Most Antarctic bivalves are small and protect their young by holding fertilized eggs or larvae in their mantle cavities for varying periods. Nourishment for these early growth stages is provided by yolk reserves rather than by planktotrophy. The anomalously large Antarctic scallop, Adamussium colbecki, has unprotected planktotrophic larvae that are spawned during the austral spring. Successful recruitment of these larvae, in populations which are most abundant in oligotrophic habitats, may be associated with episodic pulses of organic material. Reasons why planktotrophy persists in A. colbecki are suggested by a comparison with another large Antarctic bivalve, Laternula elliptica. The latter has protected lecithotrophic larvae that are released at the beginning of the austral winter. This comparison suggests that unprotected larval development persists in A. colbecki because of unusual anatomical and ecological adaptations among the adults of the Adamussium lineage that have been evolving in the Southern Ocean since the early Oligocene.