Amythas membranifera Benham 1921

Amythas membranifera Benham, 1921

Figs 4, 5 A–E, J–L

Amythas membranifera Benham, 1921: 102–105, pl. 10, figs 124–132.

Amythas membranifera – Monro 1939: 141–142, fig. 24.

ANTARCTICA • 1 ♂ (BL = 55 mm, 20 AU); Terra Nova Bay; -74.67597º S, 164.24592º E; depth 400 m; 30 Jan. 2014; Schiaparelli leg.; station: DR5; XXIX PNRA expedition (2013-2014); MNA- 07940 • 1 ♂ (BL = 55 mm, 19 AU); same collection data as for preceding; MNA-07939 • 1 ♂ (BL = 13 mm, 17 AU); same collection data as for preceding; MNA-07935 • 1 ♂ (BL = 45 mm, 18 AU); same collection data as for preceding; MNA-07934 • 1 spec., undetermined sex; Princess Elizabeth Land; -67.05000º, 74.48333º; depth 437 m; station: 103; BMNH 1941.3.3.126–127.

BODY. Body (Fig. 4 A–C) short and thick. Length 13–55 mm, width 9–20 mm; girth of body spindleshaped, increasing to the middle of thorax, decreasing thereafter.

PROSTOMIUM. Prostomium trilobed, middle lobe anteriorly broadly rounded; any additional structures (nuchal organs, ridges, horns etc.) absent.

BUCCAL TENTACLES. Buccal tentacles numerous, short, smooth, attached behind a large-folded membrane. Lower lip broad, embracing mouth laterally and slightly latero-dorsally, covered by warts. Paleae totally absent.

BRANCHIAE. Three pairs of branchiae, the outermost one originating from TC1, the innermost one from TC2, and the middle one from S2 (Fig. 5C). Branchostyles cirriform, short, organized in a straight line and showing a wide middle gap. Branchostyles and branchophores covered with warts. There are no visible nephridial papillae or nephropores.

NOTOPODIA AND NEUROPODIA. 17 TC, notopodia with capillary chaetae from S3; anterior notopodia small, increasing in size from first to third pair; elevated or modified notopodia absent; 14 TU, all thoracic neuropodia well developed, with almost equal size, middle ones slightly bigger than anterior and posterior ones. Ventral shields distinct anteriorly and completely disappearing before the end of thorax. Subdivision of thorax absent. 17–20 AU; shape of neuropodia gradually changing throughout in most specimens, from tori to pinnula with marked size reduction on the thorax/abdomen transition; uncini at the margin of neuropodia. All neuropodia without cirri; typically rudimental abdominal notopodia absent.

UNCINI. Uncini pectinate (Fig. 5 J–L) with 4 teeth in double row, similar from TU1 to last AU; prow absent.

PYGIDIUM. Anal cirri absent.

TUBE (Fig. 4 F–G). Tubes of this species appear to be formed by progressive additions of small quantities of homogeneous fine-grained sediment to form an irregular banded pattern (Fig. 4F). The interior part of

the tube has a homogeneous and transparent membranous lining that isolates the body of the worm from the outer layer and that can easily be peeled off from it (Fig. 4G).

This species was reported from fjords along the West Antarctic Peninsula (WAP) (Grange & Smith 2013) and in several stations from the Ross Sea (NZ NIWA “IPY-CAML” Voyage TAN0802; records retrieved from GBIF, last accession 2020 Feb. 20).

Benham in the description of the new genus Amythas, stated that there were no buccal tentacles and that these were replaced by a folded membrane (Benham 1921). Hartman (1966) and Fauchald (1977) followed Benham.As it has already been stated by Monro (1939), buccal tentacles are present in Amythas as in all other Ampharetidae. The whole construction of buccal tentacles shows perfect resemblance to Terebellidae Johnston, 1846 and, at the same time, to Ampharetidae with an everted pharynx (see Jirkov 2016: fig. 3) which provides an indication of their homology and prostomial origin. The position of the uncini at the margin of the neuropodia was found to vary according to the individuals (e.g., Fig. 5 D–E). The smallest specimen examined (MNA-07935) has small rudimental abdominal notopodia and AU neuropodia more pinnuli-like (Fig. 5D). The morphology of the tube of this species was documented and described to occur in extant Sabellidae as well as in the ichnofossil Caprascolex antarcticus Schweitzer et al., 2005 from the Antarctic Eocene of La Meseta formation (Seymour Island, Antarctica) (Schweitzer et al. 2005: figs 3b, 4). In situ images of tubes of this species are available in Grange & Smith (2013: fig. 2c).