Ocean Watch
By Susan Scott
Starfish a great topic
for studentsI get a lot of e-mail about starfish. Often, the letters come from students who find my Web site and write something like this: "I am doing a project on starfish. Could you send me information on this topic? Thanks." Internet searches are fine, I advise these kids, but not without some textbook reading first.
While reviewing my starfish letters last week, I noticed that each writer used the term starfish when referring to these multiarmed creatures. This is noteworthy because not one of my textbooks or marine guides calls them starfish. From the oldest book, published in 1982, to the newest, published in 1998, each author refers to these creatures as sea stars.
For years now, biologists have been encouraging people to use the term "sea star" for starfish. The reasoning goes that these creatures aren't fish and, therefore, shouldn't have that word in their name. But when I use the term "sea star," most people say, "Do you mean starfish?"
The waters of animal names are muddy enough. For me, a starfish is a starfish.
In my stack of starfish letters, I also found a good question. For a research project, a student, Laura, chose starfish regeneration. Laura wrote that she knows a starfish can grow into a new individual from one torn-off or castoff arm, but she read that the new arms never grow as big as the original. She asks, "Would the starfish's ability to move and eat be affected by this?"
First, torn-off and castoff arms are two different things.
A starfish body consists of a central disk, the animal's control center, and five to 50 radiating arms. If part of a starfish's disk is injured or any of its arms are bitten or broken off, the creature can regenerate the lost parts.
Besides regrowing damaged parts, some kinds of starfish also routinely create entire new individuals by casting off an arm. These starfish, common in Hawaii, are called linckia (LINK-ee-a).
Linckia are notable because their castoff arms have no portion of the central disk attached, yet each arm still is able to grow into a complete starfish. Other starfish types need at least part of the disk attached to a castoff arm to make a new individual.
After dropping an arm, the linckia grows a new one in the empty space. The dropped arm grows a new central disk with a cluster of tiny arms radiating from it. At this stage these animals look like comets, and that's what they're called.
Because comets take a long time to achieve normal proportions, it seems that the new arms never grow as long as the original. But it's not so. We seldom see linckia with equal-length arms because these creatures are nearly always in some stage of regeneration. New starfish, however, eventually become symmetrical.
During a starfish's period of lopsidedness, it walks slower and eats less than full-grown adults. Still, linckia do just fine. They are the most common starfish in Hawaiian waters.
We host two species here. One is the green linckia, some of which are green and some of which are blue or brown. The other species is the spotted linckia, a gray or pink starfish that may or may not have red spots.
Whatever we call them, starfish are excellent choices for student projects and papers at every level. These seemingly simple symbols of the sea aren't simple at all.
Marine science writer Susan Scott can be reached at http://www.susanscott.net.
