Fission in Convolutriloba longifissura:: asexual reproduction in acoelous turbellarians revisited

Studies of the asexual reproduction of Convolutriloba longifissura (Acoela, Acoelomorpha) revealed that there is no longitudinal fission of the whole animal as has been described by Bartolomaeus and Balzer (1997). Instead, the first step is a transverse fission. This results in the detachment of the caudal fourth of the mother animal. The detached part forms what we call the butterfly stage, which initially has no mouth and no eye fields. This stage gives rise to two new individuals by a longitudinal fission. Within 2-3 days the eye fields and a mouth develop in each of the two progenies formed in this way. In the meantime the mother individual grows and develops the three typical caudal lobes. The mother animal can repeat this process resulting in three individuals every fourth day. The finding of this new pattern of reproduction in the Acoela has prompted us to review the various ways by which asexual reproduction occurs in the group. The peculiar combination of few cases but high diversity of asexual reproduction in the Acoela is discussed from an evolutionary point of view.