Frontohornera frontalis n. sp.

(Figs 2J, K, 16–18)

Material examined. Holotype: ZIRAS 1/50707, colony detached from nodule, YMG R.V. Yuzhmorgeologiya

cruise YMG4–13, Stn 292, 27 July 2014, 13.03353° N, 132.49292° W, 4829 m. Paratype: ZIRAS 2/50708, colony attached to nodule particle, YMG R.V. Gelendzhik cruise GLD4–11, Stn 210, 20 April 2012, 13.35817° N, 133.27183° W, 4815 m. Additional material: YMG4–07, Stn 124; GLD4–11, Stn 215; YMG4–14, Stns 329, 334. Total specimens examined six.

Etymology. Latin, frontalis, frontal, alluding to the frontally borne gonozooid.

Description. Colony erect, white, to 2.81 mm high, 6.76 mm broad, dichotomously branching once in a single plane (Fig. 16A, B), or with short additional bifurcation to produce a third, short branch near colony center (Fig. 18G, H). Stem diameter 0.32 × 0.37 mm, widening to colony base (0.66–0.76 mm). With distinct frontal and abfrontal faces (Fig. 16), entire surface interior-walled, textured irregularly with minute dimples, granules, ridges and furrows (Fig. 17A–G), with longitudinal linearities only on stem of colony; conspicuous ribbing (nervi) and grooves (sulci) lacking. Pores simple, tiny, sparse.

Autozooids in 2–4 series, their peristomes produced frontally upwards, relatively short along frontolateral and central parts of branch, longest along abfrontolateral margin, with simple circular apertures, no apertural spines (Fig. 17A–F, I). Peristomes somewhat dimorphic (Fig. 17F), short and uniformly tubular with circular apertures in central face of branches, but strongly elongated, with aperture having shape of teardrop, isosceles triangle or irregular oval along branch margins. Peristome wall interiors with short, thin spinules proximal to aperture (Fig. 17I). Cancelli most numerous on central part of branch distal to gonozooid (Figs 16A, 17A, E, J), otherwise sparse, shallow, of varying size. Abfrontal face of colony with outlines of autozooidal tubes (Figs 16B, 17G), their longitudinal contours converging proximad.

Gonozooid frontally borne, adjacent to second dichotomy (Fig. 16A), subglobular, surface like that of colony but with slightly denser prickled granules (Fig. 17H, J, K), frontal peak and distal sides of chamber converging towards short ooeciostome that curves frontalwards (Fig.17H, J, K), ooeciopore transversely oval (Fig. 17L).

Ancestrular zooid not seen in isolation; earliest growth stages encountered show lowest peristome of colony (presumed ancestrular peristome), which appears to be wholly erect, though concealed within basal skirt of extrazooidal calcification (Fig. 18G, I). Peristome of daughter zooid above it in series, but in another colony ancestrular and two daughter peristomes occur as cluster of three (Fig. 18B).

Measurements (mm). Holotype, ZIRAS 1/50707 (Fig. 16A, B): ZL 0.569–0.763 (0.663 ± 0.071). Branch center: PeL 0.072–0.197 (0.122 ± 0.042); PeD 0.141–0.162 (0.153 ± 0.008); ApD 0.129–0.141 (0.137 ± 0.004). Branch margin: PeL 0.194–0.526 (0.374 ± 0.102); PeD 0.110–0.146 (0.135 ± 0.011); ApL 0.132–0.176 (0.156 ± 0.016) (n = 7); ApW 0.122–0.147 (0.132 ± 0.008) (n = 7). Gonozooid (n = 1): GZL 0.744; GZW 0.511; GZH = 0.377; OsL 0.167; OsW 0.132; OsH 0.196; OpL 0.073; OpW 0.071.

Non-type specimen GLD4–11, Stn 210 (Fig. 18G–I): AnPeD 0.144 (n = 1).

Remarks. Frontohornera frontalis n. sp. is represented in the collection by six colonies of varying ages and sizes. Only one reproductively mature colony was encountered. All were found attached to polymetallic nodules.

Distribution. Recorded from six stations within coordinates 13.03353– 14.29353° N, 131.87598– 134.23457° W, at depth range 4815–5166 m.