Heligmaster, 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 : 19-20

Heligmaster n. gen.

Etymology. Combination of the Greek “ heligma ” for “curl” and “aster” for star referring to the strong curvature of the arms.

Diagnosis. Body stellate to strongly stellate (R/r=4.3–8.6) with elongate strongly curled arms.Abactinal surface composed of thin membranous skin through which distal ambulacrals and pyloric stomach are visible in life.Abactinal plates sit on delicate oval to circular shaped glassine imbricate plates, variably spaced, each bearing dermis-covered hyaline spines. Single marginal plates series present. Each plate composed of one or two elongate spines with jagged to sharp tips sitting on a broad, rounded base, quadrate to round in outline. Marginal plates, approximately twice the size of each subambulacral spine, with one marginal present for every 3 subambulacrals, separated from one another by distinct space, surface covered by soft dermis. Subambulacrals with variable spine number, similar in shape to those on superomarginals, also sitting on low, rounded-convex base. Furrow spines variably webbed or unwebbed, 1 to 6 per plate, mostly 4 to 6. Ambulacral ossicles with strongly angular, arrow-shaped head, flat neck (or ishtmus) with quadrate shaped base. Podial pores remain biserial throughout, but drawing together very closely on distalmost armtip (approximately 10 plates from terminal) giving the appearance of switching from biserial to monoserial near terminus.

Placement and Establishment of Heligmaster . Morphological characters of the two specimens described herein support the new genus Heligmaster , which includes two new species, within the Myxasteridae . Ambulacral and other skeletal terminology follows Turner & Dearborn (1972) and Breton (1992). The ambulacral ossicles ( Fig. 6C ) display the flattened angular proximal-facing head (or capitulum), the shaft (or isthmus) with flattened surface and round-quadrate shaped base. Other myxasterid characters observed include the delicate, glassine, imbricate plates present in the abactinal surface and the spines and/or spine clusters which sit on the low, rounded bases. Although adambulacral ossicle morphology is not identical, Heligmaster demonstrates the same type of arrangement showing tissue-filled spaces alternating with the individual adambulacral ossicles along the arm.

Heligmaster shows characters that distinguish it from the other genera in the Myxasteridae . Most notably, Heligmaster shows a full marginal plate series, as defined by Blake (1978) as extending from the interradius to the terminal plate. This character is absent or weakly developed in other genera within the Myxasteridae ( Alton 1966; Mah et al. 2012). Heligmaster also does not appear to have an osculum nor the associated array of osculum-associated plates and spines as is observed in other myxasterids.

Heligmaster ’s general appearance invited comparisons with other taxa showing superficial similarity, notably the Antarctic ganeriid Cuenotaster and members of the Benthopectinidae . Ambulacral ossicles provided the most immediate distinguishing characteristic of these taxa from Heligmaster . In both Cuenotaster and the Benthopectinidae , the ambulacral plate lacks the pronounced arrow-shaped, proximal facing head, has an angular ridge rather than flat isthmus or neck as well as a non-quadrate shaped basal ambulacral segment. Benthopectinids have a well-developed and full series of both superomarginal and inferomarginal plates that are absent in Heligmaster . Cuenotaster , like Heligmaster has only a single row of marginal plates, which are paxillae-like and have been described in some accounts as the inferomarginal series (H.E.S. Clark 1963). Other accounts (A.M. Clark 1966) have indicated superomarginal plates but, as similar to or identical with the abactinal plates. Cuenotaster differs in having a calcified fenestrate abactinal skeleton as well as a strongly paxillar marginal plate with an elongate shaft-like peduncle that bifurcates along the lateral side of the arm. These characters are absent in Heligmaster .

Comparisons with other Myxasteridae . Myxaster and Asthenactis display between 6 and 10 rays with both showing a relatively well-developed fenestrate abactinal skeleton composed of trilobate and/or quadrilobate plates, as well as transversely arranged, webbed ( Asthenactis ) or unwebbed ( Myxaster ) furrow spines [also referred to as the actinolateral spines by Alton (1966) and others].

Pythonaster and Heligmaster both have five rays, but Pythonaster has more heavily developed imbricate abactinal plates arranged as transverse series along the arms, have elongate spines around the osculum, transverse furrow spines along the tube foot rows, and in lacking elongate, pointed marginal spines. Heligmaster has smaller, flattened, imbricate abactinal plates, which do not occur in transverse series, osculum is absent or weakly developed with no distinct spination, furrow spines sit in a palmate array parallel with the tube foot furrow and large superomarginal plates bearing one to many pointed, elongate spines are present. Rays in Heligmaster also show a distinct curl and more developed skeletal structure than Pythonaster .

Comments. The presence of a distinct marginal series in Heligmaster sets it apart from the other members of the Myxasteridae . Although Heligmaster shares characters (e.g. the ambulacrals, the imbricate abactinal plates) with other myxasterids further work could establish Heligmaster as a distinct group within the Velatida , possibly providing further character support a relationship between the Velatida and the other modern Asteroidea.

Feeding & Ecology comments. All three specimens of Heligmaster n. gen. had their pyloric stomachs extended and showed a black or dark substance on the surface that was similar in color to the substratum, suggesting that they feed on microbial microfilm or other nutrients from the deep-sea surface. All i n situ observations of Heligmaste r n. gen. were on vertical Mn-basalt faces similar in to the position observed for Pythonaster spp. when observed in situ (Mah unpublished data).

Evaluation of Heligmaster spp. in the context of their habitat and collection areas did not suggest any clear environmental trends which could provide the basis for the morphological differences between H. kanaloa n. sp. and H. pele n. sp.

Key to Heligmaster species

- Furrow spines, 3–5, webbed ( Fig. 6F ). Abactinal spines short, 1 per plate. Marginal plates with 3–9 spines. Subambulacrals with 1–3 spines. More weakly stellate (R/r=4.3 at R=7.0)................................. Heligmaster kanaloa n. sp.

- Furrow spines, 1–6, unwebbed, elongate (at R=2.6 and 14.7) ( Fig. 7F ). Abactinal spines, 1–4, mostly multiple spinelets. Marginal plate spines 15–20. Subambulacrals with spines, 6–8. More strongly stellate (R/r=7.7–8.6)......................................................................................................... Heligmaster pele n. sp.

Figures

FIGURE 6. Heligmaster kanaloa n. gen., n. sp. USNM 1457376 A. Abactinal disk surface. Scale bar=2.0 mm. B. Lateral view, whole body. Scale bar=5.0 mm. C. Cutaway of ambulacral and adambulacral ossicles. Disk is on right side. H=ambulacral head, N=ambulacral neck, B=ambulacral base, Ad= adambulacral plate. Scale bar=1.0 mm. D. Lateral view of abactinal surface, superomarginal, inferomarginal paxillae. Scale bar=2.0 mm. E. Actinal surface showing furrow spines. Scale bar= 5.0 mm. F. Close-up of furrow spination, tube foot furrow. Scale bar=1.0 mm.

FIGURE 7. Heligmaster pele n. gen., n. sp. Holotype MCZ 151023, Scale bar=5.0 mm unless otherwise noted. A. Abactinal body view. B. Close up disk surface showing madreporite, spines and papulae. C. Close up on marginal paxillae, furrow spines. D. Actinal surface showing mouth and tube foot grooves. E. MCZ 151015 paratype, (R=2.2) Smaller specimen showing disk characters, especially spination. Scale bar=0.5 mm. F. Close-up on holotype showing unwebbed furrow spines. Scale bar=2.0 mm. G. In situ observation. no scale.