Sexual maturity of the spider crab Libinia ferreirae was assessed for each sex, based on body dimensions and observations of gonad condition. A total of 346 crabs were analysed, of which 68% were females. Immature and adult individuals were recognized based on their allometric growth and gonad development. Abdomen width (AW) vs carapace width (CW) and propodus length (PL) vs CW were the relationships that best separated allometric groups of females and males, respectively. For females, gonad and allometric morphological maturity were, respectively, 38.77 and 39.43 mm of CW, which is close to the carapace size of the smallest ovigerous female (38.08 mm). For males, gonad maturity was 34.86 mm of CW and three allometric phases were observed: immature (IM♂), adolescent (AD♂) and adult morphometrically mature (MM♂). The IM♂ phase showed lower values of CW and PL than the AD♂ phase, without spermatophores inside the vas deferens; the AD♂ phase exhibited higher CW values than IM♂, but lower CW and PL values than the MM♂ phase, and the presence of spermatophores in the vas deferens; the MM♂ phase had higher values of CW and PL than the AD♂ phase and spermatophores in the vas deferens. Therefore, females showed synchronic morphometric, gonadal and functional maturity, while in males, gonadal maturity was attained before morphometric maturity, which probably could be a reproductive strategy for this species.

Morphometric and gonad maturity of Libinia spinosa on the coast of Mar del Plata, Argentina, were assessed. In both sexes various morphometric features were measured to determine the morphometric maturity, and through macroscopic inspection of gonads and analysis of its content, different stages of gonad maturity were described and the physiological maturity was estimated. In females, the carapace width ranged from 8 to 56.5 mm while in males from 4.6 to 81.97 mm. The size at which 50% of females were morphometrically mature was calculated at 40.6 mm. Based on the allometric growth of the abdomen, two morphometric groups of females were recognized as juvenile and adult phases. In males, results of principal component analysis showed that the most suitable variables that efficiently represent the size and sexual differentiation factor were the carapace width and cheliped length respectively. The size at which 50% of males were morphometrically mature was estimated at 58.9 mm. Based on the allometry of the cheliped growth, two morphometric groups were recognized as immature and mature phases. Males maintained under laboratory conditions moulted and growth was related to regular and terminal moults. Males reached their terminal moult at a wide size-range, from 48.5 mm to 61.4 mm of carapace width. This terminal moult coincided with the size of morphometric maturity. In both sexes four stages of gonad development were described, two corresponded to immature phases and two to mature phases. The size at which 50% of females and males reached gonad maturity was LC50 = 40.33 mm and LC50 = 33.6 mm, respectively. In females, both morphometric and gonad maturity occurred at a similar size. In males, gonad maturity is acquired prior to morphometric maturity, thus three different types of males were characterized: (1) those with small carapace width and cheliped, without spermatophores in their vas deferens; (2) those with large carapace width and small cheliped, bearing spermatophores in their vas deferens; and (3) those with large carapace width and cheliped, bearing spermatophores in their vas deferens.