Ultrastructure of gametogenesis in a chemosynthetic mytilid bivalve (Bathymodiolus childressi) from a bathyal, methane seep environment (northern Gulf of Mexico)

The ultrastructural features of gametogenesis have been described in the me thane-seep mytilid bivalve Bathymodiolus childressi Gustafson, Turner, Lutz & Vrijenhoek, 1998 collected from the Gulf of Mexico in August 1995. This is the first ultrastructural description of gametogenesis in any methane-se ep bivalve. B. childressi is gonochoric, and both the testis and ovary cons ist of acini surrounded by inter-acinal tissue composed of adipogranular ce lls that serve a nutrient storage function. Oocytes develop in close associ ation with squamous follicle cells although the follicle cells do not appea r to play a primary role in yolk synthesis. During the vitellogenic phase, biosynthesis of four types of organelles occurs, including lipid droplets, yolk granules, cortical granules, and unknown inclusions, which are exocyto sed as part of egg envelope biogenesis. Vitellogenesis appears to be largel y autosynthetic involving the Golgi complex and rough endoplasmic reticulum . Sperm differentiation resembles that reported in other mytilid bivalves a nd includes the differentiation of proacrosomal granules and a flagellum du ring the early stages of spermatogenesis. Mature sperm have a cap-like acro some, substantial subacrosomal material, a bullet-shaped nucleus, and four to live mitochondria in the midpiece. The general features of gametogenesis closely resemble:those reported in shallow-water, seasonally reproducing m ytilids, Despite the relatively stable methane seep environment and presume d continuous availability of nutrients, reproduction parallels that of litt oral mytilids, suggesting some phylogenetic constraints on the capacity for variability in gametogenic processes: in the family Mytilidae.