Potamethus Chamberlin, 1919  

Potamethus sp. 1

Voucher. Sea Lion: station 40MFB.

Diagnosis. Large species; voucher is the only specimen encountered in this study; complete, with branchial crown; body 35 mm long, crown 17mm long, body 2mm wide; 8 thoracic and 39 abdominal chaetigers; chaetigers very crowded and very wide in posterior abdomen given the specimen somewhat swollen look towards the pygidium; colour in alcohol light tan to pink.

Branchial crown with 8 pairs of radioles, basally connected by a well-developed palmate membrane; radioles with pinnules to the tip; ventral filaments absent.

Collar most developed ventrally with distinctly elongate posterior peristomial ring, laterally and dorsally as a low lobes; dorsally forming two pockets, not overlapping at midline. Peristomial eyes not observed. Glandular ridge on chaetiger 2 present.

Collar uncini limbate notochaetae ranged in longitudinal rows. Thoracic superior notochaetae long, broadly limbate about 10 per fascicles and numerous, shorter mucronate chaetae arranged in 2 rows (around 9 chaetae per row). Thoracic uncini avicular with very long handles, breast as a pronounced swelling, about 14 per fascicle, arranged in single row, each with main fang and 4 small, diminishing in size teeth as seen in profile; with a row of smaller, hooked-shaped companion chaetae.

Abdominal neurochaetae composed of thin capillaries, around 6 per fascicle and row of 6 mucronate chaetae (narrower than thoracic mucronate chaetae); posterior neurochaetae change shape dramatically; in posterior abdominal segments neurochaetae as shorter and longer thickened (almost spine-like) narrowly limbate capillaries with very distinct long, thin mucrons. Abdominal uncini of anterior and mid abdominal chaetigers long handled, similar to thoracic unicni, around 20 per fascicle arranged in single row anterior and mid abdominal segments, reducing in numbers posteriorly.

Pygidium indistinct, very small triangular lobe; pygidial eyes not observed. Fragments of tube observed, composed of fine mud, greyish in colour, smooth without embedded particles.

Remarks. The FI specimen belong to genus Potamethus as they possess a girdle on chaetiger 2, and distinctly elongate posterior peristomial ring of the collar, although the form of thoracic uncini is distinctly avicular not acicular as otherwise common for species with girdle on chaetiger 2. Potamethus scotiae was described from Princess Martha Coast, around 2500m depth by Pixel (1913), however her original description was very brief (concentrating mostly on branchial crown and shape of collar, while at the same time providing drawings for chaetae only) and, therefore, of relatively little value. Nevertheless Pixell (1913) pictured and described only 1 type of abdominal neurochaetae, not 2 as clearly obvious in FI specimens. Knight-Jones (1983) re-examined holotypes and re-defined genus Potamethus. Based on drawings she provided for P. scotiae, the collar is not the same as for the one found in Falkland specimens. Knox and Cameron (1998) also provided description of P. scotiae, this time from the Ross Sea. The collar corresponds well with that of genus Potamethus, but they reported only one type of abdominal neurochaetae, as well as large number of radiolar pairs in comparison to FI specimens (15 versus 8).

List of species in genus Potamethus:

P. breviuncatus Hartmann-Schröder, 1977

P. dubius (Eliason, 1951)

P. elongatus (Treadwell, 1906)

P. filiformis Hartmann-Schröder, 1977

P. japonicus (Johansson, 1922)

P. malmgreni (Hansen, 1878)

P. mucronatus (Moore, 1923)

P. murrayi (McIntosh, 1916)

P. scotiae (Pixell, 1913)

P. singularis Hartman, 1965

P. spathiferus (Ehlers, 1887)