Starfish, Brittle Stars, and Sea Urchins
Starfish are perhaps the most familiar of the echinoderms, and many adapt well to aquarium life. Most people are quite surprised to realize that they are active, interesting predators, fully capable of exhibiting a wide variety of behaviors in the aquarium. Many are quite useful scavengers, but all are predatory in nature and will consume mollusks, coral polyps, and other sedentary invertebrates.
When a picturing a starfish, most people think of the simple reddish orange animal so often seen as a dried curio in beachfront shops. However, many starfish are fantastic in appearance and coloration. The redknobbed starfish typifies these — it has brick red dorsal spines, set off brilliantly against a white background. Native to the Indo-Pacific region, this perennial favorite reaches a length of 12 inches and is capable of consuming large mollusks. It is best fed by placing a piece of clam, scallop, or muscle directly below the animal, although it is quite active and capable of finding food on its own.
Although starfish are quite adept at sensing and finding food, they do, in general, respond slower than do most fish and therefore will be outcompeted unless care is taken to see that food is placed directly below each animal.
Brittle Stars bring the word “bizarre” to mind instantly, even to those well acquainted with the sea's curiosities. They react very quickly to the scent of food, and their long, slender arms thrash wildly about as they begin to explore. It is quite a sight to see a tank housing several of these normally sessile creatures suddenly come to life. The animals quickly move across the substrate, unerringly toward the source of the odor that has aroused them.
Brittle stars are harmless to most other creatures and are extremely valuable scavengers in that their very thin arms can get into the tiniest of crevices between coral heads and other places where bits of uneaten food might go unnoticed.
These spine-covered, slow-moving invertebrates are often collected in tide pools and are worldwide in distribution. The spines of all sea urchins are potent weapons, and many secrete venoms whose chemistries are not well studied. Hot-water baths seem to assist in alleviating the sting caused by most species.
Many unusual species are commercially available, including the longspined sea urchin (
Why do sea urchins seem to orient their spines toward a shadow that passes overhead?
Sea urchins respond to overhead shadows or objects as though they were predators and direct all the spines to face the possible threat. Although some fish are adept at clipping off the spines or turning the urchins over to expose the softer parts of the body, the defense is, in general, foolproof.
Despite their slow-moving ways, sea urchins are quite active and adept at getting in to all nooks and crannies of the aquarium. Be sure to check that they do not wedge themselves too tightly into small corners, or tumble backwards into coral and become stuck.