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

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.

Keywords: Hemigrapsus sanguineus; diet quality; gut morphology; individual variation; progastric carapace region.

FIGURE 1

(a) Progastric region widths (in black) and carapace widths (in white) of two examples of Hemigrapsus sanguineus individuals used in the study. Vertical blue dotted lines facilitate the comparison between the two external stomach marking widths. (b) The same two crabs pictured in Part (a), but with the progastric region of the carapace outlined in black dotted lines. While the two individuals are nearly identical in carapace width, there is a >10% difference in the width of the progastric carapace region.

FIGURE 2

Cardiac stomach width (in blue) and progastric region width (in red) as a function of carapace width. Regression lines are included in black for both cardiac stomach width and external stomach marking width.

FIGURE 3

Residual cardiac stomach width as a function of residual progastric region width. A regression line is included in red. Outliers excluded from the final analysis have been highlighted in red.

FIGURE 4

Residual cardiac stomach width (a) and residual progastric region width (b) as a function of sampling date in Hemigrapsus sanguineus for individuals collected from Goshen Point in Waterford, Connecticut. Boxes represent the interquartile range, whiskers represent 1.5 × the interquartile range, open circles denote outliers, and black bars represent medians. The dashed red lines are polynomial regression curves fitted to the data.

FIGURE 5

Residual cardiac stomach width (a) and residual progastric region width (b) by location in Hemigrapsus sanguineus. Upper case letters denote sites at which residual external stomach marking width or residual stomach width did not differ significantly based on Tukey's HSD tests for each metric independently (p < .05). Outliers falling outside the maximum and minimum y‐axis values were excluded from the graph to enhance trends. Boxplots are as explained in the Figure 3 caption.