Reevaluating scorpion ecomorphs using a naïve approach

Abstract

Background: Ecomorphs create the opportunity to investigate ecological adaptation because they encompass organisms that evolved characteristic morphologies under similar ecological demands. For over 50 years, scorpions have been empirically assigned to ecomorphs based on the characteristic morphologies that rock, sand, vegetation, underground, and surface dwellers assume. This study aims to independently test the existence of scorpion ecomorphs by quantifying the association between their morphology and ecology across 61 species, representing 14 families of the Scorpiones order.

Results: Without a priori categorization of species into ecomorphs, we identified four groups based on microhabitat descriptors, which reflect how scorpion ecospace is clustered. Moreover, these microhabitat groups, i.e., ecotypes, have significantly divergent morphologies; therefore, they represent ecomorphs. These ecomorphs largely correspond with the ones previously described in the literature. Therefore, we retained the names Lithophilous, Psammophilous, and Pelophilous, and proposed the name Phytophilous for vegetation dwellers. Finally, we sought to map the morphology-ecology association in scorpions and found that the morphological regions most tightly associated with ecology are at the extremities. Moreover, the major trend in ecomorphological covariation is that longer walking legs and relatively slender pedipalps (pincers) are associated with sandy microhabitats, while the inverse morphological proportions are associated with rocky microhabitats.

Conclusions: Scorpion ecomorphs are validated in a naïve approach, from ecological descriptors and whole body anatomy. This places them on a more solid quantitative footing for future studies of ecological adaptation in scorpions. Our results verify most of the previously defined ecomorphotypes and could be used as a current practice to understand the adaptive significance of ecological morphology.

Keywords: Ecomorphological diversity; Ecomorphology; Microhabitat; Scorpions.

Fig. 1

PCoA map of scorpion ecospace. The ecospace is depicted by species (dots) colored according to ecomorph affiliation. Dots are colored based on the four clusters obtained with the R package mclust. For illustration purposes, and because it explains approximately the same variation as the second PCoA dimension, we plotted the third PCoA dimension in the vertical axis. Moreover, only species with a cos² correlation higher than 0.70 with both dimensions for the pelophilous and phytophilous ecomorphs, and a cos² correlation higher than 0.45 for the psammophilous and lithophilous ecomorphs are labeled. Percentage values refer to the variation explained by each axis. See Additional file 3: Fig. S3 for contributions of microhabitat variables to axis composition

Fig. 2

Phylogenetic relationships of the scorpion species included in this study. Names of the families and superfamilies are indicated. Paraphyletic families are indicated with an asterisk. Colors correspond to microhabitat ecomorph affiliation. Tree topology is based on Santibáñez-López et al. (2019) [32] and Sharma et al. (2015) [31]

Fig. 3

Phylogenetic PLS between scorpion ecology (Y-axis) and morphology (X-axis). Morphological traits with the highest covariation with ecology are labeled on the X-axis. Scorpion illustrations correspond to sand burrowing microhabitats (on the left, minimum extreme of the morphology PLS vector) and microhabitats found on and under rocks (on the right, maximum extreme of the morphology PLS vector). Colors correspond to microhabitat ecomorph affiliation

Fig. 4

Ecomorph means (filled dots) rotated to their principal components, and 95% confidence ellipses around them. Illustrations approximate the mean morphologies of each ecomorph, and colors correspond to ecomorph affiliation