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Quantitative and qualitative aspects of reproduction in the giant scallop Placopecten magellanicus (Gmelin) were compared between populations in the Damariscotta River (13 to 20 m depth) and the Gulf of Maine (170 to 180m depth). Gonad weight (p < 0.005) and gonad index (p < 0.01) were significantly greater in the shallow-water population dunng the period of gametogenesis. Oocytes of scallops in the shallow-water population developed gradually and spawning occurred in the fall. In spite of reduced gonadal production, scallops at the deep-water site completed gametogenesis. Oogenesis in the deep-water population was characterized by a greater initial rate of oocyte growth, resorption and a possible minor spring spawning, a period of redevelopment with continued resorption, and a later, more abrupt fall spawning. Gametogenesis was generally less synchronous among individuals in the deep-water population and oocytes were less dense withn gonadal tissue. The reduction in gonad production at the deep-water site was manifested as a reduced fecundity (number of eggs), rather than reduced egg size, as mean oocyte bameters were similar for both populations. In spite of apparent energehc hmitations, scallops at 180 m produce and spawn apparently viable ova, but in considerably lower numbers than those from shallower depths.

Reduced fecundity in a deep-water population of the giant scallop Placopecten magellanicus in the Gulf of Maine, USA