|Functional morphology and evolution of isopod genitalia|
Wilson, G.D.F. (1991). Functional morphology and evolution of isopod genitalia, in: Bauer, R. et al. (Ed.) Crustacean Sexual Biology. pp. 228-245
In: Bauer, R.; Martin, J. (Ed.) (1991). Crustacean Sexual Biology. University of Columbia Press: New York. 355 pp., more
Isopod Crustacea are ubiquitous and have a long evolutionary history. Their success may have been aided by their mating systems, of which an important feature is internal insemination. This work presents an overview of the function of isopod genitalia, with some discussion of their evolutionary aspects. Isopods have varying degrees and kinds of precopula, and the actual copulation has been described in detail only a few times. Mating generally takes place during the isopodan biphasic molt, although some groups have extended receptive periods. The nonmotile, "pennantlike" sperm are grouped into "spermatophores" that may be necessary for sperm transfer during internal insemination. Primitively, the male genital papillae are on the coxae of the last walking legs, but in most isopods they have moved onto the last thoracic sternite or, in some taxa, onto the pleotelson. Sperm transfer may be mediated by the appendix masculina of the male second pleopod, although this appendage's morphology shows a great deal of diversity. In the simple and primitive form, the second pleopod has a rod on the medial ramus. The copulatory function of this rodlike appendix masculina is not clear. In at least three independent lineages, the first two pleopods form a "funnel" system that acts as an extension of the penile papillae. The Asellota also have a unique copulatory apparatus on the second pleopod, the "arm and hammer" form, that in the successful superfamily Janiroidea has a highly constrained function owing to interlinked parts and a continuous sperm conduit through both anterior pairs of pleopods. The female genitalia primitively open on the coxae of the sixth thoracic limbs, although the opening is on the sternum in most groups. Some taxa, however, have coxal plates that replace the sternum, so the derivation of the oopore may not be apparent. Most isopods are suspected of practicing sperm holding, and instances of well-developed spermathecae in the oviduct are known. In the Asellota, the female genitalia are separated into two separate functions: an oopore to release the ova and a spermathecal duct that receives sperm from the male. Although the pore for the asellotan spermathecal duct is directly associated with the tissues of the ventral oviduct, this duct opens on the dorsal surface in the evolutionarily derived Janiroidea. The janiroidean spermathecal duct receives the male appendix masculina directly, and the accessory structures seen in other Asellota are not present. While most isopods can copulate only during the brief period during the molt to the brooding stage, the highly evolved copulatory system of the Janiroidea allows the female to mate over much longer periods. The mating system of these asellotes may be a preadaptation to low population densities that permitted their astounding evolutionary radiation in the deep sea.