TEMPORAL VARIATION IN GAMETOGENIC CYCLES OF VESICOMYID CLAMS

Sulfide-rich cold seeps in Monterey Canyon support dense communities o f vesicomyid clams, which harbor chemosynthetic bacterial endosymbiont s. Because these animals rely upon nonphotosynthetic food sources, the ir life histories may be decoupled from seasonal phytoplankton product ivity. We examined this hypothesis by investigating temporal changes i n gametogenesis in two vesicomyid clam species. Clams were collected f rom two cold seeps (600 m and 900 m depths), and their gonads were rem oved and processed for histological analyses. Oocyte diameters and rel ative abundances of tissue types were measured in thin sections taken across the gonad. Mean oocyte diameters of Calyptogena kilmeri increas ed from August to November, 1994, but did not change by March, 1995. R elative proportions of ovarian tissue types (reproductive vs. somatic) also changed during this period, indicating reproductive seasonality. Analyses of Calyptogena pacifica tissues showed no significant change within a 3-month period. Existence of seasonal cycles for vesicomyid clams suggests that either (1) chemosynthetic production is seasonal o r (2) reproductive output is tied to factors other than food productio n, such as synchrony of spawning to maximize fertilization success, or larval survival.