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Morphology and function of the male reproductive tract, female spermatheca and patterns of sperm storage were assessed in the crab Libinia spinosa using histological methods. Testes are characterized by the presence of peripheral spermatogonia and different sequences of sperm maturity. Spermatophores begin to be packed in the last portion. The vas deferens consists of three sections: anterior, with undeveloped spermatophores and free sperm; median, with well-developed spermatophores; and posterior with granular secretions. Female spermathecae are of the ventral type, with a velum separating dorsal and ventral chambers. Live individuals were kept in the laboratory and arranged in pairs. An experiment was conducted toward the end of the reproductive season, in which males with the right gonopod excised were placed with receptive females. After mating, females were killed and the spermathecae dissected for histological study and observation of the pattern of sperm storage. Spermatozoa were found forming discrete sperm packages. New ejaculates can fill the entire spermatheca or be restricted to the ventral chamber; sperm are rounded, with a distinguishable acrosomal core. Old ejaculates are restricted to the dorsal chamber and are of irregular shape and larger size; an acrosomal core was not distinguishable. The secretions produced by the glandular epithelium of the dorsal chamber of the spermathecae are likely to have a role in the removal of dead sperm.

Few studies conducted in crustaceans have demonstrated how habitat features could shape the mating systems.

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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 LC 50 ¼ 40.33 mm and LC 50 ¼ 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.

Caridean shrimps are unique among decapod crustaceans in showing a great diversity of sexual systems, including gonochorism, protandry, protandry with primary females and simultaneous hermaphroditism. Crangon franciscorum (Stimpson, 1859) (Crangonidae), an ecologically and economically significant species from the Pacific coast of North America, has been assumed to be gonochoristic. Earlier population studies concluded that average lifespan is longer in females than in males, and that the latter die in or emigrate from estuaries after mating. This was believed to explain a recurrently observed ''shrinking'' of average male size starting during the fall of the first year of life. We investigated an alternative hypothesis according to which this species is a protandric hermaphorodite, and disappearance of large males from the population reflects sex change rather than death or emigration. We present several pieces of evidence in support of the alternative hypothesis: (i) ovarian development in males undergoing sex change, (ii) presence of atrophied vas deferens in secondary females, and (iii) observation of sex change in individuals kept in captivity. Our results are supported by histological study of the gonads, careful description of secondary sexual characters, and monthly sampling of an estuarine population (Grays Harbor, Washington). Tracking an identifiable year-class (1980) and combining field and laboratory data allowed us to assemble a life history schedule, including growth and a calendar of significant reproductive events. We discuss the implications of our results for the interpretation of survey data and studies on population dynamics.

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