| Juvenile gnathiid isopods are one of the commonest ectoparasites of coral reef fishes on the Great Barrier Reef, Australia, and are the main food item of cleaner fish that remove parasites from the bodies of other fishes. Little is known about gnathiid host specificity, foraging behaviour, or evolutionary ecology. My thesis addresses this gap by providing insight into the processes that drive diversification in these ectoparasites. The close association between parasites and their hosts often results in rapid evolutionary change, with the parasites becoming specialists on particular host species as part of an intense evolutionary arms race. I investigated host preference and specialisation in an undescribed gnathiid species that was allowed to choose among host fishes from three different families to feed on. This gnathiid species, which was isolated from a Iabrid host fish, showed a strong preference for labrid fishes, rarely feeding on pomacentrids or apogonids. Gnathiids that fed on labrids had higher survival than those that fed on apogonids, and males moulted to the adult stage more quickly. This suggested that host specialisation might be occurring between these ectoparasites and their host fishes at the host fish family level. I then constructed a phylogeny of gnathiid taxa with the genetic marker ITS2, which showed that there is, in fact, no match between gnathiid taxa and host fish taxa (at the species or family level). Thus, host preference and specialisation do not appear to have obvious evolutionary consequences for divergence in the parasites in this system. The phylogeny also revealed that nocturnal and diurnal gnathiids at one site were genealogically distinct from each other, although closely related. To investigate this further, I quantified morphological differences between individuals of the nocturnal and diurnal clades and found that nocturnal gnathiids have longer antennules and larger eyes, suggesting that they use different senses than diurnal gnathiids to locate hosts. Behavioural tests showed that both nocturnal and diurnal gnathiids use olfaction and vision to detect stimuli, but that nocturnal gnathiids use olfaction more often in dark conditions, whereas diurnal gnathiids used vision more effectively when there was some ambient light. In a separate experiment that measured phototaxis, I showed that nocturnal gnathiids were far more sensitive to low light levels than diurnal gnathiids. These findings are discussed with reference to predation pressure from cleaner fish and ecological speciation, and I conclude by suggesting that evolutionary divergence in gnathiids is influenced by natural selection acting on ecological traits such as predator avoidance and host detection. Finally, I describe a simple molecular technique for identifying hosts by sequencing bloodfeeding parasites. |
