. The Biological bulletin. Biology; Zoology; Marine biology. NEW REPRODUCTIVE STRUCTURE IN BRACHYURA 337 must receive sperm from more than one male (either in a single molting/mating cycle or by storing sperm across cycles), and (3) the retained sperm must be used in fer- tilization (Parker, 1970). All female brachyurans have ei- ther ecto-mesodermally (spermatheca) or ectodermally derived (thelycum) sperm storage organs (HartnoU, 1968). Diesel (1991) divided spermathecae into two morpholog- ical types; dorsal and ventral. The ventral type, occurring in the Calappidae, Geryonidae, Leucosiidae,
. The Biological bulletin. Biology; Zoology; Marine biology. NEW REPRODUCTIVE STRUCTURE IN BRACHYURA 337 must receive sperm from more than one male (either in a single molting/mating cycle or by storing sperm across cycles), and (3) the retained sperm must be used in fer- tilization (Parker, 1970). All female brachyurans have ei- ther ecto-mesodermally (spermatheca) or ectodermally derived (thelycum) sperm storage organs (HartnoU, 1968). Diesel (1991) divided spermathecae into two morpholog- ical types; dorsal and ventral. The ventral type, occurring in the Calappidae, Geryonidae, Leucosiidae, Partheno- pidae, Parathelphusidae, Corystidae, Ocypodidae, Maji- dae (see Diesel, 1991 for references), and Menippidae (Wilber, 1989), is a 'cul-de-sac' design with oviduct and vagina opening ventrally in close proximity to each other. The last male to inseminate a female should have the advantage (last male sperm precedence) because his sperm are placed nearest the oviduct, physically blocking access of rival sperm to the emerging oocytes. In dorsal-type spermathecae, the dorsally opening oviduct and the ven- trally opening vagina are widely separated from each other, and the first male to inseminate the female is hy- pothesized to have the advantage (first male sperm pre- cedence); oocytes encounter his sperm first. The dorsal type occurs in Portunidae and Pilumnidae and was pre- dicted in the Cancridae (Diesel, 1991). Sperm competition creates two opposing selective forces on males: (1) mechanisms for pre-emption (replacement or displacement) of stored rival male sperm, and (2) ad- aptations that resist pre-emption by rival males (Parker, 1970). The gonopods of the snow crab, Chionoecetes op- ilio. are equipped with backward pointing processes and it is possible that they are used to remove sperm previously placed in the female's spermathecae (Beninger et ai, 1991); such a mechanism has been demonstrated in dam- selflies (Waage, 1979). Brachyuran sperm plugs may pre- vent s
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