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

## TL;DR

This study shows that the external progastric region of the carapace in Hemigrapsus sanguineus cannot reliably predict stomach size or diet at the individual level.

## Contribution

The study challenges the use of external morphology as a proxy for stomach size in individual crabs.

## Key findings

- The progastric region width increases faster with body size than stomach width.
- Progastric region trends differ from stomach width across sites and over time.
- The progastric region may not reliably predict stomach size or diet at the individual level.

## Abstract

Stomach morphology can provide insights into an organism's diet. Gut size or length is typically inversely related to diet quality in most taxa, and has been used to assess diet quality in a variety of systems. However, it requires animal sacrifice and time‐consuming dissections. Measures of external morphology associated with diet may be a simpler, more cost‐effective solution. At the species level, external measures of the progastric region of the carapace in brachyuran crabs can predict stomach size and diet quality, with some suggestion that this approach may also work to examine individual diet preferences and specialization at the individual level; if so, the size of the progastric region could be used to predict trends in diet quality and consumption for individuals, which would streamline diet studies in crabs. Here, we tested whether external progastric region size predicts internal stomach size across latitude and time of year for individuals of the invasive Asian shore crab Hemigrapsus sanguineus. We found that the width of the progastric region increased at a faster rate with body size than stomach width. In addition, the width of the progastric region followed different trends across sites and over time compared to stomach width. Our results therefore suggest that the progastric region may not be used as a proxy for stomach size variation across individuals.

Diet studies often rely on internal measurements of gut morphology to estimate diet, but this involves time‐consuming dissections and animal sacrifice. The progastric region of the external carapace in brachyuran crabs has been shown to be an effective predictor of diet across species, but not at the individual level. Here, using Hemigrapsus sanguineus as a model species, we demonstrate that the progastric region may not be able to predict diet at the individual level.