Patterns of recruitment of sessile invertebrates in two subtidal habitats

Abstract Recruitment of sessile marine invertebrates onto small panels at two subtidal sites in southern Australia varied considerably on all spatial and temporal scales. Recruitment at West Lakes was consistently higher than at Edithburgh, usually by an order of magnitude or more. There were strong seasonal trends in the number of recruits and the number of colonizing species per panel at West Lakes, but these trends were unclear at Edithburgh. Small panels received fewer recruits than did larger panels, although the number of recruits per cm 2 did not change. Individual species showed similar patterns of recruitment. The temporal pattern varied from year-round settlement, with seasonal peaks (most serpulids), to regular recruitment at the same time each year ( Didemnum sp., Botrylloides leachii Savigny), to irregular, intense peaks of recruitment ( Electroma georgiana Quoy and Gaimard, Ciona intestinalis (Linnaeus)). Even those species that showed regular peaks in recruitment showed great variation between years in the magnitude of the peaks. The spatial pattern of recruitment of individual species varied from random ( Spirorbis spp.) to strongly aggregated ( Elminius modestus Darwin, Mania ione Gray). Aggregated patterns of recruitment were probably due to settlement near adult conspecifics ( Elminius modestus , Balanus amphitrite Darwin), or to “swarms” of larvae settling together ( Hydroides norvegica (Gunnerus)). There was little synchrony between the two sites. Major peaks in recruitment at one site were not accompanied by peaks at the other site, and even a good year for a particular species at one site was not necessarily matched for the species at the other site, providing little support for the concept of generally “good” or “bad” years for a coastal region as a whole. Replicated panels that were immersed at the same time varied greatly in their recruitment. This was partly explained by the distance between panels: the similarity between a pair of panels sometimes decreased as the distance between them increased. This relation rarely explained >20% of the variation in similarities, however. Small panels were generally less similar to each other than were large panels. The increase in similarity as panels became closer together could not always be explained by gregarious behaviour of larvae, and was probably due to small-scale patchiness in the distribution of larvae in the plankton. There remained a large amount of variation in recruitment that could not be explained by the behaviour of individual species or by patchiness in the plankton.