Abstract
Abstract Protobranch bivalve species of the family Nuculidae pass through either a planktonic lecithotrophic larval stage or a direct non-planktonic development. Oogenesis of the three sublittoral species examined is synchronous. Deposition of egg masses by Nucula delphinodonta and spawning by Nucula annulata and Nucula proxima occur only during summer months. Among the four bathyal and abyssal species, Ennucula similis , Ennucula granulosa , Deminucula atacellana , and Brevinucula verrilli , oogenesis is asynchronous and there is no discernable pattern of periodicity of spawning. Absence of periodicity in reproduction in these deep-sea species is confirmed by examination of individuals from dredge samples taken at different times of the year. The median apparent fecundity among both sublittoral and deep-sea species is directly related to size (i.e. shell length) and age. Among the Nuculidae the median apparent fecundity is greater among sublittoral than bathyal and abyssal species. The geographic distribution of a species depends on its capacity to disperse. The dispersal of the planktonic lecithotrophic larvae of the sublittoral species N. annulata and N. proxima is limited to the continental shelf of the northwestern Atlantic by inshore bottom circulation and because these very small planktonic larvae ( D. atacellana .
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