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For many species, ocean acidification (OA) is having negative physiological consequences on their fitness and resilience to environmental change, but less is known about the ecosystem effects of these changes. Here, we assess how OA conditions predicted for 2100 affects the biological functioning of an important habitat-forming species Mytilus edulis and its susceptibility to predation by a key predator, the gastropod Nucella lapillus. Change in three physiological parameters in Mytilus were assessed: (1) shell thickness and cross-sectional surface area, (2) body volume and (3) feeding rate, as well as susceptibility to predation by N. lapillus. Shell thickness and cross-section area, body volume and feeding rate of Mytilus all reduced under OA conditions indicating compromised fitness. Predation risk increased by ∼26% under OA, suggesting increased susceptibility of mussels to predation and/or altered predator foraging behaviour. Notably, predation of large Mytilus - that were largely free from predation under control conditions - increased by more than 8x under OA, suggesting that body size was no longer a refuge. Our results suggest OA will impact upon ecosystem structure and functioning and the continued provision of ecosystem services associated with Mytilus reefs and the communities associated with them.

Model organisms are an important part of experimental biology, especially in the field of evolutionary biology (Gasch et al., 2016), where taxa such as Drosophila spp., Caenorhabditis elegans, and Daphnia spp. allow the investigation and interpretation of some of evolution's biggest questions (Kellogg & Shaffer, 1993).Traditionally, work on model organisms has often been restricted to the laboratory (Barata et al., 2000). Laboratory experiments are useful for controlling natural environmental variation but also often result in individuals being studied in isolation (Kohler, 2002). This may exclude social aspects of the environment and other potentially important biological and environmental interactions (Morin, 1998). Since laboratory studies only capture a small part of the dynamic natural environment (Grodwohl et al., 2018), it can be difficult to know whether the results obtained in the laboratory are applicable to real-world field conditions (Ieromina, 2014;Poorter, 2016).

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