Distribution of sea anemones in the Southwest Atlantic: biogeographical patterns and environmental drivers

Abstract

In this paper, we explore spatial patterns of shallow-water sea anemones in the Southwest Atlantic (SWA). The following hypotheses were tested: (1) the distribution of sea anemones (Actiniaria and Corallimorpharia) corroborates SWA biogeographic provinces, (2) their distribution presents a richness peak at mid-latitudes, and (3) temperature is the main abiotic driver that structures their species composition. Occurrences/registries were obtained mainly from published sources. We adopted Sorensen dissimilarity and cluster analyses to compare fauna compositions. To verify latitudinal richness patterns, we used a Hill’s numbers approach through rarefaction and extrapolation techniques. Partial redundancy analysis was performed to identify which drivers contribute more to the distribution. The sea anemone faunas are congruent with the following SWA provinces: Brazilian province (BP, 0 to 33° S), Argentinian province (36 to 43° S), and Patagonian province (43 to 56° S), and all differ from the Caribbean province (CP). Although there is a high similarity among the BP and the CP, they form significant clusters with well-supported endemism levels (> 19%). An elevated richness was found at 19–23° S, in the BP southeast (34 spp. and 13% of endemism), supporting an SWA biodiversity center. Temperature explained 64% of the distribution, and the Amazonas-Orinoco and La Plata estuaries act as soft barriers. The sea anemone distribution can be explained by features of the local current systems, combined with isolation through barriers and different ecological tolerances. Furthermore, the patterns reported herein (unprecedented for the group) are similar to those of other marine taxa. A transition zone between tropical and subtropical waters indicates diversity and species accumulation in the SWA.