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

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.