Non-contact competition in a sessile marine invertebrate: causes and consequences

In marine benthic communities, phenotypic responses to contact competition are well resolved, but the causes and consequences of non-contact competition remain unclear. Here, we used the arborescent bryozoan Bugula neritina to firstly identify whether colonies change their phenotype as a result of non-contact competition, and then understand the mechanism behind the changes. Secondly, we determined the phenotypes that change in response to non-contact competition, with focus on changes in the feeding structure, viz. the lophophore. Lastly, we used a reciprocal transplant design to test whether phenotypic responses to non-contact competition reduce its negative effects. We found that phenotypic responses to non-contact competition were mediated by the biological effects of conspecific neighbours, but were also determined by the physical effects associated with increased density. Further, we found that colonies grown in high conspecific density environments were smaller (though more elongated for their size) and had smaller lophophores than colonies from low conspecific density treatments. However, we found no evidence that such phenotypic responses constituted adaptive plasticity; instead, individuals that experienced non-contact competition always performed worse than individuals that had not, and the effects of exposure to non-contact competition were additive. Our study suggests that noncontact competition is an important and persistent process in benthic marine communities, but that phenotypic plasticity, though present, does not buffer individuals from the negative effects of this process.