Evolutionary divergence in common marine ectoparasites Gnathia spp. (Isopoda: Gnathiidae) on the Great Barrier Reef: phylogeography, morphology, and behaviour

The blood‐feeding juvenile stages of gnathiid isopods are important ectoparasites of marine fishes on the Great Barrier Reef (GBR), and are a major component of the diet of cleaner fishes. We report here that these gnathiids have undergone evolutionary diversification, both geographically and temporally (into diurnally and nocturnally active taxa), which has been accompanied by changes in their morphology and behaviour. To perform this analysis, we sequenced a portion of the nuclear ribosomal ITS2 for 47 gnathiids collected from 29 host fishes of 11 species at three locales spanning 2000 km on the GBR. Maximum parsimony and Bayesian phylogenetic analyses both revealed four major clades. There was some degree of geographical structuring in these clades, but there was no evidence supporting host fish specialization, as gnathiids collected from the skin of different teleost taxa did not resolve into distinct clades. The topology of the phylogeny also implied some structuring that was dependent upon collection time (day or night), so we investigated whether there were also behavioural and morphological differences between taxa active at these different times. Nocturnal gnathiids had significantly longer antennules and larger eyes than diurnal gnathiids – two traits presumably adaptive for nocturnal activity. Behavioural tests showed that both nocturnal and diurnal gnathiids use olfaction and vision while foraging, but that nocturnal gnathiids used olfaction more often in dark conditions, and that they were able to perceive movement under extremely low levels of light. Diurnal gnathiids used vision more effectively when there was some ambient light. Our results thus suggest that both phenotypic and genotypic divergence in gnathiids may be influenced by natural selection acting on ecological traits, such as predator avoidance and host detection. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94 , 569–587.