Rafting of benthic macrofauna: important factors determining the temporal succession of the assemblage on detached macroalgae

Abstract

Rafting on biotic and abiotic substrata has been reported for many benthic marine invertebrates. Here, I describe important characteristics of common floating substrata and review published studies examining the succession of the assemblage on detached macroalgae to identify the most important factors determining this succession. Floating substrata differ in survival time (i.e. before they disintegrate) and in food value, with abiotic materials having high survival times but low food value and some biotic substrata (seagrasses, fresh wood) having high food values but short survival times. Large macroalgae with pneumatocysts may combine high survival times and high food values. Substratum survival and food value have consequences for the species composition of the rafting macrofauna. In general, suspension feeders dominate rafting assemblages on abiotic substrata, while grazing and boring species abound on macroalgae and wood. The succession of the rafting assemblage can be subdivided into three phases during which different processes predominate. During the initial phase some of the original colonizers (if present) disappear from the substratum, either due to active emigration or predation. This short, initial phase is followed immediately by the colonization phase, during which new organisms colonize the rafting substratum. Colonization may be rapid and intense in the beginning but then slows down. Towards the end of the colonization phase, some rafting organisms may reproduce and their offspring recruit within the parental raft. Results from two long-term studies confirmed that the proportion of species with direct development increased with duration of rafting. These successional changes will be most pronounced for large biotic substrata, rendering these particularly suitable for long-distance dispersal of organisms with direct development.