Rhipidipathes helae Horowitz, 2022

Horowitz, Jeremy, Opresko, Dennis, Molodtsova, Tina N., Beaman, Robin J., Cowman, Peter F. & Bridge, Tom C. L., 2022, Five new species of black coral (Anthozoa; Antipatharia) from the Great Barrier Reef and Coral Sea, Australia, Zootaxa 5213 (1), pp. 1-35 : 10-13

Rhipidipathes helae Horowitz sp. nov.

( Figs. 1–2 and 5 ; Tables 1 and Supplementary Table 1 )

Material examined: MTQ G80117, Australia, Great Barrier Reef, Bowl Slide, Schmidt Ocean Institute R / V Falkor Northern Depths of the Great Barrier Reef expedition FK200930, ROV SuBastian dive S0394, collected on October 5, 2020, 18.3865° S, 147.6705° E, 119 m depth, collector Jeremy Horowitz.

Diagnosis: Corallum flabellate, branched; branches and branchlets extensively anastomosing. Terminal branchlets 0.5 to 1 cm in length and 0.08 mm in diameter, arranged bilaterally, are irregularly alternate, opposite, or subopposite, and slightly protrude from the colony plane. Spines on branches perpendicular or distally inclined. Circumpolypar spines 0.23 to 0.29 mm tall and hypostomal and interpolypar spines maximum of 0.11 mm tall. Spines on terminal branchlets are distinctly curved apically and rarely basally. Five to six axial rows of spines can be counted in one view. Surface of spines extensively tuberculated, especially from about the midpoint to the apex. Polyps roundish, 0.8 mm in transverse diameter. Interpolypar space 0.1 to 0.2 mm, with 10 polyps per cm.

Description of holotype: Colony flabellate and about 20 cm wide and 20 cm high based on estimations from in situ images ( Figs. 5A–B ). Collected sample about 7 cm wide and 9 cm tall. Longest branches are ~ 8 cm in length and have 0.1 cm basal diameter. Terminal branchlets are 0.5 to 1 cm in length, arranged bilaterally, and are either irregularly alternate, opposite, or subopposite. Branchlets often not strictly bilateral but slightly protrude from the general plane of the colony forming ~120° interior angles. Most terminal branchlets form ~80° distal angles and are slightly curved distally ( Fig. 5B ). There is extensive fusing among branches and terminal branchlets ( Fig. 5C ). Four anastomosing branches/branchlets can be counted in a 5 cm 2 fragment of a colony. Terminal branchlet basal diameter is 0.08 mm, distance between neighboring terminal branchlets ranges from 1 to 3 mm, and about 10 branchlets can be counted per cm of a branch, counting branches in both rows.

On a branch 0.2 mm thick, polypar spines are anisomorphic with circumpolypar spines ranging from 0.23 to 0.29 mm tall and hypostomal and interpolypar spines reduced to 0.11 mm. Abpolypar spines are uniform in height, ranging from 0.13 to 0.15 mm ( Fig. 5D ). Polypar spines on branches are positioned mostly perpendicular to the axis or slightly distally inclined and abpolypar spines are more distally directed than polypar spines, creating ~45° distal angles ( Figs. 5D–E ). Six to 10 conical and apically directed tubercles can be counted in lateral view of the polypar spines, including those on the edges ( Fig. 5E ) and three to six tubercles can be counted in lateral view of abpolypar spines, with the proximal surface of all spines being mostly smooth ( Figs. 5C–D ). Tubercles become elongated and strongly appressed to the surface of the spine as they increase in size, reaching a maximum size of 0.03 mm (measuring the distance from the base of the tubercle to the apex of the tubercle). Tip of largest tubercles up to 0.004 mm above the spine surface ( Fig. 5E ). On thin branchlets (0.08 to 0.095 mm in diameter), circumpolypar spines are 0.13 mm tall, oriented perpendicular to the axis, and are distinctly curved upward ( Fig. 5D , right image). On thin branchlets abpolypar spines are 0.11 mm tall, distally directed and are curved upward. On thin branchlets, a maximum of three tubercles can be counted in one lateral view of the surface of polypar and abpolypar spines, with the proximal surface of all spines being mostly smooth. On branches and terminal branchlets, five to six uneven axial rows of spines can be counted in one view.

Polyps are pink, roundish with equally developed tentacles, and sagittal tentacles positioned slightly lower than lateral tentacles ( Fig. 5B ). On terminal branchlets and branches, polyps occur in one row; however, polyps can be arranged in several rows along thicker branches near the base of the colony. Polyps are ~ 0.8 mm in the transverse diameter and spaced 0.1 to 0.2 mm apart, resulting in 10 polyps per cm. Tentacles are approximated from in situ images to be 0.15 mm in length, when extended.

Comparative diagnosis. This is the third nominal species in the genus Rhipidipathes . Rhipidipathes reticulata ( Esper 1795) and Rhipidipathes colombiana ( Opresko & Sánchez 1997) are similar in most features (See Table 1 for comparison of three species); however, R. colombiana has limited anastomosing branchlets, hypostomal spines that are only minimally reduced in size, and spines with almost no tubercles while R. reticulata has greater anastomosing branchlets, reduced hypostomal spines, and possesses tubercles on polypar spines. The new species is morphologically and phylogenetically most similar to R. reticulata by having highly anastomosing branchlets and terminal branches, reduced hypostomal spines and clear presence of tubercles on spines. The new species is different from R. reticulata by having thinner terminal branchlets (0.08 vs 0.22 mm in the type) and has spines on terminal branchlets that are distinctly curved, mostly upward but sometimes downward, not found in R. reticulata . See comparison of spines on a terminal branchlet between R. helae sp. nov. ( Fig. 5D , right image) that possesses upward curved spines and R. reticulata holotype ( Fig. 5F ) that possesses straight spines. Additionally, the new species has a greater number of tubercules on polypar and abpolypar spines than R. reticulata (six to 10 vs three to seven tubercles in one view of a polypar spine, and three to six vs zero to three tubercles in one view of an abpolypar spine).

Etymology: From the Norse, “hel”, goddess of death, who is depicted wearing a headdress of curved deer antlers that resemble the distinctively curved spines of the new species.

Distribution. Known only from the Great Barrier Reef at 119 m depth.

Figures

FIGURE 1. Locations where specimens were collected shown on a bathymetric surface map of the Great Barrier Reef and Coral Sea. Blue boxes represent specimens hand collected while on SCUBA, and red circles represent specimens collected via a remotely operated vehicle.

FIGURE 2. Maximum likelihood phylogeny of the Antipatharia based on a 50% complete matrix containing 1,047 loci. Taxa in blue and imaged represent species described in this study.

FIGURE 5. Rhipidipathes helae sp. nov.: holotype, MTQ G80117, A–E. A, in situ images of colony; B, collected sample; C, branches showing anastomosis; D, sections of branch (left) and terminal branchlet (right) showing curved polypar (left sides of branch and terminal branchlet) and abpolypar (right sides of branch and terminal branchlet) spines; E, skeletal spine showing tubercles; F, Rhipidipathes reticulata (Esper), holotype, SMF 5885, section of terminal branchlet showing straight spines.