Spatial patterns of the gastropod Olivella minuta reveal the importance of tide-dominated beaches and the subtidal zone for sandy beach populations

Abstract

The main ecological hypotheses on spatial patterns of intertidal sandy beach populations predict that they are mainly influenced by morphodynamic features, with most population parameters (e.g., number of individuals, reproductive effort, recruitment rates) increasing from wave-dominated reflective to wave-dominated dissipative beaches. Yet, these hypotheses did not take into account tide-modified and tide-dominated environments due to the lack of data from these habitats. Also, they were based on species inhabiting the intertidal and supratidal zones while those living in the intertidal–subtidalbeach gradient (marine component of the Littoral Active Zone) remain largely overlooked. We investigated differences in abundance and size of the sea snail Olivella minuta in the intertidal and subtidal zones of two beach types: a wave-dominated dissipative beach and a tide-dominated sand–mud flat. Samplings were performed at three different times (September and December 2011, and March 2012). Abundance was higher at the tidal flat, with a high number of small (young) and large (old) individuals. On the other hand, individuals from the dissipative beach mainly belonged to larger size classes. Furthermore, we observed that individuals were more homogeneously distributed in the across-shore gradient of the tidal flat, with a similar number of individuals inhabiting both intertidal and subtidal zones, whereas most individuals (especially young ones) from the dissipative beach were found in the subtidal zone. Overall, our results support the importance of beach morphodynamics as a main driver of sandy beach populations along the Littoral Active Zone and emphasize the relevance of tide-dominated environments for coastal species. Importantly, our results demonstrate the ecological role of the subtidal zone for sandy beach macrobenthic populations and the necessity of performing ecological investigations along the whole Littoral Active Zone.