Heterochely and handedness in the shore crab Carcinus maenas (L.) (Crustacea: Brachyura)

A large sample ofadult male Carcinus maenas was 79% right-handed and 21% left-handed. A separate sample of 207 intact adult males was divided into left-handed and right-handed crabs and four measurements were taken from all major and minor chelae. Correlation and regression analyses against carapace width on log-transformed data showed that major chelae of right-handed crabs grow proportionately higher with increasing size and the ideal mechanical advantage increases; concurrently, the fingers of the minor chelae grow proportionately longer. The data for left-handed crabs showed greater variability, especially for minor chelae, providing evidence for the concept that left-handedness arises by reversal of handedness following loss of the major chela from the right-hand side. Records of handedness in large samples of non-ocypodid heterochelous brachyuran crabs show a preponderance of right-handedness.     

The evolution of armament strength: evidence for a constraint on the biting performance of claws of durophagous decapods

Performance data for the claws of six sympatric species of Cancer crabs confirmed a puzzling pattern reported previously for two other decapod crustaceans (stone crabs, Menippe mercenaria, and lobsters, Homarus americanus): Although biting forces increased, maximum muscle stresses (force per unit area) declined with increasing claw size. The negative allometry of muscle stress and the stress at a given claw size were fairly consistent within and among Cancer species despite significant differences in adult body size and relative claw size, but were not consistent among decapod genera. Therefore, claw height can be used as a reliable predictor of maximum biting force for the genus Cancer, but must be used with caution as a predictor of maximum biting force in wider evolutionary and biogeographical comparisons of decapods. The decline in maximum muscle stress with increasing claw size in Cancer crabs contrasts with the pattern in several other claw traits. Significantly, three traits that affect maximal biting force increased intraspecifically with increasing claw size: relative claw size, mechanical advantage, and sarcomere length of the closer muscle. Closer apodeme area and angle of pinnation of the closer muscle fibers varied isometrically with claw size. The concordant behavior of these traits suggests selection for higher biting forces in larger crabs. The contrast between the size dependence of muscle stress (negative allometry) and the remaining claw traits (isometry or positive allometry) strongly suggests that an as yet unidentified constraint impairs muscle performance in larger claws. The negative allometry of muscle stress in two distantly related taxa (stone crabs and lobsters) further suggests this constraint may be widespread in decapod crustaceans. The implications of this performance constraint for the evolution of claw size and the "arms-race" between decapod predators and their hard-shelled prey is discussed.     

The significance of chelae in the agonistic behaviour of the white‐clawed crayfish,a Ustropotamobius pallipes

As in other benthic decapods, crayfish have chelae that are important for intra‐specific agonistic encounters. In Austropotamobius pallipes, scars and mutilations are present mostly on the chelae, these being the main targets of agonistic contacts. Because males participate in more aggressive interactions than females do and compete with other males for acquiring females in escalating contests, selection for large chelae should be stronger in males. Thus, in A. pallipes (i) chelae are longer, wider and higher in males than in females; (ii) chelar size in males increases allometrically with cephalothorax length; (iii) male specimens are more often deprived of at least one cheliped and (iv) large males have the highest frequency of scars. Males with one regenerated cheliped, when opposed to a competitor with both large chelipeds, display the same motivation to fight, but perform less chelae threat displays than normal crayfish and immediately get a lower hierarchical rank. In these contests, Resource Holding Potential (= chelar size) is highly different between the crayfish and the information of this asymmetry is correctly transferred between the two opponents.     

Moving in fast waters: the exaggerated claw gape of the New River crayfish (Cambarus chasmodactlyus) aids in locomotor performance

Humans are inherently fascinated by exaggerated morphological structures such as elk antlers and peacock trains. Because these traits are costly to develop and wield, the environment in which they are used can select for specific sizes or shapes to minimize such costs. In aquatic environments, selection to reduce drag can constrain the form of exaggerated structures; this is presumably why exaggerated morphologies are less common in aquatic environments compared to terrestrial ones. Interestingly, some crayfish species possess claws with an exaggerated gape between their pinching fingers, but the function of this claw gape is unknown. Here, I describe and test the function of the exaggerated claw gape of the New River crayfish, Cambarus chasmodactylus. Specifically, I test the hypothesis that the claw gape aids in movement against flowing currents. I found that both claw size and gape size were sexually dimorphic in this species and that males have disproportionately larger gapes compared to females. By experimentally covering their claw gape and testing crayfish locomotor performance, I found that individuals with their gape blocked were 30% slower than crayfish with a natural gape. My results highlight a unique adaptation that compensates for wielding an exaggerated structure in aquatic environments.     

Size,but not sex,predicts pinch force and exoskeleton mechanical properties in crayfish of the genus Faxonius

Studies of animal weaponry and defensive structures rarely take into consideration their underlying mechanical properties. We measured the compressive strength and thickness of the exoskeleton of the claw (chela) in two North American crayfish species, Faxonius virilis and F. limosus. We performed similar measures on the carapace, a body region not directly involved in agonistic contests. Males of both species generated significantly stronger maximum pinch forces than females. However, these differences can be attributed to differences in claw size between the sexes. The thickness (ultrastructure) of the claw exoskeleton was a significant predictor of its compressive strength and likely explained the difference in compressive strength we observed between the two species. Neither claw thickness nor claw compressive strength was correlated with maximum pinch force. Additionally, we found that crayfish body size was a strong predictor of carapace compressive strength and thickness, whereas sex was not. The claw had greater compressive strength and thickness than the corresponding values for the carapace. Our study shows that the mechanical properties of the crayfish exoskeleton are largely a function of size and highlights the need to integrate mechanical properties into studies of animal morphology and performance.