Mutualisms play a critical role in ecological communities, however the importance and prevalence of mutualistic associations can be modified by external stressors. On coral reefs, elevated sediments are a major stressor, reducing the health of corals and damaging reef resilience. Here, we investigated the influence of sediment stress on the mutualistic relationship between small damselfishes (Dascyllus aruanus and Pomacentrus moluccensis) and their coral host (Pocillopora damicornis). In an aquaria experiment, corals were exposed to sedimentation rates of ~100 mg cm-2 day-1, with and without symbiont fish to test whether: (1) fishes influence the accumulation of sediments on coral hosts, and (2) fishes moderate partial colony mortality and/or condition. Colonies with fishes accumulated much less sediment compared to colonies without fishes, and this effect was strongest for colonies with D. aruanus (five-fold less sediment than controls) as opposed to P. moluccensis (two-fold less sediment than controls). Colonies with symbiont fishes also had up to 10-fold less sediment-induced partial mortality, as well as higher chlorophyll and protein concentrations. These results demonstrate that fish mutualisms vary in the strength of their benefits and indicates that some mutualistic or facilitative interactions might become more important for species health and resilience as stress levels increase.

Datasets were collected through a combination of field and aquaria experiments on fish behaviour and coral physiology.