Temporal variation in gametogenic cycles of vesicomyid clams

Abstract

Summary Sulfide-rich cold seeps in Monterey Canyon support dense communities of vesicomyid clams, which harbor chemosynthetic bacterial endosymbionts. Because these animals rely upon non-photosynthetic food sources, their life histories may be decoupled from seasonal phytoplankton productivity. We examined this hypothesis by investigating temporal changes in gametogenesis in two vesicomyid clam species. Clams were collected from two cold seeps (600 m and 900 m depths), and their gonads were removed and processed for histological analyses. Oocyte diameters and relative abundances of tissue types were measured in thin sections taken across the gonad. Mean oocyte diameters of Calyptogena kilmeri increased from August to November, 1994, but did not change by March, 1995. Relative 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 or (2) reproductive output is tied to factors other than food production, such as synchrony of spawning to maximize fertilization success, or larval survival.