Hippasteria capstonei, Mah, 2022

Mah, Christopher L., 2022, New Genera, Species and Occurrences of Deep-Sea Asteroidea (Valvatacea, Forcipulatacea, Echinodermata) collected from the North Pacific Ocean by the CAPSTONE Expedition, Zootaxa 5164 (1), pp. 1-75 : 51-53

Hippasteria capstonei n. sp.

FIGURE 17 A–G

Etymology: This species is named in honor of the Okeanos Explorer Pacific survey expedition named Campaign to Address Pacific monument Science, Technology, and Ocean NEeds or CAPSTONE (from 2015 to 2017) under the auspice of which, this species was observed and collected.

Diagnosis. Body shape weakly stellate (R/r=1.9), arms short, interradial arcs weakly curved. Abactinal surface covered by abundance of large bivalve pedicellariae, equal to 3 or 4 granules in length and short, conical pointed spines. Superomarginal plates with conical, pointed spines, 1 to 5 present on a raised platform on the superomarginals with a sculpted pattern on the surface. One or two, exceptionally three large, conical bullet-shaped spines located centrally on each actinal plate. Furrow spines three per plate, round in cross-section, evenly spaced, blunt tipped. One or two enlarged, blunt tipped subambulacral spines present in transverse series.

Comments. Although the specimen was not tested with molecular phylogenetic methods, the bullet shaped spines and the bivalve pedicellariae support this species as a member of the “ Hippasteria heathi clade” as outlined by Mah et al. (2014), which includes Hippasteria falklandica , H. heathi , H. mcknighti , and H. tiburoni .

Hippasteria capstonei n. sp. differs from other known species in the H. heathi clade in that there is the possession of multiple numbers of short, conical spines, the raised, sculpted surface on the superomarginal plates and the more weakly stellate body shape. Marginal plate shape, elongate marginal spines and the weakly stellate body shape differentiates this species from H. falklandica and the pointed spine shape in H. tiburoni are different from the bullet shaped spines in Hippasteria capstonei n. sp. Hippasteria heathi and especially H. mcknighti both share similar characters, including identical types of bivalve pedicellariae. However, the spine and granule types in H. heathi and H. mcknighti are more elongate and more bullet-shaped than those in Hippasteria capstonei n. sp. which are shorter and more conical.

Feeding Observations. This species was observed climbing up and feeding upon Narella sp. , an octocoral in the Primnoidae , in a fashion typical of corallivorous goniasterids ( Fig. 17B ) climbing along the stalk and systematically feeding on the polyps. The base had been denuded leaving approximately the upper 80% of the colony still displaying the capsules apparently intact. However, upon closer examination, it became clear that the capsules on the lower half of the colony upon which the star had been perched were devoid of the internal polyp, leaving only the empty external covering. Capsules on the distal tip of the Narella colony, upstream of where the star was present, were not yet fed upon and still retained living polyps within them. The basal portion of the stalk appears to have been denuded, however it is unclear however if these were eventually devoured by the star or may have simply fallen off lacking living tissue. There was no indication of whether the star simply prefers digesting the polyp tissue followed by the capsule or if the capsule is inedible. The capsules in Narella and other members of the Primnoidae are composed of calcium carbonate sclerites ( Cairns & Bayer 2008).

Occurrence: Johnston Atoll region. 1993.94 m

Description. Body thick, pentagonal to stellate (R/r=1.9) in outline but with short arms, broad, weakly curved interradii ( Fig. 17A, B, C ). Arms upturned.

Abactinal surface arched, inflated with water in life (surface deflated upon removal from water). Plates round to irregularly polygonal, abutted forming jigsaw-like surface.Abactinal plates extend to terminal plate on arm. Plate boundaries obscured by membrane and spines. Surface covered by large, cone-shaped pointed spines, variable in size from 0.3 mm to 1.0 mm in length, heterogeneous across the abactinal surface. Spines abundant, approximately 2 spines per 1.0 mm line ( Fig. 17A ). Pedicellariae, large (1.5–2.0 mm in length), bivalve, sitting approximately 0.1 above the surface ( Fig. 17A inset, D), present primarily around the edge of the disk with fewest pedicellariae proximally. Some pedicellariae closer to central disk are relatively short compared to distal ones, approximately 75% of the length. Larger spines and plates present centrally on abactinal plates with smaller spines present around periphery, surface otherwise smooth and bare. Papulae large, finger-like approximately same length as larger spines, projecting through round openings, present along radial and basal interradial regions. Madreporite flattened, concave, embedded into surface. Anal region centrally located, covered by membrane, flanked by five large conical blunt spines.

Marginal plates 16 to 18 per interradius, with superomarginals and inferomarginals identical in number demonstrating 1:1 correspondence but slightly offset showing zig-zag contact ( Fig. 17A ). Superomarginal plates, trapezoidal interradially becoming more quadrate distally, forming mostly an abactinal-lateral edge with one to five large conical pointed spines with three or four present interradially decreasing to a single large spine per plate distally. A smaller spine or bivalve pedicellaria regularly present on superomarginal plates near the base of the larger, conical spines. Superomarginal plate central surface adjacent to spines are covered by a raised, distinct bump-like structure ( Fig. 17D ). Sculpted area is smooth, devoid of accessories, slightly discolored compared to surrounding plate but with no sign of breakage. Periphery of superomarginal plates covered by approximately 25–60 (mostly 40) pointed, coarse granules, numbering approximately 8 per with, 12 per length ( Fig. 17D, E ). Inferomarginals twice the width of corresponding superomarginal interradially, each with two to seven large, thick, conical, pointed spines, one or exceptionally two small bivalve pedicellariae, and 1 to 6 small, pointed granules on an otherwise smooth inferomarginal surface. Inferomarginal periphery composed of approximately 20–60, mostly 35–55 coarse, pointed granules ( Fig. 17F ), approximately ten along the width, 12–15 along the length. Shallow fasciolar groove present around the superomarginal and inferomarginal plates as well as around the contacts between the abactinal/actinal surfaces with the marginal plates. Terminal plates large, approximately the size of two adjacent superomarginal plates, surface round. convex with smooth surface.

Actinal surface composed of rounded quadrate to irregular polygonal plates in chevron formation ( Fig 17C ). One or two, exceptionally three large, conical bullet-shaped spines located centrally on each plate ( Fig. 17C, F ). On plates adjacent to the adambulacral series, large bivalve pedicellariae transversely present, so large that they bisect the plate upon which they sit, adjacent to or replacing each spine up to base of or midpoint along arm. Each large actinal spine surrounded by 2 to 12 smaller, accessory pointed granules adjacent to the larger bullet shaped primary spines. Actinal plate surface is otherwise smooth, bare.

Furrow spines three per plate ( Fig. 17G ), round in cross-section, evenly spaced, blunt tipped. One or two enlarged, blunt tipped subambulacral spines present in transverse series relative to tube foot groove, each approximately twice the thickness and about 15% longer than the furrow spine. Three to six small (~1/10 length of subambulacral spine), accessory pointed granules present near base of each, large spine.

Color in life was uniform light orange on abactinal and actinal surface ( Fig. 17B ).

Material Examined. Holotype. USNM 1457387, South Johnston Seamount, Johnston Atoll region, North Pacific Ocean. 15º02’N, 170º52’W, 1994 m, Coll. C. Mah & C. Kelley, NOAA Ship Okeanos Explorer using ROV Deep Discoverer, EX 1706 . 25 July 2017. 1 wet spec. R =3.3 r=1.7. EX 1706_IMG_20170725 T 003303Z_ ROVHD. jpg

Images Examined

South Johnston Seamount , Johnston Atoll region. 15.03905368, -170.8744186, 1982 m. EX1706 _IMG_20170725 T011410 Z_ ROVHD.jpg

Figures

FIGURE 17. Hippasteria capstonei n. sp. USNM 1457387. A. Abactinal view. Scale bar=5.0 mm. Inset showing pedicellariae and surface details. Pedicellariae=1.5 mm length. B. In situ observation of holotype feeding on primnoid coral, Johnston Atoll, 1982 m. C. Actinal view. Scale bar=5.0 mm. D. Abactinal-lateral view showing pedicellariae and spination. Scale bar=2.0 mm. E. Lateral view showing spines and other accessories. Scale bar=2.0 mm. F. Actinal close up, showing spination, etc. Scale bar=2.5 mm. G. Adambulacral spination, including furrow and subambulacral spines. Scale bar=1.0 mm.