Most polychaetes can regenerate to some degree – from regenerating lost appendages to posterior body segments. Some polychaetes reproduce asexually by breaking into two or more groups of segments, and some reproduce very efficiently this way by “multiple fragmentation”, in which each segment becomes a new individual (for example Dodecaceria (Cirratulidae)).
Polychaetes are almost all dioecious, although they do not have distinct gonads. Instead, patches of the peritoneum lining the coelem (in one or many segments, depending on the taxon) divide to produce prospective gametes, which then break off and fully mature into eggs or sperm in the coelom. Eggs and sperm are released from the worm’s coelom through nephridial or coelomic ducts to the outside, or through breaks in the worm’s body wall during spawning. Most polychaetes have external fertilization, although some species brood their young. In order to maximize fertilization some primarily-benthic polychaetes (characteristic in the families Nereidae, Eunicidae, and Sillidae) spawn while swarming at the top of the water column. To do this, these worms transform into a swimming, sexual form quite different from the benthic form (called epitoky). One way this is done, as seen in members of the families Nereidae and Eunicidae, is by completely transforming the whole body into a sexual individual. Some notable modifications include enlarging swimming parapodia at the anterior end and often developing large eyes. Epitoke formation is stimulated by hormones in the brain, which are found only in older worms. Another method is to bud off the posterior portion of the body to form the sexual epitoke. This happens in the Syllidae. A third method of forming a sexual form is for a hind portion with the gametes to break off. This is not a true epitoke because the swimming section is not a complete worm. Palola viridis (Eunicidae) is an example of a species that makes this type of swarmer. Lunar cycles trigger the swarming event, and this species swarms in such huge numbers that natives of the samoan islands, where they are found, collect and feast on them (Kozloff 1990; Brusca and Brusca 2003)
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