Do the threats of predation and competition alter larval behaviour and selectivity at settlement under field conditions?

The temporary ability of larvae to select optimal substrata during settlement can strongly influence the success of sedentary benthic invertebrates. Larval responses to cues from algae, epilithic microbial assemblages and conspecific species are well recognised. Comparatively, little is known about how the threat of predation or of competition influences settlement. We used a manipulative field experiment to test if the presence of a predator or spatial competitor affects settlement patterns of two intertidal mussel species, Perna perna and Mytilus galloprovincialis. By deploying treated artificial units of settlement habitat (larval collectors) in 'no choice' and 'choice' combinations, we determined whether the observed settlement patterns resulted from active larval choices (preference/avoidance). Among treatments where no choice was offered, settlement was generally greater on the predator treatment, lower on treatments with epilithic biofilm or competitor presence, and least on collectors without biofilm. In all but one case, the observed proportions of settlement in the 'choice' tests were significantly different from that expected by random association based on the 'no choice' tests. Choices related primarily to the absence of biofilm and to the threat of predation, with little influence of competitor presence. Although active preferential behaviour was confirmed, patterns of selectivity differed between the two study sites. Settlers of both species preferred the predator treatment at one site, possibly suggesting a predator swamping strategy. In contrast, settlers at the second site avoided the presence of predators. This suggests that settlement behaviour may be contingent on site-specific factors and demonstrates a greater degree of flexibility in settlement behaviour than expected.