Interactions between juvenile marine fish and gnathiid isopods: predation versus micropredation

Theory suggests that micropredators can be virulent and that they will impact smaller hosts more than larger ones. We examined the interactions between micropredatory gnathiid isopods and juvenile damselfish Acanthochromis polyacanthus, the only fish on the Great Barrier Reef without a pelagic larval stage. Compared to most other fishes, A. polyacanthus can potentially interact with reef-based micropredators much earlier in life. To determine whether gnathiid isopods feed on juvenile A. polyacanthus, 150 juvenile fish sub-sampled from 20 fish broods were surveyed for ectoparasites and micropredators. Gnathiids were associated with 5 A. polyacanthus broods with mean standard lengths (SL) between 4.2 and 21.1 mm. Gnathiids were also found attached to 5 individual A. polyacanthus juveniles <10 mm SL. To determine if infection is detrimental, and/or if juveniles eat gnathiids, we exposed juveniles from a range of sizes (7.2 to 23.5 mm SL) to an individual third stage gnathiid Gnathia falcipenis for 6 h. Gnathiids fed on 29% of fish and gnathiid feeding success was significantly reduced by time in captivity. In 99% of these infections, gnathiids were not eaten afterwards, indicating that micropredation and predation were mutually exclusive. In 40% of trials the fish ate the gnathiid before the gnathiid could feed on the fish, and the probability of gnathiids being eaten was significantly greater for larger fish. Gnathiids only caused mortality in fish <10 mm SL. These data indicate that larger juvenile A. polyacanthus were more likely to eat gnathiids, which preempted micropredation, and less likely to die after gnathiid infection than were smaller juveniles.