Infestation dynamics of gnathiid isopod juveniles parasitic on the coral-reef fish Hemigymnus melapterus (Labridae)

The dynamics of infestation by parasitic juveniles of gnathiid isopods were investigated at Lizard Island in the summer of 1997/1998 to determine when , and at what rate, they infest fishes. Variation in gnathiid abundance on wild-caught fish (Hemigymnus melapterus) between dawn and sunset was examin ed, and unparasitized H. melapterus in cages were exposed to gnathiids in t he field for 4 h (8 h for fish sampled at 06:00 h) at five different times of the day and night (10:00, 14:00, 18:00, 22:00, 06:00 hrs) on three reefs . To control for any potential effect of the cleaner-fish Labroides dimidia tus, which consumes large numbers of gnathiids each day, sampling was carri ed out on reefs from which all cleaner-fish had been removed. Gnathiid abun dance decreased during the day. Standardized abundance per wild-caught fish was 1.9 times higher at dawn than at sunset. Gnathiids successfully infest ed fish in cages. Sixty-one percent of the fish in cages were infested with greater than or equal to 1 gnathiids, with 51% of the fish having between 1 and 4 gnathiids after 4 h exposure. Gnathiids infested fish both day and night. The log(10) (abundance +1) of gnathiids per caged fish varied signif icantly between time periods, with higher gnathiid numbers on fish sampled at 18:00, 22:00, and 06:00 hrs than on fish sampled at 10:00 and 14:00 hrs. In contrast to gnathiid abundance on wild-caught fish, the number of gnath iids on caged fish sampled at sunset was as high as that at dawn. The estim ated mean (SE) cumulative number of gnathiids per caged fish [mean size = 1 3.7 cm (+/-0.25)] per day was 7.8 (1.1); this is similar to the estimated m ean number of 7.3 gnathiids on similar-sized wild-caught fish at 6:00 hrs. The high infestation rate of gnathiids on caged fish in the late afternoon contrasted with the low numbers on wild-caught fish at this time, suggestin g that factors other than infestation behaviour may be responsible for the low numbers of gnathiids on wild-caught fish in the afternoon.