An experimental field test of susceptibility to ectoparasitic gnathiid isopods among Caribbean reef fishes

Susceptibility to infestation by a gnathiid isopod (Gnathia marleyi: Crustacea: Isopoda) was examined among 16 species from 9 f amilies and 3 orders of common Caribbean reef fishes off St. John, United States Virgin Islands. Fish were placed in cages during times of peak gnathiid activity. Individuals from most (n = 14) species were compared against a single species (French Grunt, Haemulon flavolineatum) that served as a standard and effectively controlled for the effects of habitat and variation in gnathiid abundance on exposure to and the likelihood and intensity of host infestation by gnathiids. All species were susceptible to infestation by gnathiids, with individual hosts harbouring up to 368 gnathiids. However, there was significant variation in levels of infestation among the 14 comparison species. Controlling for body size, nocturnal species from the families Haemulidae and Lutjanidae had the highest gnathiid infestation. Our finding that haemulids and lutjanids are particularly susceptible has important implications for the role of gnathiids in Caribbean reef food webs, given the role members of these families play in trophic connectivity between reefs and associated habitats. To our knowledge this is the first manipulative field study to examine variation among potential hosts in susceptibility to an ectoparasite in any terrestrial or aquatic system and is the greatest number of teleost hosts documented for any gnathiid species.