The pectinid scallop Pedum spondyloideum occurs almost exclusively on massive coral colonies of the genus Porites. These scallops have reduced the effects of recent heavy levels of predation by the starfish Acanthaster planci on their coral hosts on the Great Barrier Reef. Scallops repel foraging starfish and other echinoderms on contact by repeated expulsion of jets of water. In coral populations subject to heavy predation pressure, this paramutualistic behavlour has resulted in significantly greater survival of host Porites in comparison with congeners without scallops. The survival of corals protected by scallop populations may be important in enhancing recovery and preserving the population structures of these long-lived species. The scallop's expellent water jets function primarily in the removal of wastes. Pedum spondyloideum's secondary application of the jets against A. planci and other foraging organisms may thus exemplify an exapted means of host defence. (DeVantier and Endean, 1988.)
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