Coralline algae enhance the settlement of the vermetid gastropod Dendropoma irregulare (d'Orbigny, 1842) in the southwestern Atlantic.

The Abrolhos reefs are the largest and the richest coral reefs of Brazil, where corals, crustose coralline algae (CCA), and vermetids are the main reef-building organisms. In the Abrolhos Archipelago, the vermetid Dendropoma irregulare (d'Orbigny, 1842) associated with the CCA Porolithon pachydermum (Foslie) Foslie, represents a significant portion of the reef edges. This study aimed to analyze the seasonal influence of the substrate and hydrodynamic exposure to vermetid D. irregulare settlement, and to verify if the biological cues might be acting in the vermetid settlement. Seasonal in situ settlement experiments were performed, testing different substrata (live coralline algae, dead coralline algae, and artificial substrate) in three study sites (Mato Verde, Caldeiros, and Siriba) and a seasonal exposure index was calculated. We found that P. pachydermum enhance settlement of D. irregulare in the Abrolhos reefs. The vermetid settlements were significantly higher on (1) substrates with live CCA, and (2) in the presence of substrates with live CCA cues. The highest vermetid settlements, which best reveal the positive effect of substrate cues (live coralline algae), were observed at Siriba during summer and autumn–winter, being more than twice that recorded in Mato Verde. It is suggested that features associated with live coralline surfaces may attract the settlement of the juvenile vermetid. This study revealed a significant positive correlation between settlements of the vermetid on live CCA and the degree of exposure to hydrodynamics, where the highest densities were observed in a moderate condition of exposure. Responses to the preferential settlement on live coralline algae surfaces in the present study are probably due to a complex of cues acting simultaneously, which includes the physical (hydrodynamics) and biological cues (biofilms). This study provides the first attempt to examination of the settlement factors of the vermetids, contributing to the role that live coralline may play as “nursery habitats” in the Abrolhos Archipelago. Because of its ability to induce settlement of the crawling juvenile vermetid, CCA play a fundamental role in the life cycle of many marine invertebrates, including D. irregulare . [ABSTRACT FROM AUTHOR]