Phalacrostemma timoharai

Description. Holotype complete, body reddish-brown after preservation in alcohol. Body gradually tapering to elongated cauda (Figs 6 A – B, 7 C – E). Holotype 10.1 mm in body length, 1.3 mm maximum width. Opercular lobes completely fused, with distal end perpendicular to longitudinal axis (Figs 6 A – B, 7 C – E, 8 A – B). Operculum with 18 pairs of outer paleae, arranged spirally (Fig. 6 C); outer paleae golden, as simple spines with acute tips. Thecae compact with straight and not expanded margins (Figs 6 A – B, 7 C – E, 8 A – B, 9 A – C). Two short inner paleae each opercular lobe, amber, with smooth surface and tapering tips (Fig. 8 C). Eight pairs of robust and tapering opercular papillae, peripheral to outer paleae (Figs 6 A – B, 7 C – E, 8 A – C), not extending to tips of outer paleae. Four pairs of flattened nuchal hooks, with limbation on concave side (Figs 6 B, 7 D, 8 A, C). A medial organ at dorsal junction of opercular lobes. Eyes not observed. Tentacular filaments absent (Figs 6 A, 7 D, E, 8 D). A pair of broad buccal flaps present (Figs 6 A, 7 C, 8 B). A pair of short robust palps extended beyond operculum (Figs 6 A, 7 D, E). Segment 1 (thoracic) with a pair of long tapering neuropodial cirri and capillary chaetae (Figs 6 B, 7 E, 8 A – B). Segment 2 (thoracic) with a pair of long triangular lateral lobes between branchia and neuropodia with capillary chaetae, notopodia absent (Figs 6 B, 8 A). Nine pairs of dorsal tapering branchiae present from thoracic segment 2 – 9, not meeting mid-dorsally (Figs 6 A – B, 7 C – E, 8 A), largest ones on chaetigers 3 – 7, subsequently reducing posteriorly in size. Segments 3 – 6 (parathoracic) with two types of notochaetae arranged in transverse rows; about eight robust retractile lanceolate chaetae interspersed with some fine capillaries (Fig. 9 G – H). Segments 3 – 6 (parathoracic) with two types of neurochaetae arranged transversely, 4 – 7 fine lanceolate chaetae interspersed with several fine short capillaries (Fig. 9 E). Notopodial lanceolate chaetae much stouter than neuropodial ones (Figs 6 B, 8 A, 9 E). Abdominal notopodia as erect expanded tori, with number of uncini within torus decreasing posteriorly. Each uncinus with 1 – 4 longitudinal rows of teeth (Fig. 9 F). Neuropodia with long fine capillaries with expanding thecae arranged in discrete fascicles in two tiers (Fig. 9 E). Cauda smooth, curved to abdomen and the end almost extending to buccal cavity (Figs 6 A – B, 7 C – E). Solitary tube made of small particles attached to a mollusc shell (Fig. 7 A). Variability. Paratypes vary from 2.5 – 3.3 mm in length from tip of paleae to end of parathorax (holotype 3.3 mm), 0.5 – 1.3 mm in width, with 18 – 22 pairs of outer paleae; 3 – 4 pairs of nuchal hooks; nine pairs of opercular papillae (eight pairs in the holotype); two pairs of neuropodial cirri on segment 1 (one pair in holotype).

Figures 6 – 9

Remarks. Phalacrostemma timoharai n. sp. is characterized by having 18 – 22 pairs of outer paleae, two pairs of neuropodial cirri on thoracic segment 1 and one pair of lateral lobes on thoracic segment 2. Currently 12 species are known in the genus, either living in aggregations or as solitary individuals in deep-sea localities in the Atlantic and Indo-Pacific (Capa & Hutchings 2019). The new species Phalacrostemma timoharai n. sp. found at 1013 – 1770 m differs from P. maloga from Tasman Sea found at 502 – 548 m by having 18 – 22 pairs of outer paleae, two pairs of neuropodial cirri on thoracic segment 1, and only one pair of lateral lobes on thoracic segment 2, whereas P. maloga has 12 pairs of outer paleae, one pair of neuropodial cirri on thoracic segment 1 and two pairs of lateral lobes on thoracic segment 2. Phalacrostemma timoharai n. sp. can also be distinguished from P. abyssalis (Caullery, 1944) which has 10 pairs of outer paleae, 3 – 4 pairs of tentacular filaments and lacks buccal flaps. The new species differs from P. cidariophilum and P. lechapti Kirtley, 1994 by the number of pairs of lateral lobes on thoracic segment 2. Phalacrostemma dorothyae Kirtley, 1994, P. gloriaae Kirtley, 1994, P. paulineae Kirtley, 1994, P. perkinsi Kirtley, 1994, and P. setosa (Treadwell, 1906) all lack buccal flaps which are present in P. timoharai n. sp. The new species which only has 18 – 22 pairs of outer paleae can be distinguished from P. profundum Kirtley, 1994 which has 30 – 40 pairs of outer paleae. Phalacrostemma tenue Lechapt & Kirtley, 1998 has four pairs of tentacular filaments which are absent in P. timoharai n. sp. (Table 3).

Distribution. The type locality is in the Coral Sea CMR (Australia), and the species has also been found off the east coast of Australia (Fig. 1). Habitat. Depths 1013 – 1770 m, as solitary individuals attached to mollusc shells (Fig. 7 A – B).

Etymology. The species is named timoharai in honour of Dr Tim O’Hara of Museums Victoria, Australia, who was the mastermind and Chief Scientist of the “ Sampling the Abyss ” voyage.

Material examined. Holotype, St. 121, 23 º 35.22´– 23 º 37.02´S, 154 º 11.64´– 154 º 11.682´E, 1013 – 1093 m, beam trawl, 13 th June 2017, AM W. 50674 (tissue sample taken). Paratypes: St. 121, 23 º 35.22´– 23 º 37.02´S, 154 º 11.64´– 154 º 11.682´E, 1013 – 1093 m, beam trawl, 13 th June 2017, AM W. 52730, one spec. (mounted for SEM), AM W. 50675, one spec. (tissue sample taken). Other material examined. St. 104, 26 º 57.678´– 26 º 59.478´S 153 º 50.85´– 153 º 50.808´E, 1071 – 1138 m, beam trawl, 10 th June 2017, AM W. 50673, one spec.