Habitat traits and predation interact to drive abundance and body size patterns in macroalgae associated fauna

Habitat-forming organisms provide three-dimensional structure that supports abundant and diverse communities. Variation in the morphological traits of habitat-formers will therefore likely influence how they facilitate associated communities, either via food and habitat provisioning, or by altering predator-prey interactions. These mechanisms, however, are typically studied in isolation and thus we know little of how they interact to affect associated communities. In response to this, we used naturally occurring morphological variability in the alga Sargassum vestitum to create habitat units of distinct morphotypes to test whether variation in the morphological traits (frond size and thallus size) of S. vestitum, or the interaction between these traits, affect their value as habitat for associated communities in the presence and absence of predation. We found morphological traits did not interact, instead having independent effects on epifauna that were negligible in the absence of predation. However, when predators were present, habitat units with large fronds were found to host significantly lower epifaunal abundances than other morphotypes, suggesting large frond alga provided low-value refuge from predators. The presence of predators also influenced the size structure of epifaunal communities from habitat units of differing frond size suggesting the refuge value of S. vestitum was also related to epifauna body size. This suggests that habitat-formers may chiefly structure associated communities by mediating size-selective predation, and not through habitat-provisioning. Furthermore, these results also highlight that habitat traits cannot be considered in isolation, for their interaction with biotic processes can have significant implications for associated communities.To test whether habitat unit size (i.e., algal biomass/thallus size) interacted with frond size diversity (small, large and mixed mean frond size) to affect the abundance and size structure of associated communities, we created and deployed five types of habitat units: a single branch with small fronds, a single branch with large fronds, two branches with small fronds, two branches with large fronds and a unit with one branch with small fronds and one branch with large fronds (hereafter 'mixed frond' in text and 'mix' in plots). To test how variation in algal morphology and predation risk structure epifaunal communities, we used an orthogonal design with predation risk (three levels) and habitat unit morphology (three levels, as only double-branched habitat units were used); with 6 replicates for each combination level. After five days, all algae were retrieved by snorkelling. Each branch was collected separately by removing cage (if present), then the cable tie at the attachment point and quickly placing each branch of the habitat unit in a jar underwater (i.e., for double-branched habitat units branches were placed in separate jars). Back in the laboratory, all jars were stored in the refrigerator (~2 °C) until processing (maximum three days). Digital zooplankton scanner technology was then used to count and measure (total body length: µm) all epifauna