Weaponry Investment in the Socially Monogamous Snapping Shrimp Alpheus brasileiro (Decapoda: Alpheidae)

ABSTRACT Weapons are morphological structures used by animals in various contexts, especially in intra‐specific contests and visual displays. In snapping shrimps of the genus Alpheus , particularly the monogamous species Alpheus brasileiro , both sexes bear enlarged chelipeds, potentially conferring advantages in mate competition or territorial defence. However, little is known about sex‐specific weapon investment value (WIV) in socially monogamous species. This study investigated WIV in males and females of A. brasileiro through monthly sampling from April 2015 to March 2016 in a southeastern Brazilian estuary. Shrimps were measured for carapace and chela (propodus + dactyl) dimensions, and allometric analyses were performed to assess relative growth patterns. WIV was quantified by comparing the dry weight of the major chela to total body dry weight. Both sexes showed positive allometry for weapon growth, indicating increased energy allocation to chela development. However, males invested significantly more (≈26%) of their body weight in weaponry compared to females (≈20%). These findings suggest that males face stronger selective pressures favouring larger weapons, possibly due to intra‐sexual competition, predatory defence or display behaviours. Our results provide new insights into sex‐based differences in weapon development in Alpheus , emphasising the relevance of investigating morphological traits in socially monogamous crustaceans.