Sea anemones (Actiniaria) are both taxonomically diverse and geographically widespread, occurring across all latitudes and ocean depths. Despite their broad distribution, localized studies have previously reported that anemones are less species diverse closer to the equator. This contradicts broad species diversity patterns such as the latitudinal diversity gradient that suggests peak diversity occurs in the tropics. Using almost 250,000 occurrence records from GBIF and OBIS, this study investigated whether global data supported these unusual patterns of anemone diversity.
The authors estimated sample coverage and species diversity of anemones by plotting occurrence records across grid cells at resolutions of 200 km, 400 km, 600 km and 800 km. To account for uneven sampling efforts, species richness was standardized. Statistical analyses were then used to compare occurrence maps at each spatial scale, in order to determine species latitudinal gradients.
Temperate regions in the Northern Hemisphere (particularly in California and northern Europe) consistently emerged as the areas of greatest species richness throughout the study. However, the specific patterns of anemone diversity showed trends of bimodality, with two peaks in diversity at different latitudes depending on spatial scale. Coarser spatial scales indicated peak species diversity at 40° N and 40° S, while finer resolutions peaked at 40°–60° N and 40° S.
These results support the hypothesis that anemone diversity decreases closer to the equator at a macro level, challenging the widely observed latitudinal diversity gradient. Similar phenomena have been reported in studies of other marine species such as copepods, bivalves and brachiopods, highlighting the need for more comprehensive analyses to understand species diversity patterns in marine environments.