In caridean shrimp, small processes on the swimmerets
(pleopods) of males appear to be important in sperm transfer (Bauer, 1976).
Males of penaeoid species have very complex genitalia, called the petasma.
Is this a female-stimulating device or a sperm transfer device or both
(see Bauer, 1996b)?
Study of sperm storage structures in penaeoid shrimps reveals that in some species, such as Rimapenaeus (formerly Trachypenaeus) similis, a copulating male deposits a sperm plug that seals off the sperm storage structures (see below). Other males cannot replace the first male's sperm and thus his sperm will fertilize the female's next several spawnings (see Bauer and Min, 1993).
In Farfantepenaeus (formerly Penaeus) aztecus, the commercially important brown shrimp of the Gulf of Mexico and southeastern coast of the U.S., the male deposits a spermatophore which has a substance that swells on contact with seawater. The substance later hardens to form a solid seal between the female's two genital plates which cover the entrance to the median spermatheca, where sperm are stored. No other male can inseminate the female until after the next molt. Sperm plugs and seals are a "paternity assurance" device.
However, in the "rock shrimp," Sicyonia
dorsalis, the lack of a sperm plug allows two or more males to
potentially deposit sperm in the spermathecae of a female, with potential
impact on paternity and genetic diversity of offspring (see Bauer, 1992a).
Study of sperm storage structures has contributed to an understanding of
the evolutionary history of penaeoid shrimps (see Bauer, 1991a).
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