Chitonaster cataphractus Sladen 1889

Chitonaster cataphractus Sladen, 1889

(Figure 4 A–E)

Sladen, 1889: 283; A.M. Clark, 1962: 22; 1992: 250; A.M. Clark, 1993: 250 (as C. cataphractus)

Distribution. South Sandwich and Thurston Island, Ellsworth Land to 62˚26’S, 95˚44’E (type locality), 3612– 5350 m.

Comments and distinctions from other species. Chitonaster cataphractus is distinguished from the other species of Chitonaster based on the single spines on each abactinal and superomarginal plate, the superomarginal and inferomarginal spines in series, the absence of pedicellariae, the elongate arms and relatively small disk (Fig. 4 A–E). Other characters, such as the relatively small abactinal plates and the absence of papulae, are shared between C. cataphractus, C. felli, and C. johannae. These all appear to be characters present in taxa collected from abyssal depths (2000–5000 m). Original type specimens and newly collected specimens of C. cataphractus show disparity between small and large sized specimens as well as the number of spines on the marginal plates. The serial spination on the superomarginals and inferomarginals is present in smaller C. cataphractus but is less distinct relative to those observed in the adults and may vary in specimens as specimen size inceases. Although morphological distinctions are clear between specimens examined, characters such as presence/absence of pedicellariae among other Goniasteridae have been shown to be variable. The boundary between C. cataphractus and C. johannae should be regarded carefully as more specimens become available.

Material examined. USNM 1122198 South Sandwich Islands, 55˚05’ 35”S 25˚47’23”W, 5350 m. Coll. Moss Landing Marine Laboratories (1 dry spec. R=4.6, r=1.3); USNM 1122200 South Sandwich Islands, 55˚05’ 34.8”S 25˚ 47’ 23.28” W, 5350 m. Coll. Stacy Kim, Moss Landing (3 wet spec. R=5.4, r=1.5; R=5.5, r=1.8; R=5.7, 4=1.5), USNM 1086360 Thurston Island, Ellsworth Land, Southern Ocean. 70˚20’S, 99˚ 10’W to 70˚13’S, 98˚57’W. 3848–3980 m. (1 dry spec. R=2.0, r=0.7).

Description. Body stellate with arms, elongate, slender. R/r=2.85 to 3.6. Disk inflated, convex (Fig. 4 A).

Abactinal plates flattened, scalar, each with convex, swollen surface, abutting or only weakly overlapping other disk plates (Fig. 4 C, E). Abactinal plates relatively small compared to C. trangae with five to nine plates from superomarginal to superomarginal, at arm base. Each abactinal plate shape polygonal to round in outline. Fasciolar grooves weakly present. Carinal series present, but not conspicuous relative to other abactinal plates. Plates and spines irregularly arranged. Each plate largely bare or with one, occasionally two, conical-pointed to blunt (in smaller specimens, R=2.0) or more cylindrical, blunt spine or spines (at R=5.5). Spines more regularly present on plates adjacent to superomarginal plate series, sometimes absent from disk plates (e.g., Fig. 4 A, C). Peripheral accessories absent. In larger specimens abactinal plates are more polygonal becoming more irregularly elongate on arm plates and continuing to arm terminus (Fig. 4 D). Abactinal body wall is relatively thin. Pulpy tissue present over abactinal surface that is more clearly visible on wet specimens. Papulae are absent from abactinal surface, an observation verified from dry and wet specimens (Fig. 4 E).

Superomarginal and inferomarginal plate series slightly offset from one another both with lateral facing (Fig. 4 C). Accessories, such as granules, etc. are absent from marginal plates. Superomarginal plate series in larger specimens, irregular varying in shape from quadrate to rectangular, becoming more polygonal and irregularly round distally on the arm.

Superomarginal plates range from 30 (at R=2.0) to approximately 54–58 (at R=5.5) per interradius (counted from armtip to armtip). Superomarginal plates are bare except for one or two spines per plate, conical and pointed in smaller specimens; blunt and cylindrical in larger individuals (Fig. 4 C). Spines occur in distinct linear series. Inferomarginal plates bare, save for one or two spines per plate, each forming a ventrolateral series (Fig. 4 C). Similar to the superomarginal series, spines are more conical and pointed in smaller specimens, becoming more cylindrical and blulnt in larger individuals. Inferomarginal plate series, number 36 interradially in small specimens, approximately 58–62 in larger individuals. Inferomarginal plates with quadrate outlines that become more elongate interradially becoming more equidistant distally. Terminal plates relatively large, equivalent in thickness to two superomarginal plates and three to four abactinal plates.

Actinal intermediate regions very reduced and restricted to disk and/or arm base (Fig. 4 B). Ten to 15 actinal interradial plates present between adambulacral and inferomarginal plate series. Each plate bare of accessories (i.e., no granules or spinelets, etc.) but with one large blunt, conical to cylindrical spine per plate. Tissue similar to that one abactinal plate also present.

Adambulacral plates, squarish in outline with curved to straight contacts between plates, each plate bare with no accessories. Two to three furrow spines, round in cross-section, blunt tipped in transverse series. Three furrow spines are present in larger specimens (e.g., R=5.5) but alternate in smaller specimens.

Pedicellariae are absent from all body surfaces.