Evoplosoma mystrion, Mah, 2023

Mah, Christopher L., 2023, New Goniasteridae and in situ observations significant to deep-sea coral predation, Memoirs of Museum Victoria 83, pp. 1-35 : 19-21

Evoplosoma mystrion n. sp.

urn:lsid:zoobank.org:act:927B5868-A059-4A3B-839E-

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Figure 8a–f

Etymology. The specific epithet mystrion is derived from the Greek for “spoon”, alluding to the shape of the pedicellariae on the actinal surface.

Diagnosis. Body stellate (R/r=3.48), arms elongate, interradial arcs acute. Pedicellariae absent from abactinal, marginal surfaces. Granules with round to spinose tips, forming continuous cover over abactinal, marginal and actinal plate surface. Superomarginal and inferomarginal series each with a single prominent, conical spine present in series along arm, although two or three per plate in each interradius. Pedicellariae present on actinal surface, paddle or spoon-shaped, numbering 2–10 on plates adjacent to the adambulacral groove and on the adambulacral plates adjacent to the subambulacral spine. None known on abactinal or marginal surface. Furrow spines, two or three (mostly three), thick, blunt. Subambulacral one, thick, approximately twice the thickness of a furrow spine. Pedicellariae with rectangular valves on most adambulacral plates adjacent to subambulacral spine.

Comments. A hippasterine species placed within Evoplosoma based on the presence of spatulate pedicellariae, complete spinose granulate covering of the body surface, especially over the marginal plates, the prominent abactinal, marginal and actinal spines, and the overall stellate body shape.

This species shares several characters with E. besseyae , including the continuous coarse, granular cover present on abactinal, marginal and actinal surfaces as well as similar numbers of furrow and subambulacral spines. This species is most similar to the Atlantic Evoplosoma watlingi Mah 2015 , Evoplosoma scorpio Downey 1981 , and Evoplosoma virgo Downey 1982 , which all display a continuous cover of pointed granules or spinelets over the entire body surface, including the marginal plates. Evoplosoma virgo lacks any large spines on the abactinal or marginal plates. Evoplosoma scorpio differs in having a greater number of furrow spines (8–10), and in displaying interradial marginal plates with a single large spine in addition to smaller subsidiary spines (4–8), in association with the large primary one. In E. mystrion n. sp. furrow spines are fewer (two or three) and larger, and consistently large conical spines are present in each interradius. Abactinal spination in E. scorpio also displays more irregularly distributed, widely occurring abactinal spination versus E. mystrion n. sp., which is present as a singular carinal series with adjacent ordered radial spine series. Evoplosoma watlingi has a widespread and evenly spaced distribution of conical spines on the abactinal surface, five or six furrow spines and two subambulacral spines per plate, in contrast to E. mystrion , with only two or three furrow spines, a single subambulacral spine, and an incomplete distribution of pointed spines on the abactinal surface.

Occurrence . Tasmanian Seamounts, 915–1182 m. Description. Body stout, stellate (R/r=3.48) in shape. Arms triangular, elongate, gradually tapering. Interradial arcs acute.

Abactinal, marginal and actinal surface covered by granule-invested integument. Individual granules, round to irregularly polygonal, evenly spaced. Abactinal surface mostly obscured by integument, plates concealed. Prominent conical spines abundant except interradially, especially adjacent to superomarginal contact. Large round tubercles present proximally and on central disk region. Angular granules covered with more strongly developed tissue, differing from round granules present elsewhere and on central disk, covered by a thin translucent mucus. No pedicellariae observed. Madreporite weakly inset on abactinal plates, convex. Sulci well developed.

Marginal plates quadrate, surfaces flush with abactinal and actinal plate surfaces. Approximately 24–26 plates per arm side, 48–52 per interradius, arm tip to arm tip. Each plate with roughly equal dimensions (L=W), surface covered by granules, round to pointed, smooth, evenly spaced, 50–200 (mostly 70–100) per plate surface. Granular layer appears continuous with those on abactinal and actinal surface, such that they incompletely obscure contact between marginal, abactinal and actinal plates. Superomarginal plates each with a single prominent, conical spine along the upper plate surface adjacent to abactinal plate contact, rising well above the plate surface and taller than those on the abactinal plates. Spine base lacking granules. Inferomarginal plates have 2–6 similar, prominent, conical spines, interradially decreasing to a single spine along arms. Interradial spines approximately 10% shorter than those on arms, clustered on plate centre rather than upper edge. Variably, some distalmost inferomarginal plates lack spines. Shallow grooves present between superomarginal and inferomarginal plates. No pedicellariae observed.

Actinal surface covered by 7–15 (mostly 8–10) irregularly polygonal granules encased in a translucent, very thin skin or mucus. Abactinal plates polygonal to quadrate, present in 3–5 series, of which only one or two extend completely onto the arms. Granules abundant and continuous, plate boundaries weakly defined. Pedicellariae, paddle-shaped, alveolar, present only on 2–10 proximal actinal plates adjacent to adambulacral plates along each tube foot groove, 4–20 per interradius.

Furrow spines, two or three, blunt tipped, but round to oblong in cross-section. Single subambulacral spine, approximately twice as thick as a single furrow spine. Proximal on the plate, a large single tong-like pedicellariae with rectangular valves. Remainder of adambulacral plate surface adorned with 4–7 pointed to blunt, irregularly polygonal granules, with a single large tubercular granule adjacent to the subambulacral spines.

Oral plate furrow spines 10, thin to triangular in cross-section, with a single, enlarged spine projecting into the mouth with a tip rounded, tong shaped. Oral plate surface with large blunt subambulacral spines corresponding to the furrow spines, similar in appearance to those on other adambulacral plates. Oral plate surface with 4–6 (mostly five) polygonal granules, quadrate in cross-section on either side edge of the diastema between oral plates in each interradius. Polygonal granules, low, blunt, 2–4 on remainder of oral plate surface. Thin, membranous skin similar to those elsewhere covering oral plates.

Material examined. Holotype. NMV F307999 Pedra southern flank, Tasmanian seamounts, Tasmania, Australia. 44.2678° S, 147.109° E, 915–1182 m. Coll. A. Williams, A.A. Weber, R-L Erickson, 25 Nov. 2018. 1 wet spec, R =115.0, r= 33 mm.