The sexual systems of the caridean shrimp species Thor dobkini and T. f10ridanus were investigated by observations on sexual characteristics and population structure from samples in various locations in Florida. Comparison of the sexual systems and reproductive characteristics
of these species were made with those of T. manningi, whose population structure and breeding was described from samples in various locations in the Caribbean and from a previous study by Bauer (1986). Unlike T. manningi, with a sexual system composed of a mix of protandric hermaphrodites,
primary males with prehensile third pereopods, and a low percentage of primary (non-hermaphroditic) females, T. dobkini and T. floridanus are completely gonochoristic, with smaller size classes composed of juvenile primary females and mainly prehensile males. Non-prehensile individuals
with male appendices masculinae (NPM's) were abundant in one non-reproductive sample of T. dobkini and were rare but present in most samples of T. f1oridanus. In T. manningi. such individuals showed, with increasing size, a relative decrease in appendix masculina size,
degeneration of the ejaculatory duct, development of the female character of incubatory basipod flanges on the anterior pleopods, and, in many individuals, a maturing ovary filling with vitellogenic oocytes. These individuals are male-phase hermaphrodites in T. manningi, and they lose
all male characteristics and become breeding females (Bauer, 1986). Detailed morphometric and other direct observations on the third pereopod and sexual characteristics of NPM's in T. dobkini and T. floridanus, however, showed that such individuals are primary (non-prehensile)
males that do not change sex. It is proposed that the NPM's of T. dobkini were males, recruited during a non-breeding season, which had not yet developed full scale prehensility. In T. floridanus, NPM's were rare but present throughout the year; some showed feminization of the
appendix masculina and ejaculatory duct, perhaps due to parasitization by bopyrid isopods. Other NPM's in T. floridanus may simply represent a rare male morph with the ancestral condition (non-prehensile) of the male third pereopod. Measures on reproductive condition (% adult females
carrying embryos) of samples from this study tentatively supported a hypothesis of increased seasonality (restriction) of breeding with an increase in latitude when comparing different species of Thor, ranging from year round in T. manningi to definitely seasonal in the most
northerly species sampled, T. dobkini. Concomitant measures on the stage of ovarian maturation and the stage of embryo development in incubating females showed that many to most females of all three species produced successive broods continuously during the reproductive season.
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