Good Neighbors?
by Robert Rattner
A coral reef is like a crowded, bustling city where—for better or worse—residents often find themselves living in close quarters with each other. Relationships form between species, a phenomenon called symbiosis. Symbiotic relationships abound on a coral reef; the reefs’ very existence is due to a symbiotic relationship—that between hard coral polyps and zooxanthellae algae. As with people, some species help each other out, while others take advantage of their fellow reef residents. If both parties get something out of the relationship, it is called mutualism. When just one gains but does no harm to the other, this is commensalism. And where one benefits but causes harm to the other, the relationship is parasitic.
The red boring sponge (Cliona delitrix) presents both extremes of symbiosis. It is an opportunistic parasite that attaches itself to corals, then reproduces, expands, and kills the coral by dissolving away its skeleton. But the sponge also forms a mutualistic relationship with sponge zooanthids (Parazoanthus parasiticus), a type of colonial anemone that attaches to the red boring sponge but does not harm it. Colonial anemones feed on microscopic creatures carried past them in the water. As the red boring sponge feeds by drawing water through its tiny surface openings, the anemones take advantage of the provender in the passing currents. The anemones return the favor by providing protection for the sponge. They are toxic to some sponge-eaters, such as angelfishes (family Pomacanthidae) and filefishes (family Balistidae), and their very visible colors may contrast with that of the sponge, possibly serving as a warning to the fish. Anemone-covered sponges often are the best survivors.
Zoogoer 30(4) 2001. Copyright 2001 Friends of the National Zoo. All rights reserved.
