentary marine animals have a remarkable property: the ability to change their tissue structure from fluid to solid, and back again.

For bio-engineers a material with this property is like the Holy Grail, and could lead to innovations such as artificial muscles.

Sea cucumbers are echinoderms, a phylum that includes sea urchins and starfish. Echinoderms have a five-way radial symmetry, a pattern most familiar in the shape of the starfish. In sea cucumbers this character is shown in the five strips of muscle inside the body wall, running from the front to the back. Incidentally, it is these strips of muscle that are, in the West, eaten fried in butter. Known in the trade as trepang, or be^che-de-mer, in East Asia the whole body is dried and used to make soups, or used in tonics and sometimes in Chinese medicine. In Taiwan they are mainly used in banquets. In Japan, naturally, sea cucumbers are eaten raw, where they are known as namako.

Like earthworms in terrestrial systems, sea cucumbers can comprise up to 90 percent of the animal biomass in marine systems, feeding on detritus and recycling the sea bed as earthworms recycle the soil on land. They range in size from 2 cm to 2 meters, and crawl around slowly on many small sucker-tube feet. The mouth is surrounded by a wreath of 10 to 20 retractable tentacles, which hoover up mud, sand and passing plankton. "Processed" sand is passed out through the body.

Little is known about the ecology of these distinctly unglamorous organisms, but scientists in the Galapagos are worried that severe over-fishing of sea cucumbers may have dangerous knock-on consequences for the rest of the marine ecosystem. Fishing of sea cucumbers in the Galapagos started only in 1988, and after the Ecuador government banned their harvesting in 1992, the pressure on the Galapagos populations increased through poaching.

A study last year by TRAFFIC, the World Wildlife Fund's wildlife-trade monitoring body, found that some 4 million sea cucumbers have been exported from Ecuador since 1992, when the ban was imposed following the exhaustion of the supply off mainland Ecuador.

"Continued illegal fishing is posing a threat to local sea cucumber populations and threatening to affect the unique ecosystem of the Galapagos Islands," says Teresa Mulliken, TRAFFIC Research and Network Development Manager and coauthor of the report. "It's vital that the government of Ecuador bring the fishery and trade under more effective control and for consumer countries and others to provide assistance where they can."

Exploitation by poachers, lured by the demand from the lucrative Asian market, has driven one species native to the Galapagos onto the endangered list. The report named Taiwan and Hong Kong as the most important markets, where the endangered species, Isotichopus fuscus, is prized for its spiky body.

"Action is required not only in Ecuador but also in other Latin American countries where this species is or may be fished," said Martin Jenkins, coauthor of the study. "This could prevent the boom-and-bust pattern already demonstrated on Ecuador's coast and typical of many sea cucumber fisheries."

Though not enough is known about the key ecological role that sea cucumbers play in recycling the seabed, scientists are getting very excited about their unique physiological properties.

One character that echinoderms share is a calcareous skeleton: echinoderm means "spiny skin." The hard spiky body of sea urchins is their most obvious trait, but what is less obvious is that the spines of urchins -- and the entire bodies of sea cucumbers -- may be made alternately rigid and fluid.

It is this ability which interested Greg Szulgit and Robert Shadwick of the Scripps Institution of Oceanography in California. Echinoderms have a compound in their tissue called collagen, which can change, under neurological control, from "liquid" form to "solid" form and back again. This ability allows sea cucumbers to in effect liquefy their bodies and pour themselves into a crack in a rock, then wedge themselves in by solidifying their tissue, to prevent a predator from pulling them out.

Other responses to stress include ejecting water (see photograph) or more drastically, by ejecting their internal organs (which are later regenerated).

Writing in the Journal of Experimental Biology, Szulgit and Shadwick report that the change from elastic liquid form to stiff solid form is caused by the changes in the linkage of long collagenous fibers which run through the tissue.

Once the researchers work out exactly how the collagen fibers link up, they will be a step closer to isolating them and making a material with the same ability. Since the change is under neurological (that is, electric) control, the outlook is promising for a material to be made which will change its property in response to an electric current.

That material may be a future artificial muscle.



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