ABSTRACT
The vast majority of teleosts, estimated at 97%, are externally fertilizing whereby eggs and sperm are shed into the surrounding aqueous medium and fertilization takes place outside the body of the female (Pecio et al. 2007). This contrasts with insemination, whereby ripe males are able to transfer sperm to the reproductive tract of the female (Bums and Weitzman 2005). Insemination must take place in order for true internal fertilization to occur, and internal fertilization, in turn, is required for viviparity, where at least some stage of embryonic development occurs inside the female reproductive tract (Wourms 1981; see also Uribe Aranzabal et a l , Chapter 3). Prior to the work of Munehara et al. (1989), it had been assumed that all inseminating fishes were also internally fertilizing: for example, see tables in Breder and Rosen (1966). However, a novel reproductive mode termed "internal gametic association" was first shown to occur in species of the scorpaeniform family Cottidae (Munehara et a l 1989, 1991; Koya et al. 1993; see also Abe and Munehara, Volume 8B, Chapter 6). In the case of internal gametic association/males are able to transfer sperm to the female reproductive tract (insemination) where the spermatozoa enter (i.e., associate with) the micropyles of the "eggs"
(oocytes). The eggs, with their associated spermatozoa, are subsequently shed into the surrounding seawater which apparently stimulates the fertilization event (Munehara et al. 1989, 1991; Koya et al. 1993). Thus, in spite of there being internal delivery of the sperm (insemination), fertilization itself (i.e., union of sperm and egg) actually occurs externally in the seawater. Given this novel reproductive method, for those species whose female reproductive tracts have simply been shown to contain spermatozoa (Figs. 17.1 and 17.2), the more accurate term to describe this would be "insemination" until the actual time of fertilization can be determined. For many inseminating species, fertilization may indeed occur internally as ovulated eggs enter the lumen of the ovary where spermatozoa are stored. However, this entire process may take place very rapidly just prior to oviposition. Therefore confirmation of fertilization, as with histological methods, would require sacrifice of females
Fig. 17.1 Light micrographs of inseminated ostariophysan ovaries. A. Gephyrocharax valenciae (Characiformes, Characidae, Stevardiinae, Stevardiini). Standard length (SL) 32.6 mm, ANSP 112230, Venezuela. This ovary contained only previtellogenic oocytes. B. Trachelyopterus lucenai (Siluriformes, Auchenipteridae). SL 150 mm, MCP 18469, Brazil. C. Rachoviscus crassiceps (Characiformes, Characidae, incertae sedis). SL 24.1 mm, USNM 220756, Brazil. D. Xenurobrycon macropus (Characiformes, Characidae, Stevardiinae, Xenurobryconini). SL 14.9 mm, USNM 317053, Paraguay. Note the presence of intact spermatozeugmata. e, ovarian epithelium; f, flagellar portions of spermatozeugmata; I, ovarian lumen; o, oocyte cytoplasm; z, spermatozeugma; arrow, spermatozoa. Original.
