Hermaphroditism in mollusca

Hermaphroditism is defined by the production of eggs and spermatozoa by the same individual, either simultaneously or sequentially. A variety of expressions of this ambisexuality occurs among diverse animals, including: accidental (=occasional=developmental) hermaphroditism; protandry; protogyny; adolescent hermaphroditism; adolescent gonochorism; and alternating (=rhythmical); sequential hermaphroditism; at least one sex-reversal occurs in some life cycles. Heller (1993) elucidated the occurrences of hermaphroditism in Mollusca, applying the findings to the explanations and theories reviewed by Ghiselin (1974), viz., hermaphroditism is primitive, hermaphroditism allows an organism to fertilize itself, and a gene-dispersal model (low density and motility). Simultaneous hermaphroditism is more common among molluscs (especially in euthyneuran gastropods) than in sequential hermaphroditism (mostly among prosobranchs). Hermaphroditism is common among deep-sea, parasitic and Cnidaria-feeding molluscs, but it is not consistent with brooding. In contrast to those features of evolutionary ecology, the mechanisms of sex determination, differentiation and modification have received little attention, Guo et al. (1998) proposed that sex in Crassostrea gigas, a protandric species of oyster, is determined by a single pair of alleles, viz., a dominant allele (M) and a "protandric female' allele (F), with MF being true males and FF being young males that undergo sex-reversal to become females. In contrast, if the hermaphroditism of the bivalve Corbicula leana is genetically determined, it is accomplished by "hermaphroditic sperm" because not only the polar bodies but also the maternal pronucleus is expelled from the egg (Komaru et.al., 1998). The masculinization effect of tributyl tin pollution on female prosobranchs (Takeda, 2000) lends support to Bacci's (1965) view that molluscs are basically hermaphroditic, as do undifferentiated protogonia in bivalve gonads (Campioni & Sbrenna, 1996). The possible role(s) of neurotransmitters and neurohormones, among other moieties, in modifying the sexuality of molluscs (after stimulation by environmental cues) needs further attention (Motavkin & Varaksin, 1989; Smith & Croll, 1997)