Morphometric correlations between dietary and reproductive traits of two brachyuran crabs, Hemigrapsus sanguineus and Aratus pisonii

Abstract

Many animals have flexible morphological traits that allow them to succeed in differing circumstances with differing diets available to them. For brachyuran crabs, claw height and gut size are diet-specific and largely reflect foraging strategies, while abdomen width reflects relative levels of fecundity. However, the link between claw size and diet has largely been documented only for primarily carnivorous crabs, while the link between diet and fecundity is strong in herbivorous crabs. We sought to determine the nature of the intraspecific relationship between claw size, dietary habits, and fecundity for two primarily herbivorous crab species, Hemigrapsus sanguineus and Aratus pisonii. Specifically, we examined whether claw size and/or abdomen width can be used as reliable measures of individual diet strategy. To test these hypotheses, we collected crabs and measured the dimensions of their claws, abdomens, and guts. By comparing these dimensions for each individual, we found that strongly predictive relationships do not exist between these traits for the primarily herbivorous species in our study. Thus, identifying external morphological features that can be used to assess diets of primarily herbivorous crabs remains elusive.

Fig 1. Dimensions for all size measurements performed as part of this study.

A) Claw dimensions measured. Here we focus only on differences in claw height, a proxy for claw strength in crabs. B) Abdomen width measured at widest point. C) Anterior edge of cardiac stomach.

Fig 2

Relationships between residual gut width for Hemigrapsus sanguineus and residual abdomen width (A), residual claw height (B), and collection site (C). All residuals reflect sizes after accounting for differences associated with crab body size. Box plots in part C show the median value (heavy black line), the first to third quartile of the data (box), 95% of the data (whiskers), and outliers that fall outside this range (circles).

Fig 3

Relationship between residual gut width for Aratus pisonii and residual abdomen width (A), major claw residual height (B), and collection site (C). Residual gut width and abdomen width reflect sizes after accounting for differences associated with crab body size, which residual claw height reflects heights after accounting for correlations with both body size and abdomen width. Boxplots are as described in the caption for Fig 2.

Fig 4

Relationship between residual gut width for Aratus pisonii and residual abdomen width (A), minor claw residual height (B), and collection site (C). Residual gut width and abdomen width reflect sizes after accounting for differences associated with crab body size, which residual claw height reflects heights after accounting for correlations with both body size and abdomen width. Boxplots are as described in the caption for Fig 2.