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 examined, and unparasitized H. melapterus in cages were exposed to gnathiids in the 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 dimidiatus, which consumes large numbers of gnathiids each day, sampling was carried out on reefs from which all cleaner-fish had been removed. Gnathiid abundance decreased during the day. Standardized abundance per wild-caught fish was 1.9 times higher at dawn than at sunset. Gnathiids successfully infested fish in cages. Sixty-one percent of the fish in cages were infested with ≥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 log₁₀ (abundance +1) of gnathiids per caged fish varied significantly 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 gnathiids on caged fish sampled at sunset was as high as that at dawn. The estimated mean (SE) cumulative number of gnathiids per caged fish [mean size = 13.7 cm (±0.25)] per day was 7.8 (1.1); this is similar to the estimated mean 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, suggesting that factors other than infestation behaviour may be responsible for the low numbers of gnathiids on wild-caught fish in the afternoon. Received: 19 August 1998 / Accepted: 1 June 1999