No abstract

Sessile communities provide habitat for feeding, reproduction and protection to a diverse mobile fauna. Along succession, the growth and overgrowth of three-dimensional sessile organisms generate structural complexity and microhabitats for mobile organisms. Most studies focus on one species or group of the sessile fauna as a habitat provider, but here we investigated the whole community, using fouling communities as a model. We tested the hypothesis that they would gain structural complexity along succession, resulting in an increase in abundance and biomass, and compositional changes of the associated mobile groups. The organisms were obtained from communities growing on PVC plates left in the water for 6, 9 and 12 months. Early succession fouling communities (6 months) were mostly flatter, dominated by encrusting bryozoans and more empty space and cover of delicate hydrozoans and filamentous algae. Advanced-succession fouling communities (9 and 12 months) showed a biomass increment and compositional changes by the increased cover of structurally complex sessile organisms, such as arborescent bryozoans and sponges. Mobile groups showed higher abundance and biomass, and a different composition at later stages. Thus, our results emphasise how the structural complexity provided by fouling organisms and the changes over succession may mediate the changes in the associated mobile fauna.

No abstract

The Amazon–Orinoco plume (AOP) is the world’s largest freshwater and sediment discharge into the ocean. Previous studies limited to mtDNA suggest that the swimming crab Callinectes ornatus Ordway, 1863 exists as two distinct genetic clusters separated by the AOP. However, questions concerning migration, diversification time, and species delimitation are unresolved. Densely sampling markers across the genome (SNPs) could elucidate the evolutionary processes within this species. Here, we combined mtDNA data and ddRAD-seq to explore the diversification patterns and processes within the swimming crab C. ornatus. We show great genetic differentiation between groups on the north and south sides of the plume but also signs of hybridization. Demographic modeling indicates the divergence between groups starting around 8 Mya following the AOP’s formation. After a period of isolation, we detect two incidences of secondary contact with stronger migration in concordance with the North Brazil Current flow. Our results suggest speciation with gene flow explained by the interplay among the AOP, oceanographic currents, and long larval dispersal. This work represents the first investigation employing ddRAD-seq in a marine invertebrate species with distribution encompassing the north and south Atlantic and sheds light on the role of the AOP in the diversification of a marine species.