Macrophytes provide a habitat for many species of marine invertebrates, the gastropods being one of the main components. This study provides new information about Sargassum-associated gastropod biodiversity, through characterization of the fauna from a highly impacted area of Brazil, investigating its variation at a small spatial scale and between two main seasons of the year, as well as its relationship with macroalgae parameters. Density of gastropods was higher during the warmest season and varied throughout sampling sites. A significant and positive, however weak, relationship between gastropod density and Sargassum dry weight was found in all localities. For all sites, a marked and unusual dominance of Bittiolum varium was observed. The high dominance of this species seems to be related to the impacts caused by shipping activities and highway construction in the 1970s and 1980s, which caused a decline in local species diversity that seems to have continued until now. Many species, both typical of these habitats and characteristic of other, nearby habitats, benefit from Sargassum sp. These macrophytes allow gastropods to establish and grow during their most vulnerable stages, as shown by the growth series and juvenile forms found for most species of gastropods. The present data highlight the importance of macrophyte habitats for gastropod biodiversity in coastal areas and call attention to the importance of raising knowledge on this fauna, especially in impacted areas, thus contributing to the conservation of these highly diverse and ecologically important macrophyte–gastropod systems.

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Macroalgae constitute an important habitat for a wide variety of marine invertebrates in coastal ecosystems, which can obtain food, refuge against predators, protection against environmental stress and sites for reproduction, and/or larval settlement (Christie et al., 2009; Thomaz & Cunha, 2010). Those small-sized associated animals have great ecological importance, as they also serve as food items for other larger coastal species such as fishes, thus acting as energy links among different levels in marine trophic webs (Donadi et al., 2017;Dulvy et al., 2002).The southeastern coast of Brazil has a great diversity of macroalgae, with about 50 species of different groups and growth forms

Marine 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.

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