Dissogony

DISSOGONY (from Greek dissos-double and gone-offspring), a special form of reproduction, first described by Chun (1892) in certain comb jellies (Ctenophora), for example in Eucharis multicornis and Bolina hyatina. Dissogony consists in the fact that the animal reaches sexual maturity and lays fertilized eggs first while still in the larval state, and then a second time after metamorphosis of the larva, in the adult state. The sexual organs formed in the body of the larva subsequently regress and are formed anew in the adult animal. Thus, dissogony is the achievement by the same individual of sexual maturity at two different stages of the life cycle, separated from each other by metamorphosis. Later, dissogony was also discovered in some hydrozoans (from Campanularidae) and siphonophores. Apparently, dissogony is also characteristic of certain worms. Thus, some annelid worms from the family Nereidae occur in two forms that differ from each other in structure. One of these forms lives on the bottom and was formerly assigned to the genus Nereis, while the other leads a swimming or pelagic mode of life, differing in the well-developed parapodia and eyes. It was previously considered a separate genus—Heteronereis. Subsequently, the bottom form came to be regarded as the young, asexual stage, while the pelagic form was considered the result of modification of the bottom form when it transitions to a sexually mature state. According to research by Hempelmann (1911), the life cycle of the genus Nereis is actually much more complex: in some cases, the Nereis form, after reaching sexual maturity and sexual reproduction, can transform into the Heteronereis form and again form sexual products within itself; thus, approximately the same type of dissogony is observed here as in comb jellies.