Reproductive and developmental phenomena in annelids: a source of exemplary research problems

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Specific traits of reproduction, of early and postembryonic development and of sexual maturation in annelids are reported, and the existing and expected contributions to this field from the study of annelids are discussed. The study of early development, as in other spiralians, reveals the existence of canonical cleavage patterns and the combined action of antithetical principles, namely determination by stereotypic sorting-out of 'determinants′ and determination depending upon the interactions between the blastomeres. A high potential of information about the processes of metameric trunk segment formation and of segment specification has only begun to be exploited in annelids. Epitoky, the formation of a free-swimming sexual form in polychaetes, involving tissue transdifferentiation and conversion of metabolism, locomotory and sensory capacities, is discussed as a mostly one-way developmental process. The study of epitoky sheds light on the metabolic relations between soma and germ cells and on related control mechanisms. The members of epitokous species are synchronized by meteorological parameters and by pheromones and are adapted to spawn under pelagic conditions. The metameric construction of the trunk predisposes for asexual reproduction by fission into trunk fragments regenerating into complete worms. This mode of reproduction is frequent among polychaetes and oligochaetes. Trunk fission has been modified in many polychaetes: posterior fragments ('stolons′) are formed, which take over the function of pelagic, epitokous sexuals and leave behind a 'stock′ which lacks somatic sexual differentiation and can bud further stolons. Naturally occurring, as well as experimentally induced regeneration, as well as the stolonization phenomenon, reveal the existence of morphogenetic gradients and of positional information along the annelid trunk.

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