Micropredation by gnathiid isopods on settlement-stage reef fish in the eastern Caribbean Sea

The transition from a planktonic larval stage to a benthic or demersal juvenile stage is a crucial event in the life history of coral reef fishes, and recruitment success has a strong influence on reef-fish population size. Post-settlement predation is thought to limit recruitment success. Most studies on post-settlement predation have focused on piscivorous reef fishes. However, recent studies in the tropical Pacific Ocean suggest that blood-feeding ectoparasites may also be an important source of predation. Here we provide further evidence that a gnathiid ectoparasite is capable of such micropredation on settlement-stage fish in the eastern Caribbean Sea. On 12 occasions in May-August, 2014 and 2015, during plankton light-trap recovery, we observed post-settlement fishes from the families Apogonidae, Bothidae, Gobiidae, Labrisomidae, and Tripterygiidae with attached ectoparasitic juvenile gnathiid isopods. In one laboratory experiment in which 10 settlement-stage French grunt, Haemulon flavolineatum (Desmarest, 1823), were fed upon by an average of one gnathiid per fish, all fish died compared to none in the control group. In a subsequent experiment in which individual settlement-stage French grunt were exposed to 0-3 gnathiids each, all 50 fish that were fed upon by at least one gnathiid died, compared with only one of 78 that were not fed on by any gnathiids. Combined with observations on damselfishes from the Pacific Ocean, these findings suggest that gnathiids are capable of micropredation on larval fish at time of reef settlement and that micropredation should be further evaluated as a factor affecting success of settlement and thus recruitment success of coral reef fishes.