The potential role of life cycle bottlenecks in shaping the marine latitudinal diversity gradient

The global response of marine species to climate warming has led to an equatorial dip and a poleward shift in species richness. Understanding the mechanisms driving such patterns is critical. While the role of ecophysiology in species biogeography has been widely debated, life cycle bottlenecks remain underexplored in large-scale biodiversity studies. Here, we highlight their importance by comparing the latitudinal distribution of fish species richness, sea surface temperature and the thermal tolerance of reproductive adults and egg stages (i.e. spawning stages) versus non-reproductive stages. Near the Equator, where the richness declines at the mean annual sea surface temperature >25°C, vulnerable spawning stages exist near their maximum thermal limits (approx. 28°C), implying very small thermal safety margins for reproduction. Recent poleward shifts in spawning habitats indicate that reproductive constraints may become key drivers of species redistribution, potentially initiating and intensifying both the equatorial richness dip and poleward shifts as temperatures rise. Therefore, vulnerable spawning stages should be an integral part of biodiversity analytics and marine conservation planning. This review steers the discussion towards an ecophysiological perspective on marine biodiversity dynamics. It also highlights data gaps across vertebrates and invertebrates, inconsistencies in existing information and the applicability of available datasets for biodiversity analyses. This article is part of the theme issue 'The biosphere in the Anthropocene'.