The morphological modifications in the reproductive system related to the success in internal fertilization
remained in the majority of amniotic vertebrate males.
In the species examined, internal fertilization might be a mechanism by which gametes can avoid direct exposure to the potentially toxic vent waters (Fretter 1988, Waren & Bouchet 1989); although the sperm are exposed to the toxins, the length of exposure is surely less than that of a broadcast spawner.
It seems likely that in all six species that undergo internal fertilization, the oocytes are fertilized during their exit from the mantle cavity.
According to Michiels, the fertilizers without partner contact are less likely to careen into a violent conflict than are hermaphrodites with full-contact internal fertilization
In species with internal fertilization
, the increase in length of the flagellum is considered to be an adaptation to increase the propulsive activity of the spermatozoa along the female genital ducts.
"That is, it acquires a new form in each new species." This trend toward rapid divergence pops up in all major animal groups where internal fertilization
predominates, he says.
is consistent with the unusual sperm morphology (sperm are elongate and have spiral mitochondria and nuclei) in all known species (Gardiner and Jones, 1985; Cary et al., 1989), and also with the presence of external ciliary tracts (Fig.
in gymnolaemate bryozoans probably involves the entry of spawned sperm into the maternal coelom, and is not the result of eggs and sperm being produced in the same perivisceral coelom by hermaphroditic zooids (see Ryland and Bishop, 1993; Temkin, 1994, 1996).
This issue is moot with brooders, who by definition retain eggs and have internal fertilization
. However, a different set of problems merits further consideration.
The more well-known form of sperm competition involves animals with internal fertilization
in which sperm from several males compete to fertilize eggs of the same species [9, 1 0].
Rather, internal fertilization
with introsperm may be basal to the Bilateria (Buckland-Nicks and Scheltema, 1995).
The sperm midpiece in many species with internal fertilization
[e.g., mammals (Phillips, 1977) and viviparous teleosts (Grier, 1975)] has a long mitochondrial sheath or a long cytoplasmic sleeve that contains many separate mitochondria.