The ability to produce more than one kind of offspring, a phenomenon called poecilogony, is a striking example of reproductive variability. Traditionally, larval nutrition has been classified as a dichotomy: If offspring obtain nutrition from their mothers (lecithotrophy), there is lower fecundity and greater chance of offspring survival than when they get their nutrition from plankton (planktotrophy). The polychaete Boccardia proboscidea (Spionidae) produces both types of embryos using three different reproduction strategies. This species is an ideal system to explore implications and consequences of different reproductive strategies without confounding factors encountered in interspecific comparisons. In this thesis I studied B. proboscidea at four different levels (intracapsular, maternal, population and phylogeographic) and I used a combination of laboratory techniques, field observations and computer simulations to test hypotheses of life history evolution and intrafamily conflict in the sea. Results show that larval fate was determined early in development, even when larvae were grown in isolation. Larval growth was significantly faster at high concentrations of nurse eggs and at high temperatures. Plankototrophs suffered high rates of cannibalism and grew faster when provided with phytoplankton, demonstrating the high cost of their remaining in a capsule. The mother actively brooded and opened her capsules (embryos could not liberate themselves) and their brooding period was negatively correlated with temperature and not completely explained by the effect of temperature on embryos’ developmental rates. Females from higher latitudes provided offspring with larger supplies of extra embryonic nutrition and brood their offspring for longer periods than females from southern latitudes. Two genetic mitochondrial markers, 16S rDNA and Cyt b, showed a single North American West Coast network which is structured, geographically, by the well-documented biogeographic break near Point Conception, CA. The southern group within this network covers a smaller range, but has larger haplotype diversity, than the northern group. Some individuals differing in reproductive type had the same haplotype, indicating independence of haplotypes and reproductive type. Results of this study suggested that evolutionary history, maternal influences and sibling conflict, are playing a role in the maintenance of this reproductive polymorphism that might be acting as a bet hedging strategy.