Diurnal variation in the abundance of juvenile parasitic gnathiid isopods on coral reef fish: implications for parasite-cleaner fish interactions

The dynamics of parasitic gnathiid isopod infestation on the fish Henzigymn us melapterus were examined at Heron Island, Great Barrier Reef, by measuri ng the abundance and feeding state of gnathiids on fish collected between d awn and sunset and by estimating the time required for gnathiids to become engorged on host fluids. A model was developed to estimate gnathiid abundan ce on fish for any given time of day and host size. Fish at dawn had 2.4 ti mes as many gnathiids compared with fish at sunset, indicating that some gn athiids infest fish overnight. Most gnathiids had engorged guts (72-86%); t he proportion of empty guts and engorged guts did not differ in three time periods of collection( < 0800 h, 0800 to 1100 h, and > 1100 h). In the labo ratory, gnathiids fed quickly with 75% of gnathiids exposed to fish for 4 h having engorged guts. The short time required for gnathiids to become engo rged and the presence of gnathiids with empty guts throughout the day sugge sts that gnathiids also infest fish during the day. Thus gnathiids eaten by cleaner fish during the day may be replaced by other gnathiids during the day or night suggesting that interactions between gnathiids and cleaner fis h are highly dynamic.