Alternatipathes venusta, Opresko & Wagner, 2020

Opresko, Dennis M. & Wagner, Daniel, 2020, New species of black corals (Cnidaria: Anthozoa: Antipatharia) from deep- sea seamounts and ridges in the North Pacific, Zootaxa 4868 (4), pp. 543-559 : 550-553

Alternatipathes venusta sp. nov.

( Fig. 6–8 )

Material examined. Holotype: USNM 1404492 ( SEM stub 406), Hawaiian Islands , McCall Seamount, 18.977ºN, 157.1107ºW, R / V Okeanos Explorer, ROV Deep Discoverer, EX1504 _L3_D2_ DIVE04 _ SPEC06 BIO, 2,638 m, NOAA, 1 Sep 2015 . Paratype: USNM 1234537 , Northeast Pacific, Gorda Ridge, Gorda Transform, 41.6701ºN, 127.317ºW , R / V Western Flyer , ROV Tiburon, Sta. T-193-A4, 2,821.1 m , MBARI, 12 Aug. 2000 .

Type locality. Hawaiian Islands , McCall Seamount, 2,638 m .

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 up to two times longer than upper pinnulated section. Pinnules generally decreasing in length proximally (> 13 cm) to distally (~ 3 cm) in a colony with a 17 cm long pinnulated section. Pinnules 4–6 mm apart on either side of axis; nine to 11 pinnules (total for both rows) per 3 cm. Polypar spines on pinnules, conical, smooth, acute, and up to 0.22 mm tall; a few are bifurcated. Abpolypar spines short, triangular, up to 0.08 mm tall. Five to six rows of spines visible in lateral view; with 4 spines per mm within each row on polypar side; 3–4 spines per mm on abpolypar side. Polyps about 5 mm in transverse diameter, with 2 polyps per cm.

Description of holotype. The holotype (USNM 1404492) is a relatively large monopodial colony with a long unpinnulated stalk and a shorter upper pinnulated section ( Fig. 6 A–B, 7A). Although the collected specimen is missing the lowermost part of the stem, in situ photos indicate that the colony was attached to the hard substrate by a basal plate. The length of the stem of the collected specimen is 29 cm, with the upper 17 cm containing simple pinnules arranged alternately in two lateral rows. The total length of the unpinnulated part of the stem was estimated from in situ images of the colony to be about 40 cm; therefore, the total length of the stem in the intact colony is estimated to have been 57 cm. The stem diameter at its broken end is 1.2 mm. A striatum was not found on the 12 cm section of stem still remaining; however, on this section the spines often occur along the crests of axial ridges.

On the collected specimen the pinnules in the lowest four pairs are broken off, therefore, their length cannot be determined. The next highest ones are 13 cm long with a basal diameter of about 0.9 mm. The pinnules in the twelfth pair from the apex are about 10 cm long. The spacing of the pinnules varies slightly along the stem (4 to 6 mm apart in each row), such that there are ten to 11 pinnules total per 3 cm. The lowest pinnule is on the left side of the colony (the polyp side being the front side of the corallum) and alternates with the one on the right. Distal angles that the pinnules form with the stem are near 60° on the lower and middle parts of pinnulated section of the stem; decreasing to about 30° near the tip. The interior angle formed by the two rows of pinnules is close to 90°on the lower part of the pinnulated section of the stem; increasing to near 180° near the tip.

The skeletal spines on the pinnules ( Fig. 7 B–C) are distinctly different on the polypar and abpolypar sides of the axis. On the polypar side they are tall, conical, smooth, with a sharp or slightly rounded apex. The abpolypar spines are short, triangular in lateral view, and rounded at the apex. On a section of pinnule where the axial diameter is 0.24 mm, the polypar spines are 0.11 mm and the abpolypar spines are 0.06 mm. On a section of pinnule where the axial diameter is 0.4 mm, the polypar spines are 0.22 mm and the abpolypar spines are 0.08 mm. The polypar spines are at right angles to the axis or are angled distally or proximally slightly. The spines are in axial rows, five to six of which can be seen in lateral view. The spacing of the spines within the rows ranges from 0.2 to 0.46 mm; on average, there are about 4 spines per mm on the polypar side and slightly less on the abpolypar side. In some places the rows of spines appear to originate on axial ridges running in a proximal to distal direction. The polypar spines, and some on the sides of the axis, are forked, and appear to eventually lead to the formation of double spines. On the stem the spines ( Fig. 7D ) are reduced in size and usually not more than 0.06 mm tall. They are often very shallowly sloped at the base in both a basal and distal direction. The number of rows and the density within the rows is similar to that on the pinnules, but the rows are not as uniform as those on the pinnules and scattered spines also occur between the rows.

The polyps near the base of the pinnules are 3.5–4 mm in transverse diameter with 2–2.5 polyps per cm. Polyps along most of the remaining sections of the pinnules are close to 5 mm in transverse diameter and the interpolypar space is about 1 mm, resulting in a polyp density of 2 polyps per cm or 6 polyps per 3 cm.

Description of the paratype. The paratype (USNM 1234537) consists only of the upper pinnulated part of a colony, the lower unpinnulated stalk was not collected. The remaining pinnulated section is about 20 cm long. This specimen is similar to the holotype in the length of the pinnules (up to 14.5 cm), in the density of the pinnules (9–10 per 3 cm), and in the size of the polypar spines (mostly 0.11 to 0.15 mm, but up to 0.19 mm tall), but it has smaller polyps (3–4 mm in transverse diameter with a density of 2.6–3 polyps per cm).

Comparisons. Alternatipathes venusta is morphologically similar to A. alternata ( Brook, 1889) . Both species form a monopodial, unbranched corallum. Although the type of A. venusta is larger than that of A. alternata and has longer pinnules, the ratio between pinnule and stem length is similar in both species. Thus, the length of the pinnules on the upper 10 cm of the A. venusta corallum are proportionally similar to those on the type of A. alternata , whose pinnulated section is only 10 cm long. The basal-most pinnules (twelfth pair from the apex) in A. alternata are 10.5 cm in length, and in A. venusta the pinnules in the twelfth pair from the apex are also about 10 cm long. Differences between the types of both species are in the length of the non-pinnulated stalk (about four times longer in A. venusta ), in the density of the pinnules (9–11 pinnules total for both sides per 3 cm vs. 7–8 pinnules total per 3 cm in A. alternata ), and in the size of the spines (polypar spines up to 0.22 mm in A. venusta and only 0.06 mm in A. alternata ).

The type species of the genus, A. bipinnata ( Opresko, 2005) has spines that are very similar to those of A. venusta in both shape and size. The polypar spines in A. bipinnata are up to 0.3 mm in height, whereas the abpolypar spines are half that size or smaller. The two species can be differentiated however, by the fact that in A. bipinnata the pinnules are much shorter (4.5 cm vs. 13 cm or more), the lowermost pinnules are developed into pinnulated branches, and the polyps are smaller (2–3 mm in transverse diameter vs. up to 5 mm).

The unpinnulated section of A. venusta is longer than its pinnulated section, a trait that is also present in the genus Umbellapathes . However, species of Umbellapathes , a lways have more than one order of pinnulated branches developing from the lowermost primary pinnules.

Etymology. From the Latin “ venusta ” meaning beautiful.

Distribution. Currently only known from the Hawaiian Islands and Gorda Ridge at depths ranging between 2,638 –2,821 m ( Fig. 8 ).

Figures

FIGURE 6. In situ photographs of Alternatipathes venusta sp. nov. holotype (USNM 1404492) photographed near McCall Seamount at 2,638 m. Photos courtesy of NOAA Office of Ocean Exploration and Research.

FIGURE 7. Alternatipathes venusta sp. nov. holotype (USNM 1404492). A. colony; B–C. skeletal spines on pinnules; D. spines on stem (B-D from SEM stub 406).

FIGURE 8. Map showing the known geographical distribution of Alternatipathes venusta sp. nov. To date, this species is only known from the Hawaiian Islands and Gorde Ridge at depths ranging between 2,638–2,821 m.