Swiftia sahlingi Breedy & Rouse & Stabbins & Cortés & Cordes 2019, sp. nov.

Swiftia sahlingi sp. nov.

Figures 1–4

Material examined. Holotype: MZUCR 2725, SIO-BIC Co2917, ethanol preserved, Mound 12, 1000 m, HOV Alvin Dive 4908, pilot Jefferson Grau, R /V Atlantis Expedition AT 37-13, 23 May 2017.

Paratypes: MZUCR 1967, ethanol preserved, Mound 12, AT15-44, HOV Alvin, Dive 4501, pilot M. Spear, 1000 m, 22 February 2009. MZUCR 2726, 2 specimens, ethanol preserved, as the holotype; MZUCR 2647, 3 specimens, ethanol preserved, as the holotype; MZUCR 2648, SIO-BIC Co 2916, ethanol preserved, Mound 12, 999 m, HOV Alvin, Dive 4907, pilot J. Patrick Hickey, 22 May 2017. MZUCR 2727, 5 specimens; MZUCR 2650 (CR-AV-4909- 4), 2 specimens; MZUCR 2730 (CR-AV-4909-5), ethanol preserved; MZUCR 2727, ethanol preserved; MZUCR 2731, fragment, ethanol preserved, Mound 12, 996– 1000 m, HOV Alvin, Dive 4909, pilot Mike Skowronski, 24 May 2017. MZUCR 2732, SIO-BIC Co 2935, dry, Mound 12, 1002 m, HOV Alvin, Dive 4917, pilot Jefferson Grau, 1 June 2017. MZUCR 2729, 2 specimens; MZUCR 2733, SIO-BIC Co 2945, 3 specimens, Mound 12, 996 m, HOV Alvin, Dive 4922, pilot Mike Skowronski, 5 June 2017.

Type locality: Mound 12 (08°55.845´N, 084°18.768´W), 996–1002 m, Costa Rica Pacific margin.

Description. The holotype is an erect, sparsely branched colony, 9 cm tall, and about 10 cm wide (Figs. 1A, 2A). It is attached to a 15-cm long rock together with a smaller colony (7 cm tall). The two colonies are about 2.5 cm apart, and arise from incrusting holdfasts about 1 cm in diameter each (Figs. 1A, 2 A–B). The main stem of the holotype is 3 mm in diameter at the base, and extends 7 mm tall, subdividing in 4 main branches that subdivide 2–3 times, branching up to five times (Fig. 1A). Branching is lateral and irregular in different planes. The branches are stout, and reach up to 2 mm in diameter including the polyp-mound. Unbranched terminal ends reach up to 8 cm in length. The flexible horny axis is light brown and somewhat translucent, with non-mineralised organic fibers. The main stem is devoid of polyps. The polyps are translucent (Figs. 1 B–C), and are about 1 mm apart. Polyp mounds are alternating, about 6 polyp mounds/cm at the end branchlets (n=8 branches) (Fig. 1A). The polyp mounds are perpendicular to the branches, raised up to 1 mm tall (Figs. 1 B–C) and about the same in width. The anthocodiae are preserved exsert, reaching up to 1 mm tall, and fully expanded, in life, up to 3 mm (Figs. 1 B–C, 2A–C).

The coenenchyme is thin. Coenenchymal sclerites consist of long, thin, warty spindles, mostly straight, 0.26– 0.60 mm long and 0.04–0.09 mm wide (Figs. 3A, 4A). Conspicuous plates with sparse tubercular sculpture, 0.153 – 0.315 mm long and 0.05–0.08 mm wide (Figs. 3A, 4C) are surrounding the polyp mound, at the neck zone of the anthocodiae and also occur in the external layer. The sclerites in the axial sheath are mostly tuberculated spindles, straight and curved; and immature sclerites (Figs. 3A, 4B). The anthocodial armature is arranged ¨en chevron¨ with points consisting of bar-like rods, straight or slightly bent, 0.28–0.44 mm long, and 0.04–0.08 mm wide (Fig. 4D), warty clubs with tuberculated or bifurcated heads, 0.19–0.39 mm long, and 0.034 –0.075 mm wide (Fig. 4E), and irregular flattened rods around the peristome and along the tentacles, up to the tips, 0.10–0.19 mm long and 0.022 –0.049 mm wide (Figs. 3B, 4E). Tentacular sclerites are colourless (Fig. 3B).

The colour of the colony is red, and preserves the colour after fixation, sclerites are red, orange and of lighter hues (Figs. 1A, 3A).

Variability. The paratypes are from 8 to 17 cm long (the largest MZUCR 2730). Specimens have up to 9 main branches that subsequently subdivide up to 8 times (MZUCR 1967). Unbranched terminal ends reach up to 8.5 cm (MZUCR 2729). Polyps are distributed as in the holotype, but more closely spaced, in some cases, i.e., 5–5.3 polyps/cm (n=20 branches); but they could be more distant at the base of the colonies, up to 2 mm apart (MZUCR 2729). Stems are normally devoid of polyps. In some cases, the lateral branches of neighbouring colonies could stick together forming a lose braid (e.g. MZUCR 1967, MZUCR 2650). The sclerite types, colours and sizes of the paratypes are similar to those of the holotype.

Habitat. The species was found living in dense clusters on top of flat authigenic carbonate ledges (Figs. 2 A–D). Many of the observed and collected colonies were close to active methane seepage, and a colony was even observed on a tube of the polychaete Lamellibrachia (Siboglinidae). Nearby the collection location were clusters of mussels (Bathymodiolus spp.), bacterial mats and branching Foraminifera.

Etymology. The species is named in memory of Heiko Sahling, a distinguished marine geologist who discovered and named Mound 12, the type locality of the new species.