A NEW GENUS AND SPECIES OF AN ORBINIID-LIKE POLYCHAETE FROM METHANE SEEP COMMUNITIES IN THE GULF OF MEXICO
Blake, J. A.
ENSR, 89 Water Street, Woods Hole, Massachusetts, 02543 USA
Writhing masses of a large polychaete have been found in association with methane seeps (500 m+) in the Gulf of Mexico. Character states suggest an orbiniid relationship. Worms large, up to 15 cm long, 7-9 mm wide anteriorly, with hundreds of crowded segments. Body widest anteriorly, gradually tapering posteriorly, not divided into distinct regions. Branched branchiae from setiger 5, continuing to posterior end, most numerous anteriorly. Branchiae with central blood vessel with numerous, fine lateral branches. Prostomium narrow, elongate, directed ventrally, bluntly rounded on anterior margin, superficially resembling capitellids; eyes absent; eversible nuchal papillae present. Peristomium single, narrow achaetous ring. Setiger 1 and those immediately following with long, laterally directed cirriform noto- and neuropodial lamellae superficially resembling tentacular cirri; these arising from single point of origin; after about 10-15 segments, podia separate, but never very far, continuing for length of body; posteriorly, both lamellae becoming shorter, with neuropodial lamella reduced to about one-fourth length of dorsal one. Posterior parapodia not shifted dorsally. Anterior notosetae longer, more numerous than neurosetae, consisting of long, orbiniid-like bristled capillaries and narrow spines with bluntly pointed tips; spines with single subapical spur or secondary tooth. Neurosetae few threadlike capillaries and about 10 large, slightly curved spines with weakly developed crenulations along margin; neuropodial spines several times thicker than in notopodia. Pygidium with terminal anus surrounded by several long, tapering cirri. These worms are allied to the Orbiniidae despite the capitellid-like prostomium, lack of distinct body regions, and laterally positioned parapodia. Some small orbiniids (e.g., Orbiniella, Microrbinia) also lack distinct body regions. Setae are orbiniid-like. Branchiae with a central blood vessel and numerous side branches occur in orbiniids and paraonids. Branched gills are known for two species of Naineris. Lack of distinct body regions and the simple nature of parapodia coupled with a more or less uniform setal morphology and distribution suggest that the seepworm exhibits plesiomorphic character states that may shed light on the relationships of orbiniids with other polychaete families.