Spermathecae of Fabricia and Manayunkia (Sabellidae, Polychaeta)

The structure of te sperm storage organs found in females of the fabriciin sabellid species Manayunkia aestuarina and Fabricia stellaris is described. In both species, adult females have a pair of prostomial spermathecae, lying anterior and dorsal to the buccal opening, in the base of the radiolar crown. The spermathecae are epidermal structures; each ends blindly with a single opening into the surrounding water. In each spermatheca of M. aestuarina, a short duct from the narrow opening extends into the larger lumen of the ellipsoid spermathecal sac. The spermathecal cells have large nuclei and numerous dark (and electron-dense) granules. No cilia are present in any part of the spermathecae. The spermathecae are either completely empty, or contain 25-50 sperm. The spermathecae of F. stellaris are more complex than those of M. aestuarina and have three distinct regions. They closely resemble spermathecae previously described from another fabriciin, Parafabricia ventricingulata. The opening into each spermatheca leads ventrally into a ciliated chamber, or atrium, which extends posteriorly and laterally into the base of the radioles. The atrium terminates at the connecting piece which in turn leads to the heavily pigmented, but non-ciliated, sperm receptacle. The sperm receptacle is the most posterior region of each spermatheca and is dorsal to the connecting piece and atrium. Several hundred sperm can be found in each receptacle of F. stellaris. Comparing these spermathecae with those of other fabriciins, I conclude that sperm storage structures and mechanisms will provide useful characters in the systematics of the Fabriciinae. Additional key words: Fabriciinae, reproduction, ultrastructure Sabellids belonging to the subfamily Fabriciinae are normally 2-4 mm in length, and can be found occurring in excess of 1.5 X 106 individuals per m2 (Lewis 1968). Such population densities are, of course, related to the reproductive mode that these sabellids show. Rouse & Fitzhugh (1994) reviewed sexual reproduction across the Sabellidae and found a range of reproductive mechanisms in the subfamily Sabellinae. However, in all fabriciin species studied to date the females brood batches of directly developing larvae. The larvae subsequently disperse by crawling out of the parental tube before building their own tubes and beginning to feed (Leidy 1883; Forsman 1956; Rasmussen 1956; Lewis 1961; Marinescu 1964; Bell 1982; Knight-Jones & Bowden 1984; Rouse & Fitzhugh 1994). How females obtain sperm to fertilize the eggs spawned into their tubes has been the subject of some speculation. A form of pseudocopulation has been postulated in Fabricia stellaris (MOLLER 1774) by Franzen (1956, 1975), but it was shown by Kahmann (1984) that transfer of sperm from male to female does not require contact between individuals. Zenkevitsch (1925) also described pseudocopulation between sexes in another fabriciin, Manayunkia baicalensis (NUSBAUM 1901). In both of these species, and in Manayunkia aestuarina (BOURNE 1883), it has been noted that females have a pair of pigmented spermathecae in the radiolar crown dorso-lateral to the buccal opening (Zenkevitsch 1925; Daly & Golding 1977; Kahmann 1984). Rouse (1992) described the ultrastructure of spermathecae in another fabriciin, Parafabricia ventricingulata FITZHUGH 1992, and in several small members of the other sabellid subfamily, the Sabellinae. The mechanisms of sperm transfer between sexes have yet to be fully elucidated in any sabellid with sperm storage by females. However, I have shown (Rouse 1995) that males of the Fabriciinae have a dorsal sperm duct that opens behind the radiolar crown. The evidence of Kahmann (1984) and the fact that no spermatophores are formed (Rouse 1995) imply release of free sperm into the water. Thus, whether or not there is a form of pseudocopulation between sexes, sperm swim through the water and somehow enter the spermathecae. In this paper I describe the structure of the spermathecae of two fabriciin species, F. stellaris and M. This content downloaded from 157.55.39.104 on Mon, 20 Jun 2016 05:53:16 UTC All use subject to http://about.jstor.org/terms Spermathecae of sabellid polychaetes aestuarina. The reasons for this study are twofold. First, the position and morphology such structures will provide additional characters for cladistic analyses of the members of the Fabriciinae (e.g., Fitzhugh 1991; Fitzhugh et al. 1994). Second, any functional analysis of the complicated reproductive process seen in the Fabriciinae requires initial morphological analysis of the structures involved.