Gametogenesis, spawning behavior, and early development in the "iceworm" Hesiocaeca methanicola (Polychaeta:Hesionidae) from methane hydrates in the Gulf of Mexico

Gametogenesis, spawning behavior, and early development have been described in the methane-seep polychaete Hesiocaeca methanicola Desbruyeres and Toulmond, 1998, the first documentation of the reproductive biology of any cold-seep polychaete species. The worms were collected at the Green Canyon site in the Gulf of Mexico (27 degrees 44.77N; 91 degrees 13.33W) in August 1997. The gonads are situated in the neuropodia in association with an intraepithelial capillary system. Oogenesis is intraovarian, with oocytes remaining in the ovary until late vitellogenesis when they are released into the coelom for final growth and differentiation. Vitellogenesis involves both autosynthetic and heterosynthetic processes and, along with ovarian structure, suggests a relatively slow process of egg formation. Sperm differentiation is intertesticular until early spermiogenesis, when spermatid tetrads complete development in the coelom. The mature spermatozoon is an ect-aquasperm consisting of a tapering, digitate acrosome, a spherical nucleus, a midpiece containing from six to eight mitochondria and glycogen stores, and two centrioles associated with the flagellum. Both sexes undergo broadcast spawning, but the males spawn exclusively through the anus, a behavior previously unknown in polychaetes. Artificial fertilization resulted in spiral cleavage and development to the trochophore stage. Oocyte size-frequency analysis of adult females indicates a wide range of oocyte sizes and vitellogenic stages, suggesting that asynchronous gametogenesis occurs.