Caught between a rock and a hard place : fish predation interacts with crevice width and orientation to explain sessile assemblage structure

Complexity in physical habitats may modify predation pressure by allowing differential access of predators to prey. Rocky subtidal environments are inherently complex with many cryptic micro habitats, such as overhangs and crevices. Here, we examine the influence of habitat complexity in mediating predation on sessile assemblage structure by experimentally manipulating fish access to a range of crevice orientations and sizes. Nine fish species/families were recorded actively feeding within crevices, but australian mado, eastern stripey, wrasses and sawtail surgeon accounted for almost 70% of all entries. Sessile assemblages were influenced by crevice width, fish predation and surface orientation, with more predation activity in larger crevices. Assemblage similarity on upward facing surfaces decreased as crevice width increased. While assemblage structure on downward and vertical surfaces was influenced by crevice width and caging separately. Thus, crevice size and orientation are important habitat complexity features that act to partition predation pressure. This may allow distinct sessile assemblages to persist, even when predation can be intense.