Substrate-associated amphipod and isopod fauna in the Cagarras Islands natural monument and surrounding waters, Rio de Janeiro, Brazil

The present study deals with the diversity and characterization of amphipod and isopod fauna associated with three different substrates, Perna perna beds, Phyllochaetopterus aggregates, and artificial substrate from the Cagarras Islands Natural Monument (MoNa Cagarras), and surrounding waters. Eight sampling stations were taken manually and/or with scuba diving between 2022 and 2023. As a result, 36 species of Amphipoda and 14 species of Isopoda were identified, totaling 50 species. Among the amphipods, there are five new records for the Brazilian coast: Ericthonius punctatus , Caprella scauroides , Carpias bermudensis, Deutella mayeri, and Quadrimaera ascensionis . Amphipods dominated across all substrates, but each substrate type exhibited a different dominance pattern at the family level. The Isopoda group was marked by the abundance of the Janiridae family, including the endemic species Janaira gracilis , and four Carpias species, as well as the family Joeropsididae. Species richness differed among substrates, being highest in Perna perna beds (15–22 species), intermediate in Phyllochaetopterus aggregates (9–17 species), and variable on artificial substrates (9–22 species). The CCA revealed three distinct associations and indicated a clear differentiation between the natural substrates: P. perna supported a diverse assemblage characterized by 10 species with nestling and tubicolous behavior; Phyllochaetopterus association grouped six species with burrowing and detritivorous habit that likely inhabit the sediment accumulated among the polychaete tubes; whereas the artificial substrate exhibited a less diverse assemblage composed of typical fouling, cosmopolitan, and tolerant species. PERMANOVA results corroborated the CCA hypothesis, indicating significant differences among the natural substrate assemblages.