Abstract The aim of this study is to investigate if Sargassum-associated herbivorous amphipods Cymadusa filosa Savigny, 1816 and Sunamphitoe pelagica (H. Milne Edwards, 1830) present differences in their population parameters at sites located at different distances from a state marina, which is the main source of pollution (especially heavy metals) in an impacted bay. The study was conducted at four beach sites within Flamengo Bay, Ubatuba municipality, northern coast of São Paulo State, Brazil. The beaches are Lamberto and Ribeira close to the pollution source and Flamengo and Santa Rita, which are more distant. We observed the predominance of juveniles in the populations of C. filosa and S. pelagica, followed by females, with the sex ratio for both species being favored toward females, and the highest densities of individuals were observed during the summer. Sunamphitoe pelagica presented lower density, smaller ovigerous females and egg volumes at Lamberto beach, indicating a possible higher sensitivity to metal pollution for this species. Cymadusa filosa showed no clear alteration of density, number of ovigerous females and egg volumes between sites. Our results emphasize the importance of studying the life history and reproductive parameters of herbivorous amphipods, showing how these parameters can be altered in contaminated areas.

ABSTRACTMarine sponges are advantageous microhabitats because of their complex architecture. The system of internal canals provides circulation of water and deposition of particulate organic matter, ensuring availability of food and shelter. Diminutive amphipods have little difficulty penetrating the spaces of sponges and remain in their aquiferous systems as one of the most abundant taxa in this association. This study evaluated the temporal variation of the gammaridean amphipod species associated with the sponge Mycale angulosa. Sponge samples were collected every three months over one year at Pontal da Cruz Beach, São Sebastião Channel, southeastern Brazil. The amphipod assembly varied over time, while the amphipod density and sponge biomass remained approximately constant. Six species contributed to the temporal variation infaunal composition, highlighting the importance of the natural history of each species.