The population biology of the three sympatric species of caprellids (Caprella danilevskii, C. equilibra and C. scaura) associated with a Sargassum
bed was studied in the northern coast of São Paulo state, Brazil.
Samplings were carried out monthly from October 2010 to February 2012.
In each month, 25 fronds of Sargassum were randomly collected
through snorkelling. The caprellids were identified, counted, classified
by sex and separated into size classes. Caprella danilevskii was the most abundant species, with 14939 specimens recorded. The body size of males was larger than that of females for C. danilevskii and C. equilibra, and the sex ratio was skewed toward males for all species. The size-frequency distribution was polymodal for C. danilevskii and C. scaura and bimodal for C. equilibra. Mature males of C. scaura and C. equilibra
were recorded in all size classes. The last size classes (from 8.3-9.4
to 12.7-13.8 mm) were dominated by mature males. Mature and ovigerous
females were more frequent in intermediate size classes. Significant
temporal variations were recorded for the three species with higher
densities in spring and summer that are related to higher algal biomass;
but other environmental factors are certainly important for explaining
caprellid density variation.