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Marine Ecology Progress Series

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MEPS 357:111-119 (2008)  -  DOI:

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

Rose Penfold1, Alexandra S. Grutter1, Armand M. Kuris2, Mark I. McCormick3, Conor M. Jones1,*

1School of Integrative Biology, The University of Queensland, Brisbane, Queensland 4072, Australia
2Department of Ecology, Evolution and Marine Biology and Marine Science Institute, University of California, Santa Barbara, California 93106, USA
3ARC Centre of Excellence for Coral Reef Studies, and School of Marine and Tropical Biology, James Cook University, Townsville, Queensland, 4811, Australia
*Corresponding author. Email:

ABSTRACT: 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.

KEY WORDS: Micropredator · Host–parasite interactions · Coral reef fish · Acanthochromis polyacanthus · Gnathia falcipenis · Gnathiid isopod · Mortality

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Cite this article as: Penfold R, Grutter AS, Kuris AM, McCormick MI, Jones CM (2008) Interactions between juvenile marine fish and gnathiid isopods: predation versus micropredation. Mar Ecol Prog Ser 357:111-119.

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