A New Scale-worm Commensal with Deep-sea Mussels in the Seep-sites at the Florida Escarpment in the Eastern Gulf of Mexico (polychaeta: Polynoidae: Branchipolynoinae)

•Some polynoid polychaetes found living commensally in mantle cavities of deep-sea mussels at the seep-sites near the Florida Escarpment, are described as a new species of Branchipolynoe, B. seepensis. They show some differences from the previously described B. symmytilida, commensal with deep-sea mussels in the Galapagos hydrothermal vents. Among the abundant biological communities recently discovered in the abyssal eastern Gulf of Mexico near hypersaline seep-sites were large dense mats of mussels (Florida Escarpment Cruise Participants 1984, Pauli et al. 1984, Turner and Lutz 1984, Denoux et al. 1985, Hecker 1985). They were collected in March 1984 during four dives of the submersible Alvin in 32663270 meters (26°02'N, M°55'W). The mussels were found in dark iron-sulfide sediments at near-ambient temperatures in an area where dense hypersaline fluids seep out onto the sea floor. The communities were similar in many respects to the prolific communities found around the Galapagos hydrothermal vents and elsewhere in the Pacific. Some polynoid polychaetes were found living commensally in the mantle cavities of these mussels. Six specimens were given to me for study by Dr. Barbara Hecker: five adults removed from the mussels by Dr. Ruth D. Turner, and a juvenile collected from washings of a clump of mussels. It was of particular interest to compare these polynoids, commensal with the unnamed, non-hydrothermal, lai^e. Gulf of Mexico mussels (to be described and named by Ruth Turner and Barry Wilson), with the polynoids commensal with the large mussels in the Galapagos hydrothermal vents. recently described under the name Bathymodiolus thertnophilus by Kenk and Wilson (1985). The Galapagos polynoids were referred earlier to a new genus and species, Branchipolynoe symmytilida, in a new subfamily of the Polynoidae, Branchipolynoinae (Pettibone 1984). The Gulf of Mexico commensals are referred herein to a new species of Branchipolynoe, B. seepensis. The types are deposited in the National Museum of Natural History, Smithsonian Institution (USNM). Branchipolynoinae Pettibone, 1984 Branchipolynoe Pettibone, 1984 Type species.•Branchipolynoe symmytilida Pettibone, 1984. The generic diagnosis is emended to include Branchipolynoe seepensis, new species, commensal with deep-sea mussels in the Gulf of Mexico. Diagnosis, emended.•Bianchiae smaU to well developed, arborescent, on all parapodia from segment 2 or 3. Prostomium bilobed, with anterior lobes subtriangular or rounded, with or without minute frontal filaments; median antenna with ceratophore indistinct, style short; palps short to long. First or tentacular segment visible dorsally as short ring or fused to prostomium. Parapodia subbiramous or biramous; notopoVOLUME 99, NUMBER 3 445 dia small, nodular or conical with projecting acicular process; notosetae few (0-3) or moderate in number (5-20). Neuropodia with or without truncate lobes enclosing conical acicular lobes; neurosetae very numerous, upper ones much stouter or only slightly stouter than lower ones. Commensal with deep-sea mussels at hydrothermal vents or hypersaline seep-sites. Key to the Species of Branchipolynoe 1. Elytra small, leaving greater part of dorsum uncovered (Pettibone 1984, figs. 1, 2A). Arborescent branchiae beginning on segment 2 (Pettibone 1984, figs. 2A, 3A). Anterior lobes of prostomium with minute frontal filaments (Pettibone 1984, ñg. 2A). First or tentacular segment visible dorsally as small ring (Pettibone 1984, ñg. 2A). Parapodia subbiramous, with notopodia small and 0 to few notosetae (Pettibone 1984, fig. 3F). Neuropodia with truncate lobes enclosing conical acicular lobes (Pettibone 1984, fig. 3F). Upper neurosetae much stouter than lower neurosetae (Pettibone 1984, ñg. 31, K). Ventral papillae on segments 11 and 12 short (Pettibone 1984, fig. 4C). Commensal with deep-sea mussels in Galapagos hydrothermal vents . B. symmytilida Pettibone, 1984 Elytra large, leaving middle third of body uncovered (Fig. lA). Arborescent branchiae beginning on segment 3 (Fig. 2B). Anterior lobes of prostomium without frontal filaments (Fig. lA). Tentacular segment fused to prostomium, not visible dorsally as separate ring (Fig. lA). Parapodia biramous, with notopodia large, conical, with moderate number of notosetae (Fig. 2C, D). Neuropodia without truncate lobes enclosing conical acicular lobes (Fig. 2D). Upper neurosetae only slightly stouter than lower neurosetae (Fig. 2H, I). Ventral papillae on segments 11 and 12 long (Fig. ID). Commensal with deep-sea mussels in hypersaline seeps in Gulf of Mexico B. seepensis, new species Branchipolynoe seepensis, new species Figs. 1-4 Material. •Gulf of Mexico, from dives of Alvin in 1984, associated with mussels near hypersaline seeps at base of Florida Escarpment, in dark iron-sulfide sediment: Dive 1343,9 Mar, 26°02'24"N, 84''54'48"W, 3266 m, from washings of clump of mussels, juvenile paratype (USNM 98782).•Dive 1344, 12 Mar, 26°02'N, 84°54' 48"W, 3270 m, from mantle cavities of mussels, holotype (USNM 98779), 2 paratypes (USNM 98780).-Dive 1346, same data, 2 para-