Predicting diet in brachyuran crabs using external morphology.

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.

Metabolic rates of the Asian shore crab Hemigrapsus sanguineus in air as a function of body size, location, and injury.

Rapid warming in the Gulf of Maine may influence the success or invasiveness of the Asian shore crab, Hemigrapsus sanguineus. To better predict the effects of climate change on this invasive species, it is necessary to measure its energy dynamics under a range of conditions. However, previous research has only focused on the metabolism of this intertidal species in water. We sampled adult crabs from three different sites and measured their metabolic rates in the air. We show that metabolic rate increases with body mass and the number of missing limbs, but decreases with the number of regenerating limbs, possibly reflecting the timing of energy allocation to limb regeneration. Importantly, metabolic rates measured here in the air are ~4× higher than metabolic rates previously measured for this species in water. Our results provide baseline measurements of aerial metabolic rates across body sizes, which may be affected by climate change. With a better understanding of respiration in H. sanguineus, we can make more informed predictions about the combined effects of climate change and invasive species on the northeast coasts of North America.

Latitudinal and temporal variation in injury and its impacts in the invasive Asian shore crab Hemigrapsus sanguineus.

Nonlethal injury is a pervasive stress on individual animals that can affect large portions of a population at any given time. Yet most studies examine snapshots of injury at a single place and time, making the implicit assumption that the impacts of nonlethal injury are constant. We sampled Asian shore crabs Hemigrapsus sanguineus throughout their invasive North American range and from the spring through fall of 2020. We then documented the prevalence of limb loss over this space and time. We further examined the impacts of limb loss and limb regeneration on food consumption, growth, reproduction, and energy storage. We show that injury differed substantially across sites and was most common towards the southern part of their invaded range on the East Coast of North America. Injury also varied idiosyncratically across sites and through time. It also had strong impacts on individuals via reduced growth and reproduction, despite increased food consumption in injured crabs. Given the high prevalence of nonlethal injury in this species, these negative impacts of injury on individual animals likely scale up to influence population level processes (e.g., population growth), and may be one factor acting against the widespread success of this invader.