Variation in emergence of parasitic and predatory isopods among habitats at Lizard Island, Great Barrier Reef

Gnathiid isopods are one of the most abundant groups of ectoparasites on coral reef fishes. They, and other isopods, have been shown to significantly affect the health and behaviour of many reef fish. Whether isopod emergence differs among habitats on coral reefs is not known. In this study, we measured emergence rates of parasitic isopods (Gnathiidea and Flabellifera) in six habitats at two sites at Lizard Island during new moon periods in March and December 2004. Isopods were collected from the periphery and centres of micro-reefs, patch reefs, continuous reefs, and from inter-reefal habitats (sand or rubble) with 1 m² emergence traps. Sites (Casuarina and Coconut Beach) were located on opposite sides of Lizard Island. Live gnathiids were collected with light traps in November 2005 to investigate species differences between sites. At both sites, the most abundant gnathiid species was exclusive to that site. More gnathiid larvae emerged at night, and emergence of fed gnathiids (pranizae) and flabelliferan isopods was almost exclusively nocturnal. Diurnal emergence was greater at Coconut Beach than Casuarina Beach. Although emergence counts were not consistently affected by parameters such as habitat, site, or sampling period, gnathiid size and feeding state were. Where significant differences existed, gnathiids were larger and more often fed over reef borders than centrally. We suggest first stage larvae (Z1) have the largest influence on total abundance and are patchily distributed in accordance with adults from which they have recently hatched. As later stage larvae depend on fish, more successful (fed) and older larvae are found on the edges of reefs where appropriate hosts may be more abundant, or predation is lower. Gnathiids were over-dispersed in all habitats investigated, including apparently homogeneous beds of coral rubble and sand. This indicates that their distributions may be better predicted by very fine scale differences in substrate or that aggregations are the result of gregariousness and may be difficult to predict on the basis of substrate. Emergence traps collected comparatively few parasitic flabelliferan isopods. This community differed greatly from the previously described community of scavenging isopods at Lizard Island. These differences are probably the result of differences in trapping methodology.