Spatial analysis of recruitment of marine invertebrates on arborescent substrata

Juveniles of various species were observed on the perisarc of the dead hydroid Tubularia larynx in the Gulf of St Lawrence, Canada, some at densities up to twenty times more than on other substrata. This suggested active microhabitat selection by settling larvae. Here the results are reviewed of three field experiments using arborescent substrata, which tested whether recruitment of invertebrates could be accounted for by a passive model of encounter, or whether activity of larvae, or settled juveniles, needs to be invoked to explain the distributions. The distribution of inert particles on arborescent structures was examined in a laboratory flume. Both branch diameter and branching heterogeneity of the structures were considered as independent variables. A comparison of laboratory and field results suggested that passive settlement processes (hydrodynamic) were sufficient to explain the recruitment patterns at scales greater than 3 cm. At small scales (ca 1 mm), however, the hypothesis of active larval and/or post-settlement exploration of the substratum could not be rejected.