Rapoport s rule in the marine realm: wrong axis, right pattern

AimRapoports rule posits that species range size increases with distance from benign conditions along environmental gradients. We asked whether, and along which axis, marine species obey Rapoports rule when ranges are quantified in three dimensions. LocationGlobal ocean. Taxon> 20,000 marine species spanning pelagic and benthic habitats across major animal phyla and fishes. MethodsWe combined AquaMaps 2.0 / AquaX modelled distributions with independently curated depth limits from FishBase and SeaLifeBase to estimate latitudinal and vertical (bathymetric) ranges for each species. We quantified latitudinal range versus absolute mid-latitude and depth range versus mid-depth, and repeated analyses by habitat and taxonomic group. We used linear and polynomial regressions and Gaussian mixture models in range-gradient space to identify and compare alternative Rapoport regimes. ResultsMarine biodiversity exhibits a classic latitudinal diversity gradient and strong concentration of richness in the upper ocean, with broader ranges at higher latitudes and greater depths. Support for Rapoports rule is weak and inconsistent for latitudinal ranges, but strong and pervasive with depth, with correlations between vertical range and mid-depth frequently exceeding 0.8 across habitats and taxa. Latitudinal and vertical ranges are positively, but only moderately, coupled, with a small subset of "3-D generalists" spanning both large latitudinal and depth extents. Main conclusionsThe marine realm appears to obey Rapoports rule primarily along the vertical, rather than latitudinal, axis. Depth-structured environmental tolerance thus emerges as a dominant constraint on marine range limits and three-dimensional biodiversity gradients.