Preliminary studies have shown that males and females of the freshwater crayfish Parastacus brasiliensis (von Martens, 1869) have an intersexed internal genitalia characterized by the existence of genital ducts of both sexes that connect to a gonad with male or female components, according to the sex. To determine the type of sexuality of this species, specimens (n = 92) from 11.7 to 40.0 mm carapace length were collected at Mariana Pimentel municipality, Rio Grande do Sul, Brazil (30°20′39″S, 51°22′39″W). Anatomical and histological analysis of the gonads disclosed the existence of three sexual forms: (1) intersexed males (n = 36), (2) transitionals between male and female sex (n = 8), and (3) intersexed females (n = 48). The transitionals present the following common morphological features: both genital apertures are present, but the female apertures are closed, as is usual in males; coexistence of oocytes and testicular acini in the same gonad (ootestis); and a longitudinal collecting tubule, large in diameter in the testicular region. The ovarian component of the gonad includes previtellogenic and primary vitellogenic oocytes. Of the 44 individuals classified as males by the analysis of the aspect of the genital apertures, 18.2% were transitionals, suggesting the existence of primary males that never change sex and a sexual system that is known as partial protandry. The existence of small females suggests that some females never go through a male phase. The histology of the gonads and genital ducts is described, and the germ cells are identified in each sex. The possible reasons for the existence of hermaphroditism in the studied population are discussed based on theoretical models.
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