Discantenna metallica n. sp.

(Figs 2I, 10–12)

Material examined. Holotype: ZIRAS 1/50675, colony detached from nodule, YMG R.V. Yuzhmorgeologiya cruise YMG4–06, Stn 67, 8 August 2006, 13.18842° N, 134.51227° W, 4820 m. Paratype 1: ZIRAS 2/50676, colony detached from nodule, YMG R.V. Gelendzhik cruise GLD4–11, Stn 224, 2 May 2012, 12.55585° N, 133.08295° W, 4778 m. Paratype 2: ZIRAS 3/50702, colony detached from nodule, YMG R.V. Yuzhmorgeologiya cruise YMG18–01, Stn 37, 29 August 2003, 13.85300° N, 129.08458° W, 4715 m. Additional material: YMG4–06, Stns 69, 82; GLD4–08, Stn 151; GLD4–11, Stns 207, 225; YMG4–14, Stns 358, 362. Total specimens examined 11.

Etymology. Latin, metallica, pertaining to a mineral, alluding to the substratum.

Description. Colony erect, white, stalked, from narrow encrusting base (Fig. 11K). Initially uniserial, becoming biserial, distal end of base developing erect column that expands into shallow circular or irregular discoidal structure, up to 3.45 mm diameter, in which zooids are centripetally arranged; colony height up to 4.94 mm.

Apical disk bereniciform (Fig. 10A, B); up to 52 zooidal peristomes, angled obliquely outwards from depressed center of disk; peripheral common bud thin, with narrow marginal lamina pierced by pseudopores and having skeletal microstructure of distally imbricated foliated crystallites. Calcified terminal diaphragms (Figs 10F, 11A, G, M) common in older, proximal zooids; diaphragms made up of fused sectors of needle-like crystallites (Fig. 11F); 2–5 tiny pseudopores apparent.

Only two gonozooids seen, each produced at edge of small capitulum of unattached stem (Fig. 11A). Stem with ~10 short peristomes of varied length, ~6 of them with apertures occluded by diaphragms. Capitulum with irregular profile in apical view owing to many peristomes broken at base; made up of four ‘sectors’, each comprising fused parts of adjacent peristomes, one of which bears gonozooid. Gonozooid broader than long (Fig. 11C–E), with broadly W-shaped margin on capitulum side (Fig. 11B) or this not evident (Fig. 11C); moderately inflated frontally either side of shortly projecting ooeciostome (Fig. 11C, I) with near-circular ooeciopore with thin slightly irregular rim (Fig. 11D, E, H, I). Gonozooid wall with greater pseudopore density.

Ancestrula suberect, protoecium inclined at 50–60° angle to substratum (Fig. 12A, C, D, E, H). Protoecium may be shortly flared at base (Fig. 12H), continuous with peristome, surface with needle-like crystallites and sparse pseudopores. Abfrontal sides of ancestrular zooid, 3–4 daughter zooids and base of colony of erect fascicle of zooids supported by basal layer and skirt of kenozooids and extrazooidal calcification (Fig. 12B, F); base of skirt with irregular margin depending on substratum; exterior surface marked by vertical striae and ridges (Fig. 12H), and moderately sparse but evenly distributed pseudopores.

Daughter zooid distal to ancestrula initiating biseriality by budding two additional zooids, their respective peristomes directed to left and right (Fig. 12E, F, H), after which 5–11 additional peristomes may be produced around base of what will become erect stem of capitulum (Fig. 12I –L), which shows chambers of seven (some as small proximal ends) in cross section in one example (Fig. 12J). Non-functioning older zooids with closure plates, near or below peristomial rim (Figs 11, 12A, K).

Measurements (mm). Holotype, ZIRAS 1/50675 (Fig. 10A–D): Colony height 2.95; capitulum length 4.15, width 3.45; stalk cross-section at point of breakage 0.35 × 0.40; stalk cross-section at junction with disk 0.71 × 0.53; stalk height (from point of breakage to disk) 1.53. Disk zooids: ZL 0.772–1.295 (0.979 ± 0.166); PeL 0.221–0.713 (0.414 ± 0.179); PeD 0.123–0.140 (0.131 ± 0.005); ApL 0.132–0.171 (0.151 ± 0.013); ApW 0.098–0.132 (0.115 ± 0.012). Stalk zooids (n = 4): ZL 0.543–0.628 (0.583 ± 0.035); PeL 0.142–0.211 (0.172 ± 0.029); PeD 0.135–0.148 (0.141 ± 0.006); ApL 0.113–0.128 (0.122 ± 0.006); ApW 0.103–0.123 (0.112 ± 0.009).

Paratype 1, ZIRAS 2/50676 (Figs 10E–I, 11C, E, H, I): Colony height 4.94. Stalk: height (substrate to disk) 3.57, cross-section (narrowest, near ancestrula) 0.45 × 0.33, cross-section at junction with disk 0.58 × 0.38. Disk zooids (n = 5): ZL 0.995–1.731 (1.342 ± 0.261); PeL 0.358–0.818 (0.456 ± 0.202); PeD 0.133–0.152 (0.143 ± 0.007); ApL 0.151–0.178 (0.164 ± 0.012); ApW 0.118–0.136 (0.130 ± 0.006). Stalk zooids (n = 4): ZL 0.698–0.953 (0.834 ± 0.126); PeL 0.078–0.103 (0.092 ± 0.010); PeD 0.138–0.161 (0.150 ± 0.011); ApL 0.128–0.147 (0.136 ± 0.008); ApW 0.122–0.130 (0.126 ± 0.003). Gonozooid (n = 1): GZL 0.406; GZW 0.483; GZH 0.352; OsL 0.114; OsD 0.117–0.077; OpD 0.067.

Paratype 2, ZIRAS 3/50702 (Fig. 11A, B, D): Gonozooid (n = 1): GZL 0.337; GZW 0.561; GZH 0.294; OpL 0.072; OpD 0.078.

Non-type specimen YMG4–14, Stn 358 (Fig. 12A–D): AnPeD 0.115 (n = 1).

Remarks. Discantenna metallica n. sp. is only the second known species. The type species, D. tumba, was described from 943–1097 m depth on the Chatham Rise, New Zealand. Inter alia, D. metallica differs from it by achieving a larger colony size (> 4 mm high and wide, cf. <2.6 mm high and wide in D. tumba) with many more peristomes in the elevated disk (~52, cf. ~24) having a diameter up to 0.17 mm (cf. 0.08 mm). Whereas tubular peristomes are arranged along only the frontal side of the stem in D. tumba (with the abfrontal side pseudoporous), such peristomes alternate around the stem in a spiral pattern in D. metallica . Stem peristomes are short in D. tumba, but can be quite elongate in D. metallica, some exceeding the radius of the disk. Interestingly, even though disk size is bigger in D. metallica, the only specimens with a gonozooid had incompletely developed capitula. Whereas the incubation chamber in D. tumba is set back from the edge of the capitulum, that in D. metallica projects a little from the margin, with the ooeciostome outermost. The gonozooid in D. metallica n. sp. resembles that in the following new genus and species, which we attribute to the Oncousoeciidae .

Distribution. Recorded from 10 stations within coordinates 12.55585– 14.37702° N, 129.08458– 134.51227° W, at depth range 4715–5238 m.