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.

Using external morphology as a proxy for stomach size in Hemigrapsus sanguineus

“…However, contrary to our hypothesis and to the findings of Quezada‐Villa et al. ( 2023 ), we found no significant relationship between residual cardiac stomach width and residual progastric region width (Hypothesis 1). We also found that the two metrics exhibited different trends over time and across sites (Hypothesis 2 and 3).…”

“…Our results may differ from those of Quezada‐Villa et al. ( 2023 ) due to our larger sample size ( n = 799) and geographic breadth. The lack of correlation between internal and external morphology and the more rapid increase in progastric region width with body size compared to the cardiac stomach (Hypothesis 1) may be explained by the fact that crabs that consume higher quality diets have smaller stomachs (Griffen & Mosblack, 2011 ).…”

“…Using external morphology as a substitute for internal stomach morphology in ecological diet studies may reduce collection time, effort, and animal sacrifice. The width of the progastric region of the carapace is an effective predictor of stomach width at the species level (Quezada‐Villa et al., 2023 ), but its relationship with stomach width at the individual level and across time and space has not been tested until now. We have shown here that both stomach width and the width of the progastric region increased with body size, though progastric width increased at a greater rate (Hypothesis 1).…”

“…Quezada‐Villa et al. ( 2023 ) identified a positive correlation between cardiac stomach width and progastric width in H. sanguineus . To determine whether our dataset followed the same pattern, we used a linear model with residual cardiac stomach width as the response variable and residual progastric width as the predictor variable.…”

“…These grooves may reflect internal segmentation, or may reflect sites of muscle attachments, including the gastric muscles (Klompmaker et al., 2019 ; Patwardhan, 1934 ; Tshudy & Babcock, 1997 ). Whatever their origin, these grooves have recently been identified as an accurate predictor of mean diet quality across crab species, with some suggestions that they may also be useful for assessing intraspecific differences in diet quality (Quezada‐Villa et al., 2023 ). However, the specimens used in that study to assess intraspecific diet did not capture the temporal and spatial variation of individual crab species, which we would expect to influence diet strategy.…”