Recruitment of the brown mussel Perna perna onto natural substrata: a refutation of the primary/secondary settlement hypothesis

The pattern of recruitment of the brown mussel Perna perna on an exposed rocky shore on the southeast coast of South Africa between March 1991 and March 1992 is reported. The densities and population size structures of plantigrades associated with 2 natural substrata, filamentous algae and established mussel clumps, are contrasted. Mussels of c500 pm shell length were present throughout the study period, but peaked in abundance dunng the winter months (June to September). Although the density of these early plantigrades vaned with both samphng date and site, type of substratum had no effect. Late plantigrades, mussels of >0.5 to 3.5 mm shell length, reached peak abundance amidst mussel clumps in winter as opposed to spring/early summer on algal turf. Sampling date, site and type of substratum all influenced the dens~ty of late plantigrades. The size distributions of the recruits found in association with the 2 substrata dlffered s~gn~ficantly on 4 of the 11 sampling dates. The recruitment pattern of P perna does not, therefore, conform with the generally accepted primary/secondary settlement hypothesis. The observed pattern lndlcates direct settlement of larvae from the plankton onto adult beds in addition to temporary attachment on filamentous algae. The relatively low intensities of recruitment observed, compared with those of other mytilids, may explain such anomalous attributes of P. perna as its lack of competitive dominance and low resilience to disturbance.