Some Pearlfishes from Guam, with Notes on Their Ecology

Abstract

From October, 1960, to July, 1961, 230 specimens of pearlfishes were collected on the fringing reefs of Guam. Four forms are represented and their distinguishing features are discussed. These specimens were taken from four species of holothurians and from the armless starfish, Culcita nouaguineae. Carapus mourlani was found only in Culcita; the other three occurred in two or more hosts. Thelenota ananas, Stichopus chloronotus, and an unidentified Holothuria contained only one species each, but Holothuria argus served as host of three pearlfish species. Sea cucumbers that produce adhesive threads as well as those that do not were utilized as hosts. Over 100 specimens of Holothuria atra were opened without finding a single pearlfish. Although this species has been reported as a host, it is probably not a preferred one. The most common species is Carapus homei, its usual host is Stichopus chloronotus. Tenuis larvae were collected from October through February. Repeated collections in the same area of Tumon Bay indicated that there was a decrease in the infestation rate after February. C. homei probably spawns in late summer, and the larvae assume the inquiline habit during the fall and winter months. Small samples from other parts of the island indicate that infestation rates vary with the locality. It is unusual to find more than one pearlfish in a single host, and the fish are not confined to the respiratory trees but are often found free in the body cavity. During the period when the tenuis larvae are present C. homei often feeds on the larvae of its own species, perhaps indicating that there is competition for hosts. This could account for the infrequent occurrence of more than one fish per host. Carapus homei also eats shrimp. Encheliophis gracilis, however, seems to feed on the gonads of its host. C. homei leaves the host at night and on four occasions was seen some distance from any probable hosts. The form called Carapus mourlani is structurally very similar to C. bomei but differs in having superficial melanophores. Since mourlani occurs only in Culcita and homei never does, there is a possibility that the observed differences are due to the effects of the host. Until this can be demonstrated experimentally it seems desirable to retain the name mourlani.