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.

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The development of secondary sexual characters, the petasma, and thelycum growth were studied in Xiphopenaeus kroyeri. In adult females, the thelycum is a single plate and its anterolateral portion is characterized by a reduced hood. The aperture resembles a transverse ridge. In immature stages, the ridge has a space between the plates, which becomes narrower as it reaches the end of development. The female gonopore is 'comma' shaped. In adult males, the endopods of the petasma are linked at the dorsomedial margin by a large quantity of cincinnuli. In juveniles, cincinnuli gradually increase in number until they join both endopods. At the end of development the petasma is T-shaped. The male gonopore is C-shaped. The relative growth of the petasma total length versus juvenile body length showed a highly positive allometry, whereas in adults the growth was isometric. For the relationship carapace length versus thelycum width, the juvenile phase of females is characterized by an isometry and the adult phase by a negative allometry.

The swimming crabAchelous spinimanusis an important fishery component of several countries in the Western Atlantic; therefore, knowledge of the different phases of its life cycle is essential for good management of this resource. The juvenile development ofA. spinimanuswas investigated in the laboratory, from megalopae captured in neuston samples at Ubatuba, São Paulo, Brazil (23°26′S 46°09′W), during the summer months of 2005–2007. Rearing was performed in a constant temperature (25 ± 1°C), filtered seawater from the collection location (35 psu) and natural photoperiod. We obtained nine stages of the juvenile phase. All the morphological characters that allow the recognition of the first juvenile stage were drawn and described, as well as the main alterations that occur in the following stages. The sexual differentiation based on the number of pleopods becomes apparent from the third stage on. Some important characters in the identification of the species at the juvenile phase are the number of segments on the exopod of antennule, on the antennal flagellum and on the palp of mandible, beyond the absence of pleopods (even rudimentary) in the first stage.