Community structure and dietary pathways for invertebrates on intertidal coral reef flats

• We investigated abundance and dietary pathways for benthic invertebrates.
• We measured isotope values of invertebrates in three feeding guilds.
• Filter feeders, grazers and detritivores were most abundant.
• Grazers and detritivores depend on benthic sources enriched in 13C.
• Filter feeders appear to depend on pelagic sources.

Intertidal reef flats cover extensive inshore areas of coral reef ecosystems but their contribution to coral reef food webs remains largely unknown. We measured benthic invertebrate abundance using quadrat surveys and traced sources supporting the production of three abundant invertebrates from different feeding guilds using stable isotope measurements of carbon (C) and nitrogen (N) along reef flats bordered by mangroves in Queensland, Australia. Filter feeding bivalves, detritivorous hermit crabs, and grazing gastropods were the most abundant trophic groups on the reef flat and occurred most commonly distant from the shoreward mangroves. Isotope values of bivalves did not vary among sampling positions and these animals depended on a consistent pelagic food source. Gastropods collected at the center of the reef flat had unusually enriched C isotope values compared to those at the reef edge, indicating increased contributions from autochthonous benthic food sources, probably benthic microalgae. The C isotope values of hermit crabs also varied among sampling positions, but were less consistently enriched. The N isotope values of both hermit crabs and gastropods were lower at the reef flat center and mangrove edge, suggesting a decreased reliance on pelagic sources away from reef edge positions. Grazers and detritivores on reef flats in this study depend on benthic sources with greatly enriched C isotope values; these sources are common on reef flats globally. Trophic sources on reef flats should receive greater consideration in coral reef food webs, especially given their widespread occurrence and use by mobile consumers.

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