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

Abstract

The ultrastructural features of gametogenesis have been described in the methane-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-seep bivalve. B. childressi is gonochoric, and both the testis and ovary consist of acini surrounded by inter-acinal tissue composed of adipogranular cells that serve a nutrient storage function. Oocytes develop in close association with squamous follicle cells although the follicle cells do not appear 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 exocytosed as part of egg envelope biogenesis. Vitellogenesis appears to be largely autosynthetic involving the Golgi complex and rough endoplasmic reticulum. Sperm differentiation resembles that reported in other mytilid bivalves and includes the differentiation of proacrosomal granules and a flagellum during the early stages of spermatogenesis. Mature sperm have a cap-like acrosome, substantial subacrosomal material, a bullet-shaped nucleus, and four to five mitochondria in the midpiece. The general features of gametogenesis closely resemble those reported in shallow-water, seasonally reproducing mytilids. Despite the relatively stable methane seep environment and presumed continuous availability of nutrients, reproduction parallels that of littoral mytilids, suggesting some phylogenetic constraints on the capacity for variability in gametogenic processes in the family Mytilidae.