Reproductive Investment in Four Developmental Morphs of Streblospio (Polychaeta: Spionidae)

Per brood and per offspring C and N investment were examined in four developmental morphs of the spionid polychaete Streblospio: S. shrubsolii (direct development, D), S. benedicti (lecithotrophic, L), S. benedicti (planktotrophic, P), and Streblospio n. sp. (planktotrophic, P). Large differences were apparent among these morphs in fecundity and embryo size. S. shrubsolii (D) and S. benedicti (L) invested about 10 x more C and N in each brood in each offspring and 30% more C and N in each brood than did the morphs with planktotrophic development. C and N concentration (μg per unit volume) was significantly greater in S. benedicti (L) than in the other morphs, though no general relationship with embryo size was evident. The C:N ratio of offspring did not differ among the four morphs. Comparisons of estimated lifetime reproductive investment made by the two developmental types of S. benedicti indicated that lecithotrophic development involved twice the C and N investment in reproduction. Positive, significant regressions were evident between embryo C and N content and embryo volume at the intermorph level. Significant intra-morph regressions were evident in all morphs but Streblospio n. sp. (P). However, the large amount of variation unaccounted for by embryo volume calls into question the use of embryo size as a predictor of parental investment in offspring.