A Mutualistic Interaction Between a Sessile Marine Clam and Its Epibionts

The jewel box clam, Chama pellucida, lives attached to shallow marine rock surfaces. The shell is normally covered by a dense growth of sessile plants and animals of diverse phyla. Removal of these epibionts seems to make detection and/or attack of Chama by the predatory starfish Pisaster giganteus more often successful in the laboratory and substantially increases Chama mortality through Pisaster in the field. Thus, Chama benefits from the presence of these attached organisms. Chama occurs commonly only in areas characterized by low mortality rate of sessile organisms. The organisms attached to Chama therefore enjoy freedom from the higher mortality rates which characterize some areas not inhabited by Chama such as sea urchin (Centrostephanus coronatus) foraging ranges. Thus, attached organisms benefit from their association with Chama. Accordingly, the interaction is mutualistic. Naturally rugose Chama shells placed in the field appear to accumulate certain epibiotic species faster than do artificially smooth shells. This result suggests that natural selection on one or both participants has favored mechanisms that facilitate their association. The interaction very likely affects the abundances of the participants. Also, by the resulting creation of the physical habitat of a Chama bed, the interaction probably affects the abundances of many other species in the community as well. Influences of mutualism in general on community composition and stability are discussed.