Animal weapons are diverse and often exaggerated structures associated with sexual selection and male–male combat. Despite their diversity, our knowledge of these structures comes primarily from studies that focus on the size of the weapons and neglect other components, such as their shape or performance. Furthermore, weapons can exhibit species-level plasticity based on nutrition and other ecological variables, but few studies have documented seasonal variation in weapon plasticity. Here, we investigate how weapon size, shape and performance differ across adult male reproductive and non-reproductive Allegheny crayfish, Faxonius obscurus. Given that F. obscurus and other cambarid crayfishes exhibit cyclical moulting between reproductive and non-reproductive states, they provide an ideal system in which to investigate such questions. We found that claws of reproductive and non-reproductive male F. obscurus differ in all three weapon components we investigated; claws from reproductive males were longer, stronger and different in shape compared with claws from non-reproductive males. Wielding longer and stronger claws is likely to confer greater competitive success during the mating season, which demonstrates an unexplored pattern of seasonal weapon plasticity. We discuss these results considering our knowledge of the seasonality of weapon phenotypes and propose several avenues of future research using cambarid crayfish as a model system.
Crayfishes are known for their ability to construct burrows, although there is intra- and interspecific variation in the complexity and reliance on such burrows. Crayfishes historically have been classified into ecological groupings based on their reliance on their burrow and the burrows connection to the water table. These ecological classifications, however, do not completely account for the behavioral diversity throughout crayfishes. One group of crayfishes that do not necessarily fit into these ecological classifications are members of the invalid subgenus Glareocola of Cambarus Erichson, 1846, which includes four species: C. brachydactylus (Hobbs, 1953), C. friaufi (Hobbs, 1953), C. polypilosus (Loughman & Williams, 2018), and C. williami (Bouchard & Bouchard 1995). All four species inhabit streams characterized by loose chert gravel, which creates ample interstitial spaces and potential access to the streambed’s hyporheic zone. We studied how one member of the Glareocola group, C. friaufi, exhibits unique morphological and behavioral adaptations that enable individuals to dig into the hyporheic zone of their natural chert substrate. We found that C. friaufi was sexually monomorphic regarding the relationship between body size and claw size, which contrasts findings from other stream-dwelling crayfish species. By evaluating the digging ability and digging performance of C. friaufi and sand, pea gravel, and their natural chert substrate, we also demonstrate how their digging ability is highly adapted to their natural chert substrate. Individuals of C. friaufi were either unable to dig or rarely dig underneath substrates other than chert. We discuss our results considering the behavioral ecology and natural history of the crayfishes within this enigmatic group.
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