Faxiella delicatula (Thomson & Rennett, 1931)

7. Faxiella delicatula (Thomson & Rennett, 1931) View in CoL ( Figs. 6A–F View FIGURE 6 )

Dicholaphis delicatula Thomson & Rennett, 1931: 30 View in CoL , Pl. 9, fig. 8–9, Pl. 12, fig. 4.

Mirostenella delicatula View in CoL — Cairns & Bayer, 2009: 28, 38–39 (listed).

Plumarella (Faxiella) delicatula View in CoL —Zapata-Guardiola, López González & Gili, 2012: 233–236, fig. 6–7.

Faxiella delicatula View in CoL — Cairns, 2016:55–58, figs. 16F, 27, 28.

Material examined: One colony ( NCPOR /HYD-SWIR/0046), Southwest Indian Ridge, MGS25 cruise, R / V MGS Sagar, stn. MGS35E–D1, 28° 03' 18'' S, 63° 35' 49'' E, 3551 m, 22 nd March 2020 GoogleMaps .

Description: Colony delicate, uniplanar, axis pale yellow. Polyps paired, about four pairs occurring every cm of branch length, curved upward ( Figs. 6A, B View FIGURE 6 ). Primary polyps larger, body wall sclerite formula 4–7: 3–4: 3–4: 3–4. Opercular scales triangular to a narrow oval shape tapering to a point like lanceolate; long and slender (L: W=2.3–3.2); 0.8–0.85 mm in length, longitudinally concave, smooth outer surface; tuberculate inner distal surface for attachment; multiple-keeled inner proximal surface. Body wall scales flat to slightly concave, rounded proximal edge. Marginal scales keeled, smooth, width 0.21–0.28 mm; distal body wall scales the width of 0.6– 0.6 mm. Body wall scales flat and smooth outer surface, inner surface covered with low tubercles, except for the proximal margin about 40–50 µm in width. Coenenchymal scales trapezoidal to rectangular, smooth outer surface, tuberculate inner surface, they are a little smaller than the body wall scales, averaging ~ 0.20–0.24 mm in width, arranged in a linearly imbricate fashion ( Figs. 6A–E View FIGURE 6 ).

Remarks: The identification of the specimen agrees with the previous descriptions by Thomson & Rennett (1931), Cairns & Bayer (2009), Zapata-Guardiola, López González & Gili (2012), and Cairns (2016). F. delicatula is distinguished morphologically from F. abietina by its fragile, uniplanar, pinnate colony structure, a base branch approximately 0.2 mm in diameter, alternate paired polyps, a pale yellow main axis, about four pairs of polyps per centimeter along the branch, upward-curving polyps with 1.2-1.5 mm in length, and a subtly flared distal end.

Substrate type: The specimen collected from highly altered ultramafic rock covered by a thick Fe-Mn crust in the ridge system suggests a geological environment where ultramafic rock, rich in magnesium and iron, have undergone substantial alteration, likely due to hydrothermal processes.

Distribution: The type locality of this species was from off Macquarie Island, Australian EEZ, at 2743 m ( Thomson & Rennet 1931); also found in the east of Pitt Island and the Bay of Plenty, North Island at depths ranging from 520 to 2743 m; Chatham Rise, Iceberg Seamount at 520–650 m ( Cairns 2016). The F. delicatula from the SWIR has been recorded for the first time at a deeper depth of 3551 m.

Molecular data: The pairwise distance between Faxiella delicatula species from the SWIR ( OM472570 View Materials ) and those from New Zealand ( MG986958 View Materials , Cairns & Wirshing 2018) is 2.1%. The high distance value indicates that these populations are probably isolated from each other.