Propensity to metal accumulation and oxidative stress responses of two benthic species (Cerastoderma edule and Nephtys hombergii): are tolerance processes limiting their responsiveness?

The chronic exposure of benthic organisms to metals in sediments can lead to the development of tolerance mechanisms, thus diminishing their responsiveness. This study aims to evaluate the accumulation profiles of V, Cr, Co, Ni, As, Cd, Pb and Hg and antioxidant system responses of two benthic organisms (Cerastoderma edule, Bivalvia; Nephtys hombergii, Polychaeta). This approach will provide clarifications about the ability of each species to signalise metal contamination. Organisms of both species were collected at the Tagus estuary, in two sites with distinct contamination degrees (ALC, slightly contaminated; BAR, highly contaminated). Accordingly, C. edule accumulated higher concentrations of As, Pb and Hg at BAR compared to ALC. However, antioxidant responses of C. edule were almost unaltered at BAR and no peroxidative damage occurred, suggesting adjustment mechanisms to the presence of metals. In contrast, N. hombergii showed a minor propensity to metal accumulation, only signalising spatial differences for As and Pb and accumulating lower concentrations of metals than C. edule. The differences in metal accumulation observed between species might be due to their distinctive foraging behaviour and/or the ability of N. hombergii to minimise the metal uptake. Despite that, the accumulation of As and Pb was on the basis of the polychaete antioxidant defences inhibition at BAR, including CAT, SOD, GR and GPx. The integrated biomarker response index (IBRv2) confirmed that N. hombergii was more affected by metal exposure than C. edule. In the light of current findings, in field-based studies, the information of C. edule as a bioindicator should be complemented by that provided by another benthic species, since tolerance mechanisms to metals can hinder a correct diagnosis of sediment contamination and of the system's health. Overall, the present study contributed to improve the lack of fundamental knowledge of two widespread and common estuarine species, providing insights of the metal accumulation profiles under a scenario of chronic contamination. Finally, this work provided useful information that can be applied in the interpretation of future environmental monitoring studies.

Epibiont assemblages on limpet shells: Biodiversity drivers in intertidal rocky shores.

Limpet shells can harbour a high diversity of species. Individuals of four limpet species (Patella depressa, Patella ulyssiponensis, Patella vulgata and Siphonaria pectinata) were collected monthly during one year in southern Portugal. Epibiont organisms were identified, counted and the percentage cover of facilitator taxa was also recorded. A total of 86 taxa were identified with abundance reaching 674 epibionts on a single basibiont shell. P. ulyssiponensis showed the highest epibiont species diversity and richness while P. depressa and P. vulgata showed similar diversity and richness. P. depressa had a more even epibiotic community mainly due to higher densities of Chthamalus sp. Overall, basibiont species was the key factor determining the epibiotic community, followed by month/season and erect algae. The presence of erect algae potentiated the epibionts diversity on limpet shells, whereas the occurrence of barnacles tended to decrease it and the presence of crustose algae had no significant effect on epibionts diversity. These findings shed further light on the biological and ecological complex relationships among keystone species inhabiting intertidal rocky shores.