The role of vision in host-finding behaviour of the ectoparasite Gnathia falcipenis (Crustacea; Isopoda).

Gnathiids are common isopod crustaceans that feed on blood and plasma from a wide range of reef fishes. Although parasitic gnathiid isopods are active during both day and night on the Great Barrier Reef, little is known about their behaviour and ecology due to difficulty with species identification. Information about nocturnal gnathiids is especially lacking because of logistical constraints. This study shows that a gnathiid species captured at night, Gnathia falcipenis, has a larger eye diameter than that of a taxonomically undescribed sympatric diurnal species, Gnathia sp. A. When tested in a phototaxis experiment with this diurnal species, G. falcipenis moved towards a light source at much lower levels of light intensity than the diurnal species. Their willingness to feed in lab tests with live host fish was also strongly linked to light conditions. This gnathiid appears to have visual adaptations that have evolved for foraging under very low light levels, and is probably completely nocturnal. These results are discussed with reference to the availability of different host fishes during the day and night, the risk of predation that gnathiids may encounter during these periods, and the possibility of an unappreciated diversity of gnathiid taxa that may have a range of foraging strategies. [ABSTRACT FROM AUTHOR]