In most fishes, fertilization occurs outside the body of the female. Regardless, fertilization occurs when a sperm penetrates or is permitted to enter the egg membrane via a funnel-shaped hole in the membrane called the micropyle. The micropyle sits above the animal pole of the egg and is too narrow to allow the passage of more than one sperm.
After sperm entry, the micropyle closes and the chorion tends to harden, which prevents polyspermy or entry of more than one sperm. Sturgeon eggs have several micropyles and polyspermy occurs. Micropyle presence and size is diagnostic of different species. A zygote is formed when pronuclei of the sperm and egg fuse (Hoar 1969; Hempel 1979; Matarese & Sandknop 1984; Jamieson 1991).
In species with external fertilization, gametes remain viable for less than a minute to as long as an hour, depending on temperature; longer viability generally occurs in colder water (Hubbs 1967; Petersen et al. 1992; Trippel & Morgan 1994). Studies of the proportion of eggs fertilized during natural spawning events indicates that at least 75% and often 90–95% of the eggs released are fertilized (Petersen et al. 1992, 2001; Marconato & Shapiro 1996). This number varies directly as a function of the volume of sperm released by the male. In water column spawners such as wrasses and parrotfishes, males can control sperm expenditure in response to female size and competition, releasing more sperm when spawning with larger females that release more eggs, or when other males are simultaneously attempting to spawn. Ejaculate volume also increases in the face of competition in internally fertilizing species such as live-bearing mosquitofish (Evans et al. 2003). In benthic, territorial spawners, however, sperm expenditure does not appear to increase when females release more eggs (e.g., sticklebacks; Zbinden et al. 2001), or in the face of competition from other males (gobies and cyprinid bitterlings; Candolin & Reynolds 2002; Scaggiante et al. 2005).
Females of some internally fertilized species are able to store sperm in the ovary. In the Dwarf Perch, icrometrus minimus (Embiotocidae), newborn males are mature and inseminate but do not fertilize newborn females. The females store this sperm for 6–9 months until they mature and ovulate (Warner & Harlan 1982; Schultz 1993). Sperm storage is widespread in poeciliid livebearers, often involving more than one male partner (e.g., Evans & Magurran 2000; Luo et al. 2005). Some species store sperm and use it to fertilize multiple batches of eggs. Females of the Least Killifish, eterandria formosa, store sperm from one copulation for as long as 10 months and use it to fertilize as many as nine different developing broods of embryos, several of which may be developing simultaneously – a phenomenon known as superfetation.
In a few species, activation of cell division is not synonymous with fertilization. Some poeciliid livebearers are gynogenetic in that females use sperm from males of other species to activate cell division, but no male genetic material is actually incorporated into the zygote (see Gender roles in fishes). In some internally fertilized species, fertilization occurs but development may be arrested after a few cell divisions and then resumes when environmental conditions are more favorable for hatching. In some annual fishes, such as the South American and African rivulines, eggs are fertilized and then buried; they spend the dry season in a resting state known as diapause (Lowe-McConnell 1987; Deserts and other seasonally arid habitats).
Internal fertilization is universal among sharks but is limited to about a dozen families of bony fishes, most notably the coelacanths; a silurid catfish; brotulids; livebearers; goodeids; three genera of halfbeaks; four-eyed fishes; the neostethids and phallostethids of Southeast Asia; scorpaenids in the genera Sebastodes and Sebastes (e.g., Sebastes viviparus); Baikal oilfishes; embiotocid surfperches; an eel pout, Zoarces viviparus; clinids; and labrisomids. An Asian cyprinid, Puntius viviparus, was originally described as a live-bearer, but subsequent examination of the type material indicated that predation on cichlid young, which were contained in the stomach, had been mistaken for developing young in the ovary (attesting to the value of depositing type material and voucher specimens in museums; Collections).
Internal fertilization requires that males possess an intromittent organ for injecting sperm. This structure has different names and is derived from different structures in different taxa. The pelvic fin of elasmobranchs is modified into claspers; the pelvic girdle, postcleithrum, and pectoral pterygial elements form the priapium of phallostethoids; the anal fin forms the gonopodium of cyprinodotoids such as goodeids, anablepid four-eyed fishes, jenynsiids, and poeciliid livebearers; brotulids and surfperches have an enlarged genital papilla. In some cardinalfishes, the female purportedly inserts an enlarged urogenital papilla into the male to receive sperm (Hoar 1969).