Phalacrostemma undetermined

Phalacrostemma sp.

Figs 16–17

NORTH ATLANTIC – Southeastern Bahamas • 1 incomplete spec.; RV Columbus Iselin, stn 47; 23°43′ N, 77°09′ W; depth 3000 m; 24 Feb. 1973; UMML-22.1194.

BODY. Incomplete specimen (UMML-22.1194), 10 mm long, 2.5 mm wide, five abdominal segments (Fig. 16).

OPERCULUM. Opercular disc truncate. Outer paleae arranged spirally, 37 paleae on each lobe. Inner paleae arranged diagonally, two paleae on right lobe, one palea on left. Outer paleae amber, cylindrical, 3 mm long (Fig. 17A). Proximal region with compact thecae, margins non-expanded (Fig. 17B). Thecae of middle region compact with non-expanded margins to thecae with margins irregular slightly expanded (Fig. 17C); tips hirsute (Fig. 17D). Inner paleae amber, cylindrical, with thecae inconspicuous; tip blunt, slightly inclined (Fig. 17E–F). Opercular stalk brown, 1.6 times as wide as long (Fig. 16). Opercular papillae in one row peripheral to outer paleae; 10–11 papillae on each lobe. Papillae conical, small, and tapered, 2–3 times as long as wide (Fig. 16E–F). First papilla twice as large as others, appears on dorsal region of opercular peduncle, at level of nuchal hooks (Fig. 16A– B). Four pairs of amber nuchal hooks, compressed, tip falcate, with short limbation. Limbation not reaching tip curvature, three times as long as tip (Fig. 17G). Palps robust, tapered, twice as long as wide (Fig. 16D). Pair of broad buccal flaps (Fig. 16C–D). Tentacular filaments absent. Median organ conical, brown, without eyespots.

THORAX. First thoracic segment with 3–4 long, triangular-shaped lateral lobes (Fig. 16D–F), and capillary neurochaetae. Second segment with two triangular-shaped lateral lobes and paired branchiae, without chaetae (Fig. 16B, F).

PARATHORAX. With four segments, all with paired branchiae (Fig. 16E). Notopodia with seven lanceolate chaetae and seven capillary chaetae (Fig. 17H). Neuropodia with capillary chaetae.

ABDOMEN. Abdominal segments brownish. Neurochaetae capillary, ornamented with irregular thecal laminar extensions (Fig. 17I). Notopodia with series of uncini with 10 rows of teeth (Fig. 17J).

Phalacrostemma sp. has outer paleae that resemble those of P. danieli sp. nov., with compact proximal thecae, median thecae with denticulate margins, and hirsute tip. However, these species differ in the morphology of the inner paleae and opercular papillae.

Phalacrostemma danieli sp. nov. has inner paleae with conspicuous thecae (Fig. 6G) and crenulate margins (Figs 6F, 7G), whereas Phalacrostemma sp. has inconspicuous thecae and smooth margins (Fig. 17E–F). The opercular papillae are as long as the nuchal hooks in P. danieli sp. nov. (Fig. 5C), while Phalacrostemma sp. has short papillae, ½ as long as the nuchal hooks (Fig. 16B, E–F). Also, the first papilla is different, P. danieli sp. nov. has a thin, conical papilla (Fig. 5F), while Phalacrostemma sp. has a wide, triangular papilla (Fig. 16B).

In addition to morphological differences, the species have been found in different habitats. Phalacrostemma sp. has been found in the Bahamas at 3000 m depth, while P. danieli sp. nov. was collected in the Lesser Antilles at 547 m depth.

Only in The Bahamas, at 3000 m depth (Fig. 22).