Hexapathes bikofskii

(Figs. 1 – 2 and 7; Supplementary Table 1 and Table 2)

Description of holotype (G 80122): Colony is monopodial and pinnulate with a slight sickle shape curvature of the stem (Fig. 7 A). Grooves and ridges on the stem are present from 1 cm from the basal plate to the first anterior pinnule. Colony is 23 cm tall and 17 cm wide. Unpinnulated section of the stem is 4 cm and the pinnulated section of the stem is 19 cm (Fig. 7 A). The specimen has two rows of lateral pinnules where the bottom pair of pinnules are subopposite and positioned perpendicular to the stem. Above the bottom pair of pinnules, pinnules are arranged alternately with distal angles ranging from ~ 80 ° at the bottom of the pinnulated section to ~ 20 ° at the top; with most pinnules having 45 ° distal angles. Lateral pinnules are curved forward and then backward so that the pinnule tips face in the opposite direction from the anterior pinnules. Lateral pinnules increase in length from the lowest pair of pinnules, which are ~ 8.5 cm, to midway up the pinnulated section where the longest pinnules are 12 cm, and then decrease towards the apex where the most distal pinnules are ~ 3 cm. Lateral pinnules are ~ 0.5 mm in diameter near the attachment point, and distances between pinnules in each row range from 5 to 10 mm (increasing in distance distally), resulting in 10 lateral pinnules counted near the bottom of the pinnulated section of the stem and six pinnules counted near the top of the pinnulated section, per 3 cm counting lateral pinnules in both rows. Anterior pinnules are simple, in one row and range from 0.8 to 1.5 cm in height, with most up to 1 cm in height. Distance between anterior pinnules range from 2 to 3 mm, resulting in 11 to 13 anterior pinnules counted per 3 cm. Spines on lateral pinnules are 0.025 to 0.1 mm in height, smooth, triangular, and distally directed (Fig. 7 B). Lateral pinnule spines are spaced 0.45 to 0.7 mm apart in each row, and two spines can be counted per mm in one row. Four axial rows of spines can be counted in lateral view (Fig. 7 B). Spines on anterior pinnules are smooth, triangular to conical, and distally directed. Spine heights are variable, ranging from 0.04 to 0.08 mm, and the distance between spines in one row is about 0.25 mm. Three to four axial rows of spines can be counted in one view of anterior pinnules. Polyps are ~ 4 to 6 mm in the transverse diameter and 6 to 8 polyps counted per three cm. Description of paratype (G 80024): The colony is monopodial and pinnulate with a slight sickle shape curvature of the stem (Fig. 7 C) and has two rows of distally directed lateral pinnules. Grooves and ridges are present along the stem from 1 cm above basal plate to the first anterior pinnule. Colony is 13 cm tall and 13 cm wide. The unpinnulated section of the stem is 4 cm and the pinnulated section is 9 cm in height. The lowest pair of pinnules are suboppositely arranged and are positioned almost perpendicular to the axis. All other pinnules are alternating and have distal angles ranging from ~ 20 degrees proximally to ~ 80 degrees distally. Lateral pinnules increase in length from the lowest pair of pinnules, which are 4 cm, to midway up the pinnulated section where the longest pinnules range from 6. 5 to 8 cm, and then decreasing towards the apex where the highest pair of pinnules are ~ 3 cm (Fig. 7 C). Distances between pinnules range from 5 to 9 mm (increasing in distance distally), resulting in eight pinnules (near the top of the pinnulated section of stem) to 10 pinnules (near the bottom of the pinnulated section of stem) per 3 cm, counting lateral pinnules in both rows. Lateral pinnules are 0.3 mm in diameter near the attachment point. Anterior pinnules are simple, in one row with lengths ranging from 0.5 to 1 cm, with most pinnules being close to 1 cm (Fig. 7 D). Anterior pinnules begin from the same height on the stem as the second lowest lateral pinnules, and extend to 5 mm above the most distal lateral pinnule. Anterior pinnules are 0.18 mm in diameter near the attachment point, and distances between pinnules range from 2 to 4 mm, resulting in 13 to 15 pinnules per 3 cm. Spines on lateral pinnules 0.05 mm in height and are smooth and triangular (Fig. 7 E). Lateral pinnule spines have distances between spines in one row from 0.48 to 0.7 mm and two to three spines can be counted in one mm in one row. Five axial rows of spines can be counted in one view. Spines on anterior pinnules are smooth, triangular to conical, and distally directed (Fig. 7 F). Spine heights are variable, and range from 0.02 to 0.07 mm and distances between spines in one row range from 0.32 to 0.45 mm. Three to four axial rows of spines can be counted in one view of anterior pinnules. Polyps are in a poor state of preservation and estimated based on in-situ images to be reddish in color and 6 mm in the transverse diameter. Comparative diagnosis. H. bikofskii sp. nov. is different than other species in Hexapathes by having only one row of simple (unpinnulated) and short (maximum of 1 cm in the new species vs maximum of 6 to 11 cm in currently described Hexapathes spp.) and straight anterior pinnules. See Table 2 for comparison of species in the genus. The new species is like H. australiensis Opresko, 2003 and H. alis Molodtsova, 2006 by having just one row of anterior pinnules; however, the anterior pinnules of both species are subpinnulated while the new species has simple anterior pinnules. The new species is like H. hivaensis Molodtsova, 2006 and H. heterosticha Kinoshita, 1910 by lacking anterior subpinnules; however, both species have two or more rows of anterior pinnules while the new species only has one row of anterior pinnules. Another difference between the new species and currently described species in the genus is the length of the unpinnulated section of the stem (ranging from 2 to 2.5 cm in described species vs 4 cm in the new species). The new species shares other features with H. australiensis for the following morphometrics: 1) distance between lateral pinnules on one side of the stem (0.5 to 1 cm vs 0.4 to 1.1 cm in H. australiensis), both of which have spaces greater than the other three described species, and 2) the basal diameter of lateral pinnules is 0.3 to 0.5 mm in the new species and 0.5 mm in H. australiensis), both of which are smaller than the other three described species. The two specimens representing the new species form a clade sister to H. heterosticha; however, the other species in the genus have yet to be sequenced. Specimens representing species in the genus Hexapathes should be sequenced to further investigate morphological boundaries between species.

Diagnosis: Colony monopodial, unbranched, and pinnulate. Pinnules arranged in two lateral rows and one anterior row. Basal-most pair of lateral pinnules subopposite, other lateral pinnules alternating. Striatum present from 1 cm above basal plate to first anterior pinnules. Lateral pinnules simple, up to 12 cm long, densities of six to 10 per 3 cm counting both rows. Anterior pinnules simple, 0.8 to 1.2 cm in length, densities of 11 to 15 per 3 cm. Polyps 4 to 6 mm in transverse diameter.

Distribution. Known only from the Great Barrier Reef and Coral Sea from 638 to 789 m depth.

Etymology: In recognition of lead author’s grandfather, Morton Isaiah Bikofsky, a high school teacher whose passion for science fuelled JH’s interest in research.

Material examined: Holotype. MTQ G 80122, Australia, Great Barrier Reef, Noddy Reef, expedition Schmidt Ocean Institute R / V Falkor Seamounts, Canyons, and Reefs of the Coral Sea expedition 200802, ROV Subastian dive S 0398, collected on October 15, 2020, 13.5174 ° S, 144.1012 ° E, 789 m depth, collector Jeremy Horowitz. Paratype. MTQ G 80024, Australia, Coral Sea, Herald Cays, expedition Schmidt Ocean Institute R / V Falkor Seamounts, Canyons, and Reefs of the Coral Sea expedition 200802, ROV SuBastian dive S 0376, collected on August 08, 2020, - 16.9095 ° S, 149.1601 ° E, 638 m depth, collector Jeremy Horowitz.