Remarks: According to CHEVALDONNÉ et al. (1998) B. seepensis from the Gulf of Mexico and B. seepensis from Mid-Atlantic Ridge appear to have recently diverged and must be considered as two isolated phylogenetic species, although no morphological differences have been found on large series of specimens.

The body is short and spindle-shaped, slightly tapering towards the posterior and anterior, flattened ventrally and strongly arched dorsally. Fully developed worms have 21 segments, the first of which is achaetous. There are 10 pairs of elytra attached to large, low elytrophores on segments 2, 4, 5, 7, 9, 11, 13, 15, 17, and 19. The elytra are moderately large and oval. They cover the lateral third of the body on larger adults and nearly cover the entire body of juveniles. The elytra are oval, opaque, smooth, soft and attached near the middle on the first pair and eccentrically attached near their anterior borders on following elytra, sometimes showing branched 'veins'. Non-elytra bearing segments have dorsal cirri, with short cylindrical cirrophores and short styles with slender tips that extend to about the tips of the neurosetae. The dorsal tubercles on the cirrigerous segments are indistinct. The branchiae are well developed and arborescent, with rather short terminal filaments. Branchiae emerge from the body in two short trunks, with a lower smaller group attached to the dorsal bases of the notopodia or the cirrophores of the dorsal cirri, and an upper larger group attached to the lateral bases of the elytrophores or the dorsal tubercles. The branches of the branchiae are somewhat flattened and spread anteriorly and posteriorly between the parapodia. The branchiae begin on segment 3 as two small groups, becoming larger with more numerous branches in the middle region, and smaller again posteriorly. The branchiae are smaller on smaller adults. On juveniles, the branchiae are very small and incompletely developed.

The prostomiun is oval and bilobed, with rounded anterior lobes that lack frontal filaments. The median antenna lack a distinct ceratophore, and are inserted in an anterior notch. The median antenna are short, conical and with slender tips. The ventrolateral palps are rather short, thick, smooth and tapered, extending beyond the prostomium. Lateral antennae and eyes are lacking. The first or tentacular segment is fused to the prostomium and not distinct dorsally. The tentaculophores are lateral to the prostomium, and are achaetous, short and bulbous, each with a pair of short cirriform tentacular cirri with slender tips, about the same length as the palps. The second or buccal segment bears the first pair of large elytrophores and elytra, biramous parapodia with smaller conical notopodia with up to 9 notosetae, and larger rounded neuropodia with numerous slender neuroseate and lower buccal cirri attached basally, similar to the the tentacular cirri. The mouth is ventral with lateral and posterior lips, enclosed in tentacular and buccal segments. A thick muscular pharynx was extended on one paratype. When dissected, 5 pairs of small, delicate, oval papillae were seen around the opening, in addition to 2 pairs of small jaws that lack denticulated bases.

The biramous parapodia have short conical notopodia and long conical neuropodia. The notosetae are moderate in number (up to 20 on adults and 5 on juveniles), short to long in length, extending nearly to the tips of neuropodia. The notosetae are stouter than neurosetae, acicular, smooth or slightly roughened distally, with rounded tips. The presetal acicular lobes of the neuropodia are slightl;y longer than the postsetal lobes. The neurosetae are very numerous, long, slender and are separated by a slight gap into upper supra-acicular and lower subacicular groups. The upper neurosetae are slightly stouter than the lower ones, minutely serrated and with blunt tips, some possessing a terminal bulb. The lower neurosetae are more slender and have hooked tips with some longer denticles. The ventral cirri, attached to the middle of the parapodia, are short, tapered, and do not reach the tips of the neuropodia.

The pygidium is rather long and bulbous, with a terminal anus and a pair of short stout anal cirri that are basally fused. The pygidium is enclosed in the parapodia of the last two posterior segments and their long dorsal cirri. Distinct segmental or nephridial papillae are nor present, except for 2 pairs of long ventral papillae on segments 11 and 12 that project posteriorly. The papillae are shorter on smaller adults, and absent on juveniles.

(Pettibone, 1986)

Commensal within the mantle cavity of mussels. Two individuals (male and female) are frequently observed within a single mussel. Sexual dimorphism with females larger than males; females having two pairs of ventral papillae on segments 11 and 12. Sex ratio 0.5-0.7: 1. Females contain mature sperm and there is evidence of internal fertilization. Inferred mode of development lecitotrophic or direct. Asynchronous gametogenesis, rapid oogenesis.

Types found associated with mussles near hypersaline seeps at the base of the Florida Escarpment, Gulf of Mexico, in dark iron-sulfide sediment at 3266 m depth (26°02.24`N, 84°54.48`W) and at 3270 m depth (26°02`N, 84°54.48`W) (Pettibone, 1986). Also recorded from hydrothermal vents at Lucky Strike on the Mid-Atlantic Ridge.

Hypersaline seeps at the base of Florida Escarpment, 26°02?24??N and 84°54?48??W; Mid-Atlantic Ridge: Lucky Strike.

Body short, spindle-shaped, tapered anteriorly and posteriorly, flattened ventrally and arched dorsally. 21 segments with 10 pairs of elytra attached on segments 2, 4, 5, 7, 9, 11, 13, 15, 17 and 19. Elytra moderatly large, oval, covering lateral thirds of body on larger specimens and nearly covering body of juveniles. Branchiae well developped, arborescent. Prostomium oval, bilobed, with rounded anterior lobes lacking frontal filaments. Median antenna lacking distinct ceratophore. Palp rather short, thick, smooth and tapered. Thick muscular pharynx, five pairs of small delicate oval papillae around opening and two pairs of small jaws lacking denticulate base. Lower neurochaetae slender and having hooked tips with some longer lateral denticles.

Up to 31 mm in length and 13 mm in width.

Paratype for Branchipolynoe seepensis Pettibone, 1986
Catalog Number: USNM 98782
Collection: Smithsonian Institution, National Museum of Natural History, Department of Invertebrate Zoology
Preparation: Alcohol (Ethanol)
Collector(s): Woods Hole Oceanographic Institution
Year Collected: 1984
Locality: At Base Of Florida Escarpment, Florida, United States, Gulf of Mexico, North Atlantic Ocean
Depth (m): 3266 to 3266
Vessel: Alvin DSR/V

Holotype for Branchipolynoe seepensis Pettibone, 1986
Catalog Number: USNM 98779
Collection: Smithsonian Institution, National Museum of Natural History, Department of Invertebrate Zoology
Preparation: Alcohol (Ethanol)
Collector(s): Woods Hole Oceanographic Institution
Year Collected: 1984
Locality: Florida Seep Site, Florida, United States, Gulf of Mexico, North Atlantic Ocean
Depth (m): 3270 to 3270
Vessel: Alvin DSR/V

Paratype for Branchipolynoe seepensis Pettibone, 1986
Catalog Number: USNM 98780
Collection: Smithsonian Institution, National Museum of Natural History, Department of Invertebrate Zoology
Preparation: Alcohol (Ethanol)
Collector(s): Woods Hole Oceanographic Institution
Year Collected: 1984
Locality: Florida Seep Site, Florida, United States, Gulf of Mexico, North Atlantic Ocean
Depth (m): 3270 to 3270
Vessel: Alvin DSR/V

Paratype for Branchipolynoe seepensis Pettibone, 1986
Catalog Number: USNM 98781
Collection: Smithsonian Institution, National Museum of Natural History, Department of Invertebrate Zoology
Preparation: Alcohol (Ethanol)
Collector(s): Woods Hole Oceanographic Institution
Year Collected: 1984
Locality: At Base Of Florida Escarpment, Florida, United States, Gulf of Mexico, North Atlantic Ocean
Depth (m): 3270 to 3270
Vessel: Alvin DSR/V

Depth range based on 84 specimens in 1 taxon.
Water temperature and chemistry ranges based on 75 samples.

Environmental ranges
  Depth range (m): 549 - 3500
  Temperature range (°C): 2.588 - 9.211
  Nitrate (umol/L): 17.840 - 29.021
  Salinity (PPS): 34.924 - 35.419
  Oxygen (ml/l): 2.846 - 6.060
  Phosphate (umol/l): 1.114 - 1.891
  Silicate (umol/l): 10.356 - 40.606

Graphical representation

Depth range (m): 549 - 3500

Temperature range (°C): 2.588 - 9.211

Nitrate (umol/L): 17.840 - 29.021

Salinity (PPS): 34.924 - 35.419

Oxygen (ml/l): 2.846 - 6.060

Phosphate (umol/l): 1.114 - 1.891

Silicate (umol/l): 10.356 - 40.606
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

Commensal with deep-sea mussles.

B. seepensis from the Gulf of Mexico and B. seepensis from the Mid-Atlantic ridge appear to have recently diverged and must be considered as two isolated phylogenetic species, although no morphological differences have been found on a large series of specimens (Chevaldonne et al., 1998).