Behavioral and Ecological Relationships of a Parasite and Its Hosts within a Coral Reef System

Abstract

The life cycle of the digenetic trematode Plagioporus sp. includes an intermediate stage that encysts in the scleractinian coral Porites compressa and an adult stage that probably resides in a coral-feeding fish. Coral polyps infected with metacercariae of Plagioporus appear as swollen nodules ranging in color from bright pink to white and have lost their ability to retract into their calices. The polyps' altered appearance and behavior was thought to increase their vulnerability to predation. This study investigated the effect of parasite encystment on coral growth and the effect offish predation on both coral growth and on the parasites' rate of transmission. Parasitized P. compressa showed a 50% reduction in growth when compared to nonparasitized P. compressa. No significant differences were found in growth of corals kept in predator exclusion cages and that of corals left exposed to fish predation in either group, parasitized or nonparasitized. Uncaged parasitized P. compressa showed a marked reduction in number of parasitic cysts, with the infected polyps being replaced by healthy ones. The regeneration of healthy polyps suggests that parasite removal is beneficial to the coral, and the reduction in cyst number suggests that the parasites' rate of transmission was enhanced by exposure of infected corals to fish predation.