Departing from an ideal: An asymmetric, bimodal and non-Equatorial latitudinal gradient of marine diversity in Western Atlantic burrowing shrimps (Decapoda: Axiidea and Gebiidea)

Aim Despite the generality of the latitudinal gradient of species diversity (LDG) phenomenon, there is growing evidence showing deviations from an idealized pattern, that is, a single peak of species richness symmetrically centred in the Equator, but the underlying causes remain little studied. We here evaluate the existence of departures from the idealized LDG in a group of marine crustaceans and the explanatory role of environmental variables. Location Coastal shelf (<200 m depth) along the Western Atlantic coast (46 degrees N-47 degrees S). Taxon Burrowing shrimps (Decapoda: Axiidea and Gebiidea). Methods We assessed the shape of the LDG in 100 burrowing shrimp species using the reported latitudinal ranges of distribution. Species richness was calculated in one-degree latitude bands using a range-through approach. The shape of the LDG was statistically evaluated in terms of latitudinal symmetry, number of modes and location. We evaluated the importance of 10 environmental variables (proxies of different hypothesis categories) predicting the LDG using a random forest model. Results Burrowing shrimps exhibit an increase in diversity towards the tropics, but departures to the idealized LDG were evident. The LDG is asymmetric between hemispheres and bimodal (two peaks within the tropics), and this trend cannot be explained in terms of sampling artifacts. A random forest model explains 92% of species richness, but the only significant variables were bottom seawater temperature, bottom seawater salinity, bottom seawater temperature range, and tidal range. These predictors were nonlinearly related to species richness, whereas only bottom seawater temperature and bottom seawater salinity showed a significant phylogenetic signal. Main conclusions The LDG of burrowing shrimps is driven by ecophysiological restrictions of species, reflecting the role of evolutionary (i.e. 'time for species accumulation' and/or 'diversification dynamics') and ecological processes (i.e. 'ecological limits'). Departures from the idealized LDG could be explained by the nonlinear responses of species richness to environmental conditions, the spatial structure of these environmental conditions, and a varying degree of phylogenetic conservatism of key ecophysiological constraints.