Predicting diet in brachyuran crabs using external morphology

Abstract

Morphological traits have often been used to predict diet and trophic position of species across many animal groups. Variation in gut size of closely related animals is known to be a good predictor of dietary habits. Species that are more herbivorous or that persist on low-quality diets often have larger stomachs than their carnivorous counterparts. This same pattern exists in crabs and in most species, individuals exhibit external markings on the dorsal side of their carapace that appear to align with the position and size of their gut. We hypothesized that these external markings could be used as an accurate estimate of the crab's cardiac stomach size, allowing an approximation of crab dietary strategies without the need to sacrifice and dissect individual animals. We used literature values for mean diet and standardized external gut size markings taken from crab photographs across 50 species to show that percent herbivory in the diet increases non-linearly across species of brachyuran crab with the external estimate of gut size. We also used data from dissections in four species to show that external gut markings were positively correlated with gut sizes, though the strength of this correlation differed across species. We conclude that when rough approximations of diet quality such as percent herbivory will suffice, measuring external carapace markings in crabs presents a quick, free, non-lethal alternative to dissections. Our results also provide important insights into tradeoffs that occur in crab morphology and have implications for crab evolution.

Keywords: Diet composition; Diet quality; Gut size; Morphological variation; Nonlethal methods; Percent herbivory; Trophic position.

Figure 1. Carapace showing external gut markings (black lines) on each of the four species used for intraspecific comparison.

External gut measurement done on each crab species (from left to right, top to bottom): European green crab Carcinus maenas, Atlantic blue crab Callinectes sapidus, Asian shore crab Hemigrapsus sanguineus, and mangrove tree crab Aratus pisonii. Black lines drawn over each carapace delineate the external gut markings as measured for these samples.

Figure 2. Correlations between metrics from each carapace picture in interspecific comparisons.

Correlations across species between percent herbivory and the carapace shape index (A), between the carapace shape index and standardized gut width (B), between percent herbivory and residual carapace shape index after accounting for standardized gut width (C), and between percent herbivory and residual standardized gut width after accounting for carapace shape index (D).

Figure 3. Relationship between percent herbivory in the diet and gut width to carapace width ratio across species.

Interspecific relationship between arcsine square root transformed percent herbivory and the ratio of gut width (GW) to carapace width (CW), or the standardized gut width. Symbols represent the different phylogenetic families included in the analysis, as given along the right. Each data point shows the average value for the different species within a given genus. Crab drawing in upper right and lower left are, respectively, of Sesarma reticulatum and Callinectes sapidus, and are included to illustrate the differences in carapace shapes of crabs in these two different regions of the graph.

Figure 4. Residual external gut markings vs. residual gut width in Aratus pisonii.

Relationship between residual external markings width and residual gut width (after accounting for body size) in the mangrove tree crab Aratus pisonii.

Figure 5. Residual external gut markings vs. residual gut width for Carcinus maenas.

Relationship between residual external markings width and residual gut width (after accounting for body size) in the European green crab Carcinus maenas.

Figure 6. Residual external gut markings vs. residual gut width for Hemigrapsus sanguineus.

Relationship between residual external markings width and residual gut width (after accounting for body size) in the Asian shore crab Hemigrapsus sanguineus.

Figure 7. Residual external gut markings vs. residual gut width for Callinectes sapidus.

Relationship between residual external markings width and residual gut width (after accounting for body size) in adult male (A), adult female (B), and juvenile (C) Atlantic blue crabs Callinectes sapidus.