Feeding ecology of the fish ectoparasite Gnathia sp. (Crustacea: Isopoda) from the Great Barrier Reef, and its implications for fish cleaning behaviour

The feeding rate of a parasitic gnathiid isopod on fish was examined. Individual fish, Hemigymnus melapterus, were exposed to gnathiid larvae and sampled after 5, 10, 30, 60, and 240 min. I recorded whether larvae had an engorged gut, an engorged gut containing red material, or had dropped off the fish after having completed engorgement; variation among sampling times and larval stages was analyzed using generalized linear mixed model analyses. The likelihood that larvae had an engorged gut increased with time and varied with larval stage. First stage (<0.9 mm) and second stage (0.9 to 1.45 mm) larvae became engorged more quickly than third stage (>1.45 mm) larvae. After 30 min, however, most (>93%) larvae had an engorged gut regardless of their larval stage. The likelihood of red material in the gut of third stage larvae increased over time (46% after 30 min, 70% after 60 min, and 86% after 240 min) while that of first and second stage larvae remained relatively low (<27%) at all times. First and second stage larvae left the fish at a higher rate (approximately 23% after 30 min and 81% after 60 min) than third stage larvae (3% after 30 min and 26% after 60 min). After 60 min, however, the likelihood that larvae dropped off fish did not change. The variation in feeding behaviour with larval stage is likely due to variation in their diet and gut volume. The rapid feeding rate of gnathiid larvae suggests that their turnover rate on fish is high. This may explain why some fish species seek cleaner fish, which eat many gnathiid larvae, so often. By repeatedly visiting cleaner fish, gnathiids may also be removed from hosts before the parasites become fully engorged.