Alternatipathes mirabilis, Opresko & Molodtsova, 2021

Opresko, Dennis M. & Molodtsova, Tina N., 2021, New species of deep-sea Antipatharians from the North Pacific (Cnidaria: Anthozoa: Antipatharia), Part 2, Zootaxa 4999 (5), pp. 401-422 : 403-405

Alternatipathes mirabilis n. sp.

( Fig. 1 )

urn:lsid:zoobank.org:act:C810F528-71BD-4604-A08F-5180236EDCB8

Material examined. Holotype: USNM 1070972 ( SEM stubs 451 and 488), North Pacific , Derickson Seamount, 53.0419°N, 161.1830°W, ROV Jason II, Dive 93 (Field Identification Number: JD-093), 4685 m, coll. A. Baco- Taylor, 20 July 2004.

Diagnosis. Colony attached, monopodial, unbranched, and pinnulate. Pinnules simple, arranged alternately in two lateral rows along upper part of stem. Lower unpinnulated section of the stem longer than upper pinnulated section. Pinnules generally decreasing in length proximally to distally. Pinnular density 10–11 per 3 cm (including pinnules in both rows). Spines on pinnules smooth, triangular in profile, moderately acute, laterally compressed, and mostly 0.04–0.06 mm tall on polypar side of axis. Polyps on pinnules mostly 5–7 mm in transverse diameter, with four to five polyps per 3 cm.

Description of the holotype. The holotype (USNM 1070972, Fig. 1A ) is an almost complete colony with an intact basal holdfast; the very tip of the stem, however, is broken off. The stem is about 46.5 cm long with a basal diameter of 2.4 mm. The lower unpinnulated section of the stem is 32.5 cm, and the upper pinnulated section covers a distance of 14 cm. The pinnules are simple, bilateral and alternately arranged; generally decreasing in length from the lower part of pinnulated stalk to the distal end. Most of the pinnules are broken off at the end; the longest remaining ones are about 15 cm in length with a basal diameter of about 0.8 mm. Forty-six pinnules occur on the corallum, 23 on each side of the stem. Within each row, the pinnules are spaced 5–6 mm apart. The resulting pinnular density is 10 (total for both rows) per 3 cm near the basal end of the stem to about 11 per 3 cm in the middle and towards the distal end. In the preserved specimen the lower rows of pinnules form an interior angle of about 60°, which increases towards the top of the corallum to 180°. The distal angle that the pinnules form with the stem is 60–80°, even at the top of the corallum.

The spines on the pinnules are very small, triangular in profile, moderately acute, smooth, and laterally compressed ( Fig. 1B, 1D ). There is only a slight difference in the size of the polypar and abpolypar spines. On sections of pinnules about 0.4 mm in diameter the polypar spines are mostly 0.04–0.047 mm tall (maximum 0.06 mm) and the abpolypar spines are 0.03–0.04 mm tall. Double spines are present, but are not common. The central axial canal is 0.31 mm in diameter on a pinnule 0.43 mm in diameter. Five or six axial rows are visible in lateral view, and a few individual spines occur randomly between the rows. In each row the spines are a variable distance apart, 0.26 to 0.79 mm; however, on average, there are 2.5–3.5 spines per mm in each axial row. The spines on the stem are similar to those on the pinnules in size and shape.

The polyps ( Fig. 1C ) occur in a single series on one side of the pinnules; they are 5–7 mm in transverse diameter. Near the distal end of the pinnules there are four polyps per 3 cm, and in the mid to basal sections of the pinnules there are up to five polyps per 3 cm.

Other material. No other specimens can be assigned to the species at this time.

Genetic data. DNA analysis (nad5-nad1) of the holotype ( USNM 1070972 ) suggests a close relationship to Schizopathes ( Chery et al. 2018) ; however, further study is needed .

Comparisons. Alternatipathes mirabilis n. sp. differs from the type species of the genus, A. bipinnata , in that it does not form branches; it has longer pinnules (15 cm vs. 4.5 cm); much smaller spines (0.04–0.06 mm vs. up to 0.3 mm); much larger polyps (5–7 mm vs. 2–3 mm); and a lower polyp density (4–5 per 3 cm vs. about 9 per 3 cm).

Alternatipathes mirabilis n. sp. differs from specimens assigned to A. alternata in having: 1) relatively thicker pinnules (0.8 mm vs. 0.5 mm basal diameter for pinnules 13–15 cm long); 2) a wider central axial canal (0.3 mm vs. 0.12–0.18 mm on pinnules 0.22–0.25 mm in diameter, excluding spines); 3) less of a difference in the relative sizes of the polypar and abpolypar spines (about one-third larger in A. mirabilis vs. more than twice as large in A. alternata ); 4) relatively longer pinnules (15 cm or more vs. 10.5 cm) on colonies having pinnulated sections of similar length (13 cm vs. 11 cm); 5), a wider distal angle of the pinnules at the top of the corallum (60–80° vs. 45° or less) and 6) larger polyps (5–7 mm vs. 3–4 mm in transverse diameter), resulting in a smaller polyp density (4–5 vs. 6–8 per 3 cm). However, it is important to note that these comparisons are based on a single specimen that is currently assigned to Alternatipathes mirabilis ; thus, the ranges reported for this species should not be considered absolute.

Alternatipathes mirabilis n. sp. differs from specimens assigned to A. venusta in that it has a wider distal angle of the pinnules at the top of the corallum (60–80° vs. 30°), no difference between polypar and abpolypar spines, much shorter pinnular spines (0.04–0.06 mm vs. 0.11–0.22 mm); slightly larger polyps (5–7 mm vs. 3.5–5 mm); and a lower polyp density (4–5 per 3 cm vs. about 6–8 per 3 cm).

Etymology. The species name is derived from the Latin, mirabilis , meaning “wonderful or strange”.

Distribution. Known only from a single specimen collected in the North Pacific at a depth of 4685 m.

Figures

FIGURE 1. Alternatipathes mirabilis n. sp., holotype, USNM 1070972: A. corallum; B. enlarged view of spines; C. polyps; D. sections of pinnules showing spines, scale bars 0.2 mm (B and D from SEM stub 488).