Delayed Reproduction and Adult Dimorphism in Males of the Amphipod Genus Jassa (Corophioidea: Ischyroceridae): An Explanation for Systematic Confusion

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Abstract
The ischyrocerid amphipod genus Jassa should comprise 19 species. Eight species that have been assigned previously to this genus should be transferred to other genera. Past taxonomic confusion is attributed to a failure to recognize that in the males of Jassa the development of a distinct secondary sex character, a thumb on the propodus of the second gnathopod, is delayed to the last molt, and production of the thumb signals the onset of sexual activity, and that thumbed males are dimorphic. It is suggested that the two thumbed male morphs may have different strategies for mating. Species of Jassa belong to one of two categories, depending on the size relations and proportion of the two thumbed male morphs in the species. In type 1 species, males exhibiting the small-thumbed "minor" morph are the smallest of the thumbed males, and therefore are rare. In type 2 species, minor form males are smaller than the large-thumbed "major" form males on the average, but considerably overlap the size range of major form males, and therefore are frequently found. In type 1 species the alternative mating strategy may be conditional; in type 2 species the alternative mating strategy may be evolutionarily stable. Jassa is the only genus of mate-gaurding amphipods known to dissociate sexual activity from physiological maturity and to have dimorphic secondary sex characters in the male. It is suggested that delay in sexual activity and dimorphism at maturity are responses to competition among males for mates. These adaptations may have developed in response to increased coloniality, wich is promoted by a sedentary life within a tube and restriction to hard substrates.