Marine species turnover but not richness, peaks at the Equator

•Marine beta diversity peaks at the equator and declines in higher latitudes.•Beta diversity increases with a temperature above 25 °C, while species richness declines.•The pattern indicates increased habitat fragmentation under higher sea temperatures.•Increasing habitat fragmentation may lead to declining species richness under warming. Turnover in species composition, also called beta diversity, can indicate natural habitat diversity and fragmentation of populations due to environmental stress, such as heatwaves. Latitudinal gradients in species diversity help synthesise local diversity into more general evolutionary and climatic patterns. Recently it has been shown that local (alpha) and regional (gamma) measures of marine species richness do not peak but dip at and peak on either side of the Equator, and decline in higher latitudes, creating a bimodal gradient. Here we show that for a dataset of 50,113 marine animal species, that species turnover peaked at the Equator. Thus, species richness declined where turnover was highest. This high turnover but dip in species richness at the Equator may indicate population fragmentation due to thermal stress. Such fragmentation could be the mechanism behind declining marine species richness in latitudes with an annual mean temperature > 25 °C. One Sentence Summary: The latitudinal gradient in species turnover but not species richness is unimodal, with a peak at the Equator.