Moving in fast waters: the exaggerated claw gape of the New River crayfish () 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, . 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.