The role of corals on the abundance of a fish ectoparasite in the Great Barrier Reef


Gnathiid isopods, common fish ectoparasites, can affect fish physiology, behaviour and survival. Gnathiid juveniles emerge from the benthos to feed on fish blood. In the Caribbean, gnathiids are positively associated with dead coral and negatively associated with live coral, due to coral predation on gnathiids. However, such interactions were unstudied in the Great Barrier Reef (GBR). Due to recent extreme weather events (two cyclones and one mass warm-water coral bleaching event, 2014–2016), it is now urgent to understand the role of corals on the abundance of these ectoparasites. Here, to understand parasite–coral dynamics at the micro-habitat level, we examined substrate associations of gnathiid isopods on Lizard Island (GBR) using demersal plankton emergence traps. Additionally, we determined whether two abundant hard coral species, Goniopora lobata and Pocillopora damicornis, predate on gnathiids in a laboratory experiment using containers with gnathiids and fragments from each coral species or dead coral as controls. The abundance of gnathiids over natural substrates was higher for dead compared to live hard coral and sand, but not live soft coral. Moreover, we found that free-swimming gnathiids decreased in containers with live coral compared to dead coral controls. This was attributed to predation as we also directly observed a coral ingesting a gnathiid. Our results suggest that dead coral is a suitable microhabitat for gnathiids, but that live coral is not since live corals can predate on gnathiids. We propose that following extreme events, such as cyclones and heat waves, gnathiids might benefit from more dead coral substrate and a decrease in predation by the reduction in coral cover on the reef. We advocate that an increase in the frequency of extreme events may have cascading effects for the fish population through changes in the population of benthos-dependent ectoparasites.

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Data availability

The datasets generated and analysed during this study are available in the Figshare repository: (Paula et al. 2021).


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The authors acknowledge field assistants and the staff of Lizard Island Research Station for their friendship and help during this study. Portuguese national funds funded this study through FCT–Fundação para a Ciência e Tecnologia, I.P., within the project PTDC/MAR-EST/5880/2014 (MUTUALCHANGE: Bio-ecological responses of marine cleaning mutualisms to climate change) to JRP and RR, the strategic project UID/MAR/04292/2013 and a PhD scholarship to JRP (SFRH/BD/111153/2015). Lizard Island Reef Research Foundation supported this study with a Lizard Island Doctoral Fellowship to JRP. United States National Science Foundation supported this study within the project OCE-1536794 to PCS and ASG. JRP is currently supported by project ASCEND—PTDC/BIA-BMA/28609/2017 co-funded by FCT–Fundação para a Ciência e Tecnologia, I.P, Programa Operacional Regional de Lisboa, Portugal 2020 and the European Union within the project LISBOA- 01-0145-FEDER-028609.

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JRP, RR, ASG and PS designed the study. JRP, DS, VP, PN, RR and PS performed the experiment and collected the data. JRP, RR, ASG and PS analysed the data. JRP, RR, ASG and PS wrote the manuscript. All authors discussed the results, their implications and commented on the manuscript at all stages.

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Correspondence to José Ricardo Paula.

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Paula, J.R., Sun, D., Pissarra, V. et al. The role of corals on the abundance of a fish ectoparasite in the Great Barrier Reef. Coral Reefs (2021).

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  • Coral reefs
  • Fish ectoparasites
  • Predator–prey interactions
  • Habitat loss
  • Demersal plankton