Macellicephala linseae Neal & Brasier & Wiklund 2018, sp. nov.

Macellicephala linseae sp. nov.

(Figure 2H, Figure 15B, E, F and Figures 16, 17; Table 2.)

Macellicephala sp. in Brasier et al. (2016) [record].

Macellicephala sp. A in Neal et al. (2017) [record].

Material examined. Holotype, NHMUK.2018.9354, Amundsen Sea, Southern Ocean, cruise JR 179, station BIO5 - EBS-2A, collected on 09/03/2008, epibenthic sledge epi-net, 73°52'46.704''S, 106°19'6.456''W, 1052 m depth. DNA vouchers (2 specimens): NHMUK.2018.9355, Amundsen Sea, Southern Ocean, cruise JR 179, station BIO5 - EBS-2A, collected on 09/03/2008, epibenthic sledge, epi-net, 73°52'46.704''S, 106°19'6.456''W, 1052 m depth; NHMUK.2018.521, Amundsen Sea, Southern Ocean, cruise JR 179, station BIO5 -EBS-3D, collected on 10/ 03/2008, epibenthic sledge, epi-net, 73°58'40.152''S, 107°25'0.372''W, 550 m.

Description (based on holotype). Small to medium sized species; holotype with 18 segments (segment 1 = tentacular segment), 8.5 mm long, 1.3 mm wide excluding parapodia and 2.5 mm wide including parapodia. Live specimens not observed. Preserved specimen creamy yellow in colour (Fig. 16A–C).

Prostomium bi-lobed, with a deep anterior notch; prostomial lobes short (not extended), anteriorly rounded (Fig. 16B, Fig. 2H). Frontal filaments absent. Eyes absent. Ceratophore of median antenna large, inserted in anterior notch (Figs. 2H and 16B); style of median antenna missing. Palps very long (extending to segment 9), very slender (only slightly thicker than tentacular cirri), smooth, tapering into slender tips.

Tentaculophores inserted laterally to prostomium, of equal size, achaetous (Fig. 2H); only ventral tentacular styles remain attached in holotype, but dorsal styles observed in DNA voucher NHMUK.2 0 18.9355, tentacular styles smooth, very slender and very long (extending to segment 9) (Fig. 16A). Facial tubercle absent. Everted pharynx with 2 pairs of jaws and 9 pairs of small triangular papillae (Fig. 16A); each jaw with single fang and smooth margin.

Nine pairs of knob-like elytrophores present on segments 2, 4, 5, 7, 9, 11, 13, 15 and 17; elytra missing.

Cirrigerous segments without dorsal tubercles; distinct dorsal cirrophores elongated, cylindrical to slightly bulbous; styles of dorsal cirri missing. Dorsal ridges appears faintly from segment 3 becoming obvious by segments 5–6, on segments 8–10 forming an oval structure, in segments anterior to segment 8 and posterior to segment 10 forming concentric structures (Fig. 16C). In posterior part of body only, similar ridges appear on parapodia of cirrigerous segments; first parapodial ridge lateral to dorsal ridge, second parapodial ridge near base of cirrophore.

Parapodia biramous. Notopodia reduced to one stout acicula (Fig. 17A). Neuropodia large (Fig. 17A), elongated, subtriangular to rectangular in shape, with integument covered acicula protruding into slender tip; prechaetal and postchaetal lobes poorly developed. Ventral cirrus on segment 2 inserted basally, where very slender and long, approaching tip of neuropodial acicula lobe; inserted medially on subsequent segments where short (much shorter than tip of neuropodial lobe) (Fig. 17A).

Notochaetae stout (Fig. 17A, C, D), few (only 1 to 2 per ramus observed in holotype), often broken off and missing entirely (observed in only 4 parapodia in holotype), short, straw-coloured, with distinct horizontal striation throughout length of its shaft (Fig. 17 C, D), with large rose-bush like alternating and interlocking spines on one side only, distal tip of notochaetae smooth and blunt. Neurochaetae short to very long, numerous, translucent, of two forms: either wide and flattened with shallow, but distinct denticles on both sides throughout most of their lengths, with pointed tip (Fig. 17B) or slender chaetae, with distinct alternating denticles and blunt tip (few in number and easily overlooked) (Fig. 15F, marked by arrow and Fig. 17B).

Nephridial papillae globular, present at ventral junction of neuropodia and body on segments 10, 11 and 12, inconspicuous in other segments, present from segment 5. Reduced parapodia of segment 18 lateral to pygidium, consisting of notopodia only. Pygidium rounded. Anal cirri not observed.

Remarks. See remarks under Macellicephala gloveri sp. nov. for further details and differentiation of M. linseae sp. nov. from similar species M. gloveri sp. nov. and M. aciculata.

Molecular Information. COI and 16S were sequenced by Brasier et al. (2016), while 18S (452 bp) was obtained in this study (Table 1). Based on COI, this species was closest to Macellicephala gloveri sp. nov. with K2P distance 0.12 and uncorrected 'p' distance 0.11.

Etymology. This species is named after Dr. Katrin Linse from the British Antarctic Survey, leading Antarctic researcher and the principal scientist of BIOPEARL II cruise.

Distribution. Pine Island Bay, Amundsen Sea at shelf and slope depths of 550 to 1052 m.