DOI: 10.5061/dryad.31zcrjdh9
title: Departing from an ideal: An asymmetric, bimodal and non-Equatorial latitudinal gradient of marine diversity in Western Atlantic burrowing shrimps (Decapoda: Axiidea and Gebiidea)

description: Aim: Despite the generality of the latitudinal gradient of species
 diversity (LDG) phenomenon, there is growing evidence showing deviations
 from an idealized pattern, i.e., a single peak of species richness
 symmetrically centered in the Equator, but the underlying causes remains
 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 (&lt; 200 m depth)
 along the Western Atlantic coast (46ºN to 47ºS) 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 hypotheses categories) predicting 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 non-linearly related to species richness, and only
 bottom seawater temperature, bottom seawater salinity showed a significant
 phylogenetic signal. Main conclusions: The LDG of burrowing shrimps is
 driven 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
 non-linear 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.
description: We gathered a
 database documenting the northernmost and southernmost limit of the
 distribution of 100 burrowing shrimp species inhabiting the coastal shelf
 (&lt; 200 m depth) along the WA coast, from Canada (46ºN) to Patagonia
 (47ºS). This represents ca. 15 % and 83 % of all valid species of Axiidea
 and Gebiidea described for the world and western Atlantic, respectively
 (World Register of Marine Species, www.marinespecies.org, accessed on July 17<sup>th</sup> 2020). The information was collected from an exhaustive literature review and fieldwork carried out from 2007 to present along the coast of Central America and South America (Hernáez, 2014, 2018a; Hernáez, Gamboa-González, & De Grave, 2015; Hernaez, Miranda, & Tavares, 2020; Hernáez & Vargas, 2013; Hernáez, Villegas-Jiménez, Villalobos-Rojas, & Wehrtmann, 2012; Hernáez & Wehrtmann, 2007; Hernáez, Windsor, Paula, & Santana, 2020). The taxonomic status of each species was validated using the World Register of Marine Species. The reported latitudinal ranges are provided in the Dataset S1. We evaluated the importance of 10 environmental variables predicting species richness (Table 1). The data was obtained from BioOracle 2.0 (Assis et al., 2018), GMED (http://gmed.auckland.ac.nz) (0.05º resolution each), and Aquamaps database v2.0 (Kaschner et al., 2008) (0.5º resolution). Values were averaged over one-degree latitudinal bins, using only pixels on the coastal shelf (&lt;200 m depth). The information of environmental variables is provided in the Dataset S2. See references in the article.
description: Check the README file and annotated instructions in the Rscript
 to reproduce all analyses presented in the article.