A comparative analysis was carried out of reproductive mode, larval development pattern, and larval growth rate under identical conditions of three members of theCapitella capitataspecies complex:Capitellasp I from New York, US,Capitellasp M from shallow hydrothermal vents off Milos, Greece, andCapitellasp S from an intertidal flat on Sylt, Germany. In addition, three populations from salmon farms,CapitellaK (Kilmelford, Scotland),CapitellaCm andCapitellaCt (Cranford, Ireland), are described for the first time. Three development patterns were observed: planktotrophic (CapitellaK and Ct populations), lecithotrophic (Capitellaspp I, M and Cm populations), and direct development (Capitellasp S). Experiments with planktotrophic larvae from theCapitella Kpopulation at different temperatures, in the presence and absence of light and with several kinds of substrate were not successful in inducing settlement, although differences in the duration of larval survival were observed. Overall, major differences in reproductive modes and larval development patterns were observed among representatives of different species and populations reared under identical culture conditions. In contrast, such modes and patterns are consistent across generations. Whereas larval type and developmental mode appear to be determined largely by genetic factors, environmental conditions have an important influence on the timing of key developmental processes, such as settling and growth.