Orectopora flabellum Grischenko & Gordon & Melnik 2018, n. sp.

Orectopora flabellum n. sp.

(Figs 6–8, 51)

Material examined. Holotype: ZIRAS 1/50673, colony detached from nodule, YMG R.V. Yuzhmorgeologiya cruise YMG4–13, Stn 316, 3 August 2014, 13.09428° N, 132.37362° W, 4933 m. Paratype: NIWA 127723, colony detached from nodule, YMG R.V. Gelendzhik cruise GLD4–12, Stn 258, 10 April 2013, 12.90213° N, 132.87047° W, 4713 m. Additional material: YMG4–07, Stns 126, 136; GLD4–08, Stns 145, 151; GLD4–09, Stns 183, 198; GLD4–11, Stns 219, 230; YMG4–13, Stn 273; YMG4–14, Stns 332, 363, 365. Total specimens examined 14.

Etymology. Latin, flabellum, a fan, used as a noun in apposition.

Description. Colony erect, flabellate, subplanar and narrowly compressed, attached by short stalk (Fig. 6A, C), white. Initially with single spreading fan of zooids, more or less bilaterally symmetrical, or shortly branching as 2–3 flattened lobes, entire fan gently concave frontally, convex abfrontally (Fig. 6B, D, E). At inception, first major lobe(s) comprising bilaterally compressed fascicle(s) of 10–12 autozooids, before subdivision into smaller lobes (Fig. 8E–O). Colony and lobes essentially 1–2-layered, comprising, in transverse section, initially contiguous, but then laterally diverging, autozooidal tubes, backed by discontinuous series of smaller kenozooidal tubes (Figs 6E, 7A, C, F, 51A, B, D, E). Surface of colony textured by series of thin sinuous growth lines (Figs 6D, 7B, E, H) and needle-like crystallites perpendicular to growth lines. Simple pseudopores opening externally, seemingly moderately sparse in SEM images, but more obvious and abundant in CT scans, more or less regularly distributed (Figs 7B, E, 51A–F).

Autozooidal tubes mostly very long, with several running length of colony; other tubes shorter, originating by lateral budding in plane of colony as fascicles broaden (Fig. 51A, C). Earliest-established peristomes more evident on colony faces by bulging of their longitudinal tubes, secondarily budded autozooids not bulging or less obviously so (Figs 6A, E, 8L). Openings of primary and secondary autozooidal tubes at colony margin evidenced by their circular to subcircular rims, some separated sublaterally by shafts of slightly smaller diameter (Fig. 7A, B, D, F), interpreted to be proximal parts of differentiating autozooids. Interior surface of zooidal tubes lined by distally imbricated foliated fabric of wedge-shaped crystallites (Fig. 7D, G, J).

Kenozooids seen in transverse section at colony margin, 1–2 on abfrontal side of each autozooid (Fig. 7A, C, F, I), with round to subtriangular rims. Interior surface ultrastructure as in autozooids. CT scans reveal kenozooids to be long, irregular structures descending behind or to the side of zooidal tubes (Fig. 51B, D, E), occasionally communicating with neighbors via interzooidal pores, with more-numerous pseudopores to outer surface. Proximally, CT scans reveal kenozooidal chambers transition from elongate tubes to layers of squatter cushion- or amoeba-shapes (Fig. 51A, B, F); each kenozooid communicating with neighbors above and below; those closer to outer surface with sparse pseudopores. Kenozooids also in axils between fascicle lobes.

Gonozooid not seen.

Ancestrula suberect, inclined at 45–55° angle to substratum (Fig. 8B, N). Protoecium (Fig. 8D) flared at base, with slightly smoother margin, continuous with peristome, surface with needle-like crystallites (interpreted as eroded planar-spherulitic fabric) and sparse pseudopores. Abfrontal sides of ancestrular zooid, one suberect zooid and base of erect fascicle of zooids supported by developing skirt of kenozooids and extrazooidal calcification (Fig. 8B, E, N); skirt with scalloped margin depending on substratum (Fig. 8P); exterior surface marked by vertical striae and ridges, and moderately sparse but evenly distributed pseudopores.

Measurements (mm). Holotype, ZIRAS 1/50673 (Fig. 6): Colony height c. 2.07 (proximal end of largest colony broken), width 3.02; three main lobes (fascicles) length 0.48–1.01, width 0.43–1.51, thickness 0.09–0.16; ‘stalk’ at point of breakage 0.33 × 0.17; ZL 1.308–1.853 (1.604 ± 0.198); PeL 0.043–0.163 (0.082 ± 0.035); PeD 0.123–0.154 (0.136 ± 0.009); ApL 0.094–0.105 (0.099 ± 0.003); ApW 0.086–0.098 (0.091 ± 0.004).

Non-type specimen GLD4–08, Stn 151 (Fig. 8D): AnPeD 0.106 (n = 1).

Remarks. Orectopora flabellum n. gen., n. sp. is represented in the Russian-sector collection by 14 specimens having varied size and maturity. The novel features of this taxon defy inclusion in a named family and have required some interpretation to describe and explain them, facilitated by light microscopy, with and without staining, plus SEM and micro-CT. The form of the ancestrular zooid is similar to that in Pandanipora n. gen., and the initial colony form, comprising a short ascending series of zooids supporting an elevated structure, is reminiscent of that in Discantenna (Gordon & Taylor, 2010).

Distribution. Recorded from 14 stations within coordinates 12.65038– 14.57215° N, 131.73948– 134.71841° W, at depth range 4713–5275 m.