Coral - January/February 2010

stressed, Sea Apples can eviscerate themselves, taking down an entire reef tank with the toxins they release— although such events are not as common as feared.

Detritivore cucumbers, such as the many Holothuria spp., tend to outgrow most marine aquariums and eat up all of the available detritus rather quickly. Surface-mopping cucumbers use feeding tentacles to mop surface bacteria, organic debris, and film from rocks and sand, while others ingest particles of fine substrate and pass them whole through their gut, digesting the surface film. Detritus feeders must feed almost constantly, as the nutritional quality of the food tends to be low. For this reason, cucumber numbers must be limited in an aquarium and they should not be added until the tank is well established and well colonized by bacteria.

Pacific pencil urchins (Eucidarius spp.), are largely carnivorous, with sponges dominating the diet.

ECHINOIDS: BALLS OF SPINES

Sea urchins may seem lifeless at first glance: a ball of spines stuck to a rock or sitting on the sand. But their unique design and often hardy nature has made them a mainstay in dealer tanks. Hugely important in marine ecosystem maintenance, sea urchins are often the primary herbivores controlling overgrowth of algae. Most species available in the aquarium trade feed on filamentous algae, macroalgae, and algal and bacterial films. Some, such as the Blue Tuxedo Urchin (Mespilia globulus), prefer coralline algae, and a few, such as the Atlantic and

OPHIUROIDS: MARATHON SEA STARS Brittle and serpent stars are fantastic additions to the home aquarium and are among the few marine invertebrates in which the presence of food stimuli seems to arouse quick curiosity and fascination. Watch a shrimp pellet slowly wobble to the floor of a seemingly barren tank. Before the pellet hits the deck, long arms and bodies start writhing, and the bottom of the tank comes to life as brittle stars hiding under rocks recognize the pellets as food. These charismatic spiny skins are quite popular in the trade and, fortunately for aquarists, most of them are durable display animals. As generalist feeders, the majority of brittle stars will accept a wide range of prey items and most common aquarium fare. Several species have been known to consume fish, such as gobies and sleeping damsels, as well as crustacean tankmates.

MORE THAN UTILITY

Though their bold colors, unusual shapes, and interesting behaviors draw the eyes of curious aquarists, most echinoderms have special water quality and dietary needs that are rarely met in the average system. As a whole, the group is sometimes believed to perform an important biological role in captive systems by removing organic wastes and contributing to the long-term success of the system. Many species of sea stars, brittle stars, and sea cucumbers are excellent additions to large, mature marine systems, since their dietary requirements of organic detritus are easily met within sand beds and aquarium dead zones. More often, however, echinoderms must be supplied with species-specific environmental and dietary conditions in order to thrive for long. They should not be viewed simply as tank janitors feeding on leftovers.

Echinoderms are a distinct group that has evolved innumerable novel traits that allow them to thrive in the world’s oceans. From the “catch collagen,” the compound that allows an echinoderm to turn its body from soup to a rigid and immovable object at will, to the water-powered hydraulics of the water vascular system, echinoderms are incredibly interesting. Aquarists stand to learn volumes about these organisms by keeping them in captivity.