Myxaster sol Perrier 1885

Myxaster sol Perrier, 1885

(Figures 1 A–B, 2)

Myxaster sol: Perrier 1885a: 866; 1885b: 69; 1894: 177; Sladen, 1889: 804; Fisher 1919: 455; Alton, 1966: 1966; Clark and Downey 1992: 338; A.M. Clark 1996: 196; Villier et al., 2004; Dilman, 2005: 455; 2008: 147; Dransfeld et al., 2007: 21, 50.

Myxaster sp.: Howell et al., 2002: 1906

Description: Arms eight or nine, broader at base becoming slender, highly attenuate and more fragile distally (Fig. 2 A,B). R/r= 3.1–4.85. Disk broad, weakly convex.

Abactinal plates imbricate, delicate and embedded in a thin-skin. Proximal plates larger with well-developed lobate flanges becoming smaller and more scale-like distally with more weakly developed flanges. Degree of imbrication of abactinal plates shows some variation between larger and smaller individuals. A larger individual (USNM E18596) has disk plates that are more closely articulated whereas those on smaller individuals (e.g, USNM 1182980) are widely articulated with large openings between the plates. In both individuals, plates on the swollen arm base regions are widely articulated with relatively large skin-filled gaps between plates (Fig. 2 E). Papulae were not observed between these skin-filled gaps. Plates more tightly imbricate and more scalar in shape distally, forming a more tightly overlapping chain-mail armor like arrangement (Fig. 2 D). Individual plates flattened proximally with a strongly stellate base present on abactinal surface (Fig. 2 G). Abactinal disk plates welldeveloped with four to six radially projecting flanges each becoming smaller and more irregularly shaped distally. Abactinal, marginal and actinal plate shape indistinct from one another. No discrete series of plates between terminal and primary plate as per definition of marginal plates by Blake (1979). Plates covered by spine clusters similar to those on abactinal surface.

Glassine spines, one to ten, present on a shared base. On larger specimens (e.g., USNM 1182980 r= 3.1 cm and larger) multiple spines are present on disk and arms. Spine arrays larger and more abundant on larger individuals and present a more hirsute appearance when viewed abactinally. Each spine-array with multiple webbed or tissuecovered spines arranged into a ring-formation radiating from the surface of low swellings present on each plate surface. Smaller specimens (e.g., YPM Y36040, r= 1.5 cm) with fewer spines per plate proximally (i.e, on arms and disk) ranging in number from one to four per plate. Distal plates with fewer spines; underlying convexity of plate surface more weakly expressed. Spines usually radiating centrally from plate sometimes in more linear to crescentic arrangement. Spines triangular in cross section (following removal of tissue) with discrete wedge-like edges on each wing.

Abactinal spine clusters more widely spaced proximally, especially surrounding osculum becoming closer and more densely distributed distally. Osculum flanked by flat, discrete featureless plates corresponding to each arm. These are ostensibly the primary disk plates. A prominent, squat, paxillae-like column with six to 25 short spinelets is present on each plate. Overall spinelet number seems to be higher on larger individuals. The number of squat paxillar columns equals the number of arms present.

A discrete but narrow interradial sulcus is present between all arms (Fig. 2 A–B), extending to primary plates around osculum. Sulci are expressed more strongly on USNM 1189026 and YPM Y36040 (Figs. 1 A,B), which are both small sized and from the same seamount. The interradial sulcus is more obscured on E18596, which is larger and was collected from farther south. Spine clusters tend to be absent from surface of sulcus. Madreporite strongly convex, raised, irregularly round, separated from surrounding plates by distinct gap.

An actinal intermediate field is apparently absent, but the proximal actinolateral regions between each pair of arms share a well-developed calcified septa present interradially. When viewed internally this calcified septum is composed of an extension from the oral plate articulated/fused with actinolateral arm plates. Approximately 50% of the interradial region is covered by a layer of translucent skin. Externally, actinolateral plates converge interradially towards the oral facing expression of the sulcus between arms.

Mouth plates large, wide (Fig. 2 C), approximately the size of three to four adambulacral plates, extending from mouth to adambulacral plates on disk’s edge. Mouth plates form concave surface with flanges. Lateral flattened flanges of mouth plates projecting into tube feet grooves (2 flanges present per tube foot groove). Distinct gap with dentate contacts between two halves of mouth plates, most pronounced facing distally. Oral plates smooth, spines or other features absent save for furrow spines. Oral plate furrow spines, eight, webbed, present on mouth plates, forming a palmate arrangement facing distally.

Proximalmost spines thickened, elongate becoming smaller distally. Furrow spines on adambulacral plates six to ten, typically eight, elongate, webbed, arranged in a palmate pattern (Fig. 2 F). Spines elongate, especially on proximal plates sometimes extending nearly three series of actinolateral plates beyond the adambulacrals, embedded in tissue, spines delicate, hyaline with distinctly angular edges, triangular to X-like in cross-section, similar to those on abactinal surface.

Adambulacral plates vertebral/cylindrical in shape, separated by distinct tissue-filled gap with raised ridge for furrow spines.

Tube feet biserial, elongate, weakly developed. Each tube foot with strongly irregular surface texture. Cardiac stomach on specimen is well-developed with convoluted folds.

Distribution: Throughout the North Atlantic Ocean (bold designates new occurrence-this paper). Off NW Africa, Bay of Biscay (=Golfe de Gascogne), Porcupine Seabight, North Atlantic east to 54º 07.95’ N, 12º 48.96’W, Bear Seamount. Tongue of the Ocean, west of Andros Island, Reykjanes Ridge, off Newfoundland, 471–2110 m.

Comments: Myxaster sol is widespread throughout deep-sea settings in the North Atlantic. Material highlighted herein represents several individuals with eight arms rather than nine arms, as identified on the holotype and in specimens described by Dilman (2005). Other characters present on eight-armed individuals otherwise conform to the character definition for M. sol. Arm number may not be a dependable character for distinguishing taxa as has traditionally been recognized (e.g., Clark and Downey, 1992). It is unclear which, if any, environmental factors affect the arm number, and examined specimens do not present a clear correlation between collection locality and specimen morphology.

Some variation between individuals appears to be size related. Some characters such as number of abactinal spines and adambulacral furrow spines seem to increase in larger individuals. However, given that M. sol is widely distributed throughout the Atlantic, the counts from the number of specimens present may not completely represent regional variation. Individual specimens collected from localities in close proximity are very similar in appearance.

Material Examined: USNM E18596 West of Andros Island, Tongue of the Oean, Bahamas. 24˚26’N, 77˚20’W, 1463 m. Coll. R/V Columbus Iselin, 28 Feb. 1973 (1 dry spec. R= 15.1, r=2.5; YPM # 36040. Bear Seamount (Atlantic Ocean), center of table top peak. 39º54.845’N, 67º26.226’W to 39º54.806’N, 67º29.347’W, 1157–1800 m. Coll. J.A. Moore aboard R/V Delaware II, 7 June 2004 (1 wet spec. R= 9.0, r= 1.5). USNM 1189026 Off Newfoundland 50° 16' 22.8"N, 49° 37' 58.8"W, 1404 m. Coll. A. Mercier and J-F. Hamel. (1 dry spec. R=11.5, r=2.9). USNM 1182980, Off Newfoundland, 53° 33' 28.7N, 2° 27' 54"W, 471 m, Coll. A. Mercier and J-F Hamel, 20 Nov. 2005 (1 wet spec. R=11.2, r=3.6); USNM 1182981, Off Newfoundland 52° 53' 24"N, 51° 42', 1085 m, Coll. A. Mercier and J-F Hamel, 13 Nov, 2006, (1 wet spec. R=9.1, r=2.6), USNM 1179112, North Atlantic Ocean, Bear Seamount, 39˚55.9’-N 67˚24.7’ W to 39˚56.1’ N 67˚ 28.2 W, 1128–1759 m, R/V Delaware II, Cr. 200804, Sta. 10, 27 March 2008 (1 wet spec. R=9.7, r=2.0), USNM 1190353, Off Newfoundland 50° 16' 22.8"N, 49° 37' 58.8"W, 1404 m, Coll. A. Mercier and J-F Hamel, 4 Dec 2010 (1 wet spec. R=12.2, r=2.6 note arms are curled).

Other Records (specimens not examined): USNM 1016070 Bear Seamount (North Atlantic Ocean), 39° 55' 37.5 N, 67° 24' 43 W, 1111–1187 m. Coll. J.A. Moore aboard R/V Delaware II, 20 May 2003 (1 wet spec. R=~3.0, r=1.8, note-specimen on loan and unavailable); SH 505118#1, North Atlantic: Goban Spur, south of Porcupine Seabight, SW of Ireland. 49˚ 27.3 N, 13˚ 21.2’W, 2045–2110 m. Coll. RRS Challenger (1 wet spec. R=8.5, r=2.5, 8 arms); SH 505119#1 North Atlantic: Goban Spur, south of Porcupine Seabight, SW of Ireland. 49˚ 29.5 N, 12˚ 48.9’W, 1465– 1431 m. Coll. RRS Challenger (1 wet spec. R=7.9, r=1.8). “Irish specimen”-No number. North Atlantic 54º 07.95’ N, 12º 48.96’W, 1500 m. Coll. Irish Deepwater Survey 2006. (1 wet spec. R=7.0, r=1.6); MNHN-EcAs 11262 Golfe de Gascogne, 48º41’N, 10º53’W, 1420–1470 m. Coll. C. Cabioch, N.O. Thalassa, 28 Oct. 1973 (1 dry spec. R=8.8, r=1.9);