Lethal and sublethal impacts of a micropredator on post-settlement Caribbean reef fishes


The transition from a planktonic larval stage to a benthic or demersal juvenile stage, “recruitment”, is a crucial event in the life history of coral reef fishes, and has a strong influence on population size. Predation by piscivorous fishes is thought to be the main determinant of recruitment success, and has received the most attention. However, recent studies suggest that recently settled reef fishes are also an important target of micropredation from blood-feeding ectoparasites which may have significant lethal and sublethal effects. In this study, we quantified the relationship between levels of infestation by gnathiid isopods and mortality rates among juveniles of three species of reef fishes as a function of body mass both within and among species. We found that a single gnathiid isopod larva could kill fish of all three species shortly after settlement, up to 0.116 g [18 mm fork length (FL)] in French grunt (Haemulon flavolineatum), 0.027 g (15 mm FL) in masked goby (Coryphopterus personatus) and 0.01 g (9 mm FL) in beaugregory damselfish (Stegastes leucostictus). For juvenile S. leucostictus, we also compared the ability of fish to defend a territory when infested with a sublethal number of gnathiids versus uninfected individuals. Uninfected fish were significantly more likely to win-pairwise contests versus infected fish. These findings suggest that gnathiids can significantly impact juvenile coral reef fish survival, and affect population dynamics well past the settlement stage.

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We thank J. Jossart, J. Kisabeth, T. Santos, D. Ashaab, and M. Nicholson, for help with collecting specimens and maintenance of the gnathiid colony. We also thank P. Jobsis, R. Nemeth, and the staff of the University of the Virgin Islands’ McLean Marine Science Center. Funding was generously provided by U.S. National Science Foundation Grants OCE-121615 and OCE-1536794 (PC Sikkel, PI), and Puerto Rico Sea Grant (R-31-1-14, PC Sikkel, PI). We are grateful to the two anonymous reviewers who provided constructive comments for improvement of the manuscript. This is contribution #198 from the University of the Virgin Islands Center for Marine and Environmental Studies, and contribution number 265 from the NWU Water Research Group.

Author information

JCS collected specimens and performed experiments. PCS conceptualized and developed the project, and acquired all funding and permits. PCS, JCS, DMH designed the experiments. JCS, DMH, TLB performed data analyses, with input from PCS. JCS led the writing of the manuscript with significant input by PCS, DMH, TLB.

Correspondence to Paul C. Sikkel.

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Early post-settlement is a potentially vulnerable life stage for coral reef fishes. Gnathiid isopods are common ecotparasites of coral reef fishes that may kill their hosts, and may also be considered ‘micropredators’. Although they can be locally abundant, few studies have examined their potential role in the success of post-settlement fishes. This experimental study demonstrated lethal and sublethal impacts of as few as one gnathiid isopod on early post-settlement stages of coral reef fishes, suggesting a greater role in their host’s population dynamics than currently appreciated.

Communicated by Nicole Phillips.

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Sellers, J.C., Holstein, D.M., Botha, T.L. et al. Lethal and sublethal impacts of a micropredator on post-settlement Caribbean reef fishes. Oecologia 189, 293–305 (2019).

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  • Parasite
  • Competition
  • Gnathiid isopod
  • Mortality
  • Recruitment