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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., Se estudió la biología poblacional de las tres especies simpátricas de caprélidos (Caprella danilevskii, C. equilibra y C. scaura) asociadas a un lecho de Sargassum en la costa norte del estado de São Paulo, Brasil. Los muestreos se realizaron mensualmente desde octubre de 2010 hasta febrero de 2012. En cada mes, se recolectaron al azar 25 frondas de Sargassum a través de snorkel. Los caprélidos fueron identificados, contados, clasificados por sexo y separados en clases de tamaño. Caprella danilevskii fue la especie más abundante, con 14939 ejemplares registrados. El tamaño corporal de los machos fue mayor que el de las hembras para C. danilevskii y C. equilibra y la proporción de sexos se inclinó hacia los machos para todas las especies. La distribución de frecuencia de tamaño fue polimodal para C. danilevskii y C. scaura y bimodal para C. equilibra. Se registraron machos maduros de C. scaura y C. equilibra en todas las clases de tamaño. Las últimas clases de tamaño (de 8,3–9,4 a 12,7–13,8 mm) estuvieron dominadas por machos maduros. Las hembras maduras y ovígeras fueron más frecuentes en clases de tamaño intermedio. Se registraron variaciones temporales significativas para las tres especies con mayores densidades en primavera y verano que están relacionadas con una mayor biomasa de algas; pero otros factores ambientales son ciertamente importantes para explicar la variación de la densidad de los caprélidos.

Caprellid amphipods are common crustaceans at Sargassum beds of southeastern Brazil and constitute an important link between primary producers and marine consumers. The goal of this study is to evaluate population and reproductive biology of two caprellid species: Paracaprella tenuis Mayer, 1903 and Pseudaeginella montoucheti Quitete, 1971. Sargassum cymosum Agardh, 1820 samples were collected monthly between October 2010 and September 2011 in a rock shore in southeastern Brazil. The caprellids were identified, counted, measured and classified as males, immature females, sexually mature females, ovigerous females and juveniles. Eggs were counted and measured. Higher densities of P. tenuis and P. montoucheti were found mainly in winter and lower densities in fall and summer. Density variation is probably related to organic matter supply and substrate availability. Both species showed continuous reproduction with reproductive peaks. Pseudaeginella montoucheti larger egg volume indicates a longer incubation period. The egg volume was directly related to female size. The number of eggs was weakly related to female size, indicating that fecundity may be driven by other factors, such as the physiological conditions. Sex ratio deviated for males suggests a higher longevity of males. The present study highlighted important data of two caprellids species that occur in algal beds of different regions.