Epitoky

Abstract: 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.

Abstract: In the gonochoric polychaetous annelid Eunice siciliensis Grube, germ cells only develop in the posterior segments of the worm; these segments subsequently differentiate into an epitokous portion separating at maturity from the infertile body part. As in the Pacific and Atlantic palolo worms, the epitokous portions die some hours after spawning. Laboratory investigations revealed that the anterior parts of spawned worms survive and regenerate genital segments. After a period of at least 8 months, paired gonads are developed in these segments. Gonia proliferating later from the germinal tissue are released from gonadal sacs into the coelomal cavities, where gamete development occurs. A reproductive epitokous portion does not mature and separate until the second year of cultivation; it is followed by caudal regeneration. These facts and the observation that only about 50% of the individuals obtained from samples of coralligene were sexually differentiated, led to the conclusion that E. siciliensis probably has a bi-annual reproductive cycle. Caudal regeneration does not depend on the presence of a prostomia hormone. The ability to form posterior regenerates is inherent to the parapodial segments of all body regions, except for mature genital segments. This is also valid for the formation of morphologically different secondary prostomia. However, for initiation and further promotion of gametogenesis and epitokous development, at least one gonadotropic factor originating from the prostomium is required. Neurosecretory cells within the brain suggest this factor(s) to be of neuro-endocrine nature. Specimens developing secondary prostomia were found both under field and laboratory conditions. In several cases, sexual development took place, and even epitokous portions were separated from such individuals. From this it appears that the gonadotropic functions are re-established by the regenerated prostomia in order to ensure reproduction in individuals accidentally deprived of the anterior end. The possible role of temperature and light as environmental factors involved in the control of normal and regenerative growth, of sexual development and reproduction is discussed.

Abstract: The phenomenon of epitoky in polychaetes, in which maturing individuals of benthic species undergo metamorphosis and then swim to the surface of the sea for spawning, has been extensively documented (see Fage & Legendre, 1927; Clark, 1961 for references). The death of individuals after spawning has been widely recorded and has been assumed to apply to all swarming species of polychaetes. However, Herpin (1925) recorded the survival of Pionosyllis lamelligera and Eusyllis blomstrandi after they had swarmed and spawned. He noted the loss of the swimming chaetae present at the time of spawning and that the individuals survived for some time. Goodrich (1933) found that individuals of Odontosyllis enopla survived swarming and considered that they might live to breed again.