Complex life histories are frequently associated with biological trade-offs, as the use of one trait can decrease the performance of a second trait due to the need to balance competing demands to maximize fitness. Here, we examine growth patterns in invasive adult male northern crayfish (Faxonius virilis) that are indicative of a potential trade-off between energy allocation for body size versus chelae size growth. Northern crayfish undergo cyclic dimorphism, a process characterized by seasonal morphological changes associated with reproductive status. We measured carapace length and chelae length before and after molting and compared these growth increments between the four morphological transitions of the northern crayfish. Consistent with our predictions, reproductive crayfish molting to the non-reproductive form and non-reproductive crayfish molting within the non-reproductive form experienced a larger carapace length growth increment. Reproductive crayfish molting within the reproductive form and non-reproductive crayfish molting to the reproductive form, on the other hand, experienced a larger growth increment in chelae length. The results of this study support that cyclic dimorphism evolved as a strategy for optimizing energy allocation for body and chelae size growth during discrete periods of reproduction in crayfish with complex life histories.

Morphometric data for the present study were derived opportunistically from adult male northern crayfish collected as part of separate experiments that took place over a two-year period. Crayfish in these separate experiments were held in the lab for behavioral measurements that included measuring activity (n = 156) and aggression (n = 84). Depending on the nature of the experiment, crayfish were held in the laboratory for various lengths of time (3-6 weeks). As a result, long-term housing allowed us to capture morphological measurements before and after molting events as they occurred. Therefore, measurements for the present study were opportunistically collected from 33 of these crayfish that happened to molt while being held in the lab, as follows: 12 in 2019 and 21 in 2020.