Morphology, performance, and fluid dynamics of the crayfish escape response

Abstract

Sexual selection can result in exaggerated morphology that constrains locomotor performance. We studied the relationship between morphology and the tail-flip escape response in male and female rusty crayfish (Faxonius rusticus), a species in which males have enlarged claws (chelae). We found that females had wider abdomens and longer uropods (terminal appendage of the tail fan) than males, while males possessed deeper abdomens and larger chelae, relative to total length. Chelae size was negatively associated with escape velocity, whereas longer abdomens and uropods were positively associated with escape velocity. We found no sex-specific differences in maximum force generated during the tail flip, but uropod length was strongly, positively correlated with tail-flip force in males. Particle image velocimetry (PIV) revealed that the formation of a vortex, rather than the expulsion of fluid between two closing body surfaces, generates propulsion in rusty crayfish. PIV also revealed that the pleopods (ventral abdominal appendages) contribute to the momentum generated by the tail. To our knowledge, this is the first confirmation of vortex formation in a decapod crustacean.

Tradeoffs and Benefits Explain Scaling, Sex Differences, and Seasonal Oscillations in the Remarkable Weapons of Snapping Shrimp (Alpheus spp.)

“…For example, the primary mode of predator escape in many decapod crustaceans is the tailflip, during which individuals contract their abdomen to propel themselves backwards (Wiersma, 1947). Tailflip velocity and acceleration in crayfish increases with abdomen length (Hunyadi et al, 2020). If the same holds in snapping shrimp, then the abdomen tradeoff that we found here could influence survival.…”

“…There is likely a smorgasbord of expenditures that we didn't test for here, some of which are undetectable in purely observational work. For example, in other crustaceans, weapons hinder locomotion and reduce survival during predator escape (Hunyadi et al, 2020). These expenditures need to be identified through future experiments.…”

“…Instead, we use the term expenditure to represent tradeoffs that could cascade to fitness costs (Kotiaho, 2001). To identify weapon expenditures that vary with size as predicted by the handicap principle, we tested if snapping shrimp individuals bearing large weapons sacrificed resources from the abdomen (the muscular segmented region of the body used for swimming) (Arnott et al, 1998;Hunyadi et al, 2020).…”

“…Reduced abdomen size could lower fitness through reduced survival, given that abdomen length is positively correlated with predator escape velocity in other benthic decapod crustaceans (Hunyadi et al, 2020). Snapping shrimp with smaller abdomens could therefore be more vulnerable to predation.…”

“…Instead, we use the term expenditure to represent tradeoffs that could cascade to fitness costs (Kotiaho, 2001). To identify weapon expenditures that vary with size as predicted by the handicap principle, we tested if snapping shrimp individuals bearing large weapons sacrificed resources from the abdomen (the muscular segmented region of the body used for swimming) (Arnott et al, 1998; Hunyadi et al, 2020). Reduced abdomen size could lower fitness through reduced survival, given that abdomen length is positively correlated with predator escape velocity in other benthic decapod crustaceans (Hunyadi et al, 2020).…”