Variability of segment number and regeneration in Spiochaetopterus solitarius. Comparison with S-costarum (Polychaeta : Chaetopteridae)

The variability in the number of segments in anterior (A) and median (B) re gions of adult-size specimens Spiochaetopterus solitarius (Chaetopteridae), was examined in the present study. Variability of region A is considered i n relation to the number of specialized setae in A4, which varies from 2 to 4; variability of region B is observed as a decrease in the number of segm ents during a limited part of the year. A process involving autotomy and re generation has been advanced to explain the variability in the number of se gments in region B. It appears that, once the number of segments in region B reaches a certain level, which is close to 30, the animal divides within region B, and the missing parts of each section are then regenerated. The m aximum number of segments in region B may be considered to be species speci fic and used as a discriminative feature in the genus. It is suggested that regeneration does not reflect asexual multiplication per se, but provides a means to facilitate the supply of energy to region B, which is responsibl e for bringing the products of the sexual organs to maturity. Consequently, regeneration is considered to complement sexual reproduction, facilitating the availability of energy to region B for reproduction. Implications of t his variability have been examined by comparing S. solitarius and S. costar um, two species closely related by their morphology, sympatry and syntopy.