Contribution of motile rubble-dwelling cryptofauna to the diet of invertivorous coral reef fishes

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One of the least understood trophic pathways on coral reefs is the linkage between highly productive cryptic motile invertebrates (herein: cryptofauna) and the predatory vertebrates that underpin reef fisheries. As cryptofauna are difficult to observe and quantify, particularly in coral rubble where they proliferate, the diets of invertivores that forage in rubble have largely been determined through relative gut content analyses. Without congruent quantification of prey diversity, biomass, and rates of predation specific to rubble, it remains challenging to develop models of productivity and energy transfer. We calculated bite rates of six wrasses (Labridae) and a goatfish (Mullidae) commonly found foraging in rubble, using in situ videography on a coral reef in Palau, Western Micronesia. Consumption rates (i.e. individuals consumed) and prey preferences were determined using tank-based feeding experiments, where individual fish were presented with a diverse rubble community characterised before and after 2 h feeding trials. The motile cryptofauna community and invertivore diet consisted predominantly of Arthropoda (83% and > 58%, respectively), with calanoid copepods and mysids positively selected and harpacticoids, decapods and ostracods negatively selected. The interspecific mean (± standard error) rate of consumption was 3.5 ± 0.2 ind. min−1 from rubble, estimated to equal 9.5 ± 0.9 mg min−1 of invertebrate biomass consumed per fish, with biased uptake of small fauna. Bite and consumption rates varied among species with the greatest efficiency in biomass uptake evident in Halichoeres melanurus. This study provides a first attempt at characterising diverse cryptofauna communities pre- and post-trial to identify feeding rates and preferences of coral reef invertivores from the rubble benthos, and highlights the importance of addressing the bioavailability of lower trophic entities in energy pathways.