In Brachyura copulation prior to the instar in which females become fully morphologically and functionally mature (i.e., produce fertile eggs) is exceptional. However, in the freshwater crab Candidiopotamon rathbunae (family Potamidae), adult males frequently mate with immature females (precocious mating). The size of female functional maturity is 22-23 mm carapace width. Immature females larger than 15 mm carapace width are observed mating, usually have sperm in their seminal receptacles, and this sperm is retained through subsequent moults. Precocious mating in C. rathbunae could potentially increase the reproductive capacity of females. It could have evolved as an adaptation to a seasonally amphibious life, during which the mating opportunities are reduced compared to a permanently aquatic lifestyle. Data from the few other brachyuran taxa with precocious mating fail to indicate a common rationale.