Susceptibility to infestation by a gnathiid isopod (Gnathia marleyi: Crustacea: Isopoda) was examined among 16 species from 9 families 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.

Amblyomma hebraeum differs from many ixodid tick species in that it is not susceptible to the direct effects of the host immune response. All stages of the life cycle are able to feed repeatedly on laboratory hosts and show no progressive declines in engorged weight. However, on cattle there are declines in the yield of engorged adult females over time, as animals become sensitized to the tick and grooming increases. In the field zebu, Bos indicus and sanga, B. indicus x B. taurus breeds of cattle become less heavily infested than B. taurus breeds. The apparent resistance of zebu and sanga breeds is the result of learned tick avoidance behaviour, skin sensitivity and the ability to dislodge ticks prior to attachment as well as grooming ability. Among other Amblyomma spp., the feeding of A. americanum is adversely affected by acquired host immunity, while that of A. variegatum is generally unaffected.