Biochemical and morphological responses of Perna perna to marine eutrophication in Rio de Janeiro, Brazil

Marine eutrophication alters the quantity and quality of organic matter available to coastal consumers, particularly suspension feeder bivalves, potentially affecting both their biochemical composition and morphological traits and making them good candidates as bioindicators. This study investigated the biochemical and morphological responses of the mussel Perna perna to eutrophication by analyzing fatty acids (FAs), carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N), and shell morphometry across contrasting coastal environments of Rio de Janeiro State, Brazil. FAs and δ¹³C and δ¹⁵N in particulate organic matter (POM), P. perna digestive glands (DG) and muscles are used to describe the origin, quantity and quality of the food and its transfer to the animals’ tissues, while nine shell biometric traits were quantified to evaluate morphological responses. Comparisons between the oligotrophic Saco de Mamangua (MAM), the mesotrophic Itaipu (ITA) and eutrophic Boa Viagem (BV) sites revealed marked differences in tissue composition and shell morphology. Low FA contents and strong isotopic fractionation at MAM indicated metabolic adaptation to starvation, whereas higher FA contents at ITA and BV reflected greater food availability. ITA showed evidence of more efficient trophic transfer, consistent with better food quality (mainly phytoplankton) than BV (also bacteria and urban POM). Shell morphology also differed significantly among sites, with the most discriminating traits associated to valve opening, suggesting potential links with feeding activity. Twelve-month monitoring at ITA and BV demonstrated higher POM variability at BV driven by tidal exchange with Guanabara Bay waters. Diatom-derived FAs 20:5ω3 dominated POM pools at both sites, whereas P. perna selectively assimilated dinoflagellate-derived FAs (22:6ω3). Winter δ¹⁵N enrichment in POM at BV indicated anthropogenic nitrogen inputs. DG composition reflected the POM δ¹⁵N over the preceding days, whereas muscle tissue remained stable, reflecting longer-term (around two months) integration and reproductive dynamics. Finally, a large-scale sampling was conducted across 12 stations along the Rio de Janeiro (Ilha Grande, MAM, Sepetiba, Búzios, Cabo Frio, Angra dos Reis, ITA, BV, Jurujuba, Rio das Ostras, Saquarema e Arraial do Cabo). Clear geographic patterns were observed and reflected the anthropogenic influence of the Rio de Janeiro megacity on muscle δ¹⁵N and FA profiles, whereas shell morphometrics only partially responded to eutrophication gradients. Together, these results demonstrate that the biochemical and morphological responses of P. perna capture environmental variability across spatial and temporal scales. The combined use of FAs, stable isotopes, and shell morphometry therefore strengthens the potential of P. perna as a biomonitor/bioindicator of eutrophication in tropical coastal ecosystems.