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Friday, May 25, 2001

Starfish have
amazing power
of regeneration

A reader writes, "I know that starfish have amazing powers of regeneration, but I have seen pictures of regenerated starfish and in some the regenerated arms are much smaller than the original arms. I read that regenerated arms never grow as big as the originals and my question is: Would the starfish's ability to move and eat be affected by this?"

My reader read wrong. Regenerated arms of most starfish grow as big as the originals. It just takes time, perhaps a year or more, before the reformation is complete.

During that time, a shortened limb might not be quite as efficient as a full length one, but starfish adapt. Some commonly have only one limb and do just fine.

The most famous of these one-armed creatures are known by their scientific name, Linckia (LIN-kee-a). This name was chosen in honor of J. H. Linck, an early naturalist who published an article on starfish in 1733.

Linckia starfish are common in Hawaii and other parts of the Pacific. Most textbooks discuss them in detail because, unlike other starfish, they can cast off an arm without any part of the central body attached and that arm will slowly grow into a complete starfish.

This new individual begins life with four tiny arms budding from the end of the cast-off stump. Because the shape resembles a shooting star, Linckia at this stage are called comets. Comets are a common sight on Hawaii's reefs.

Some starfish can replicate themselves from only one arm and about 1/5th of the central disk, the "body" of the creature, and others divide their central disk in two. Each half then regenerates its missing parts.

Starfish also reproduce the usual way, with sperm and eggs. Once a year, males shed their sperm and the females their eggs into the water. If they find one another, fertilization takes place and the resulting tiny creatures become part of the ocean's drifting animal life called zooplankton.

Eventually, the little stars settle on the ocean floor.

It can be hard to make heads or tails out of starfish because they don't have either. They do, however, have sides, a top side and a bottom side. At the center of the top is the creature's anus; the center of the bottom is an efficient mouth.

This voracious mouth is the reason we don't see usually starfish in aquariums: These creatures eat just about anything and can wreak havoc in a home aquarium.

One starfish species can locate buried prey, then dig down and catch it.

Others have tiny pincers that rise from their backs and grab hold of small fish, snails or crabs that have the bad luck to rest there.

Starfish also eat spiny sea urchins, sponges, anemones, clams and oysters.

To eat two-shelled animals, the star pulls the shells pulls apart with its arms, inserts its stomach into the opening and digests.

When you hold a starfish, it doesn't seem as if this thing could dig up, hold or pull apart anything. But when threatened starfish withdraw their suction-cup feet on their underside and stiffen their skin on the topside.

After you put the starfish down, it softens up and walks off. Some starfish walk into big trouble. The one called crown-of-thorns locates corals by smell, creeps over to them and starts eating. When these starfish are thriving, they can devastate coral reefs.

There's controversy about how to get rid of crown-of-thorns starfish during a bloom, or if we should even try. But there's one thing everyone agrees on: Never cut them up and leave the pieces in the water.

 

 


Marine biologist Susan Scott writes the newspaper column, "Ocean Watch",
for the Honolulu Star-Bulletin, www.starbulletin.com