Reference: Biol. Bull. 203: 134-143. (October 20(12) Spermiogenesis and Modified Sperm Morphology in the "Seepworm" Methanoaricia dendrobranchiata (Polychaeta: Orbiniidae) From a Methane Seep Environment in the Gulf of Mexico: Implications for Fertilization Biology KEVIN J. ECKELBARGER 1 AND CRAIG M. YOUNG 2 1 Darling Marine Center, 193 Clark's Cove Road, Walpole. Maine 04573; and 'Division of Marine Sciences, Harbor Branch Oceanographic Institution, 5600 U.S. 1 North, Ft. Pierce, Florida 34946 Abstract. Spermatogenesis and mature sperm morphol-ogy have been described along with limited observations of the ovary in Methanoaricia dendrobranchiata, an orbiniid polychaete associated with dense populations of the mussel Bathymodiolus childressi at brine pools on the Louisiana slope. Gulf of Mexico. The species is gonochoric with gonads serially repeated in numerous segments and each associated with a nexus of blood vessels at the base of the parapodia. In the female, synchronous, intraovarian egg development occurs with the release from the ovary of large, yolky eggs into the coelom at first meiotic metaphase. Sperm develop in the coelom as free-floating, plasmodial clones interconnected via an anuclear cytophore. At the end of spermiogenesis, mature spermatozoa float freely in the coelom. The mature spermatozoon differs significantly from that of shallow-water orbiniid species by possessing an elongated nucleus and a greatly elongated and curved aero-some reaching 19.5 /am in length. The spermatozoon re-sembles an ent-aquasperm and may not fertilize the eggs directly in seawater in the classical manner. We hypothesize that the unusual spermatozoon morphology in this species has evolved due to the hypoxic environment in which the adults live and that fertilization biology is likely modified in some way to minimize sperm exposure to high levels of hydrogen sulfide. An analysis of life-history features in shallow-water orbiniids is used to infer reproductive fea-tures in M. dendrobranchiata that could not be directly documented. Received 16 April 2002. accepted 10 July 2002. Introduction The reproductive biology of invertebrates from deep-sea hydrothermal vents and cold methane seeps continues to receive a great deal of attention (Tyler and Young, 1999). Hydrothermal vents and cold-water methane seep habitats show significant ecological differences, and the two kinds of communities have few species in common (Tunnicliffe. 1991). There are, however, strong taxonomic affinities be-tween seeps and vents, suggesting that divergent life-history features could evolve in closely related species. Studies on deep-sea polychaetes have been almost exclusively directed at hydrothermal vent species (reviewed in Eckelbarger et al., 2001 ) and have included only 3 of 80 polychaete fam-ilies. Polychaetes show a remarkable diversity of reproduc-tive attributes and life-history features (Schroeder and Her-mans, 1975; Wilson. 1991 ) and offer abundant opportunities to study the evolution of reproductive mechanisms. Despite the ecological importance of this group, the entire life history is known for only about 3<7r of known species (Giangrande, 1997). Recently, Eckelbarger et al. (2001) described the reproductive biology of the hesionid polychaete Hesiocaeca methanicola, the first such descrip-tion for any methane-seep polychaete. A recent series of papers describes the reproductive biology of other methane-seep invertebrates from the Gulf of Mexico, including oo-genesis (Eckelbarger and Young, 1997) and spermatogene-sis (Hodgson et al., 1998) in the neritid gastropod Bathynerita naticoidea, and gametogenesis in the seep mus-sel Bathymodiolus childressi (Eckelbarger and Young. 1999). 134
Spermiogenesis and Modified Sperm Morphology in the "Seepworm" Methanoaricia dendrobranchiata (Polychaeta: Orbiniidae) From a Methane Seep Environment in the Gulf of Mexico: Implications for Fertilization Biology
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