Astrocladus socotrana

Baker, Alan N., Okanishi, Masanori & Pawson, David L., 2018, Euryalid brittle stars from the International Indian Ocean Expedition 1963 - 64 (Echinodermata: Ophiuroidea: Euryalida), Zootaxa 4392 (1), pp. 1-27: 16-20

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Astrocladus socotrana

sp. nov.

Astrocladus socotrana  sp. nov.

( Figs. 12–16View FIGURE 12View FIGURE 13View FIGURE 14View FIGURE 15View FIGURE 16)

Material Examined. Holotype: USNM 1072535View Materials, Anton Bruun Cruise 9 Station 465, south of Socotra Island, 11°37’N, 51°27’E, 67–72 m, 18 December 1964, 1 specimen, disc diameter 21 mm ( Fig. 11–15View FIGURE 11View FIGURE 12View FIGURE 13View FIGURE 14View FIGURE 15)GoogleMaps  . Paratypes: USNM 1072536View Materials, Anton Bruun Cruise 9 Station 463, south of Socotra Island, 11°24’N, 51°35’E, 75–175 m, 17 December 1964, 2 specimens, disc diameter 32 mm ( Figure 3BView FIGURE 3) and 33 mmGoogleMaps  . USNM 1201811View Materials, same locality data as Holotype, 1 specimen, disc diameter 12 mm.GoogleMaps 

Etymology. The specific name refers to the type locality, which is also mountainous, like the aboral surface of this species.

Diagnosis. Disc closely covered with tall conical tubercles with smooth rounded apices; proximal aboral and lateral surfaces of arms with similar but smaller tubercles. Oral arm surfaces covered with flat polygonal plates with slightly raised sutures. Arms with six or seven branches.

Description of holotype. External morphology. Radial shields, center of disc, and interradial areas covered with prominent cone-shaped tubercles with smooth tops lacking any glassy spines ( Figs. 12A, CView FIGURE 12, 13A –B, EView FIGURE 13). Largest tubercles tall (<3.2 mm high), on and obscuring radial shields, becoming smaller centrally where the disc is deeply depressed ( Fig. 13A –BView FIGURE 13). Genital slits bordered by minute papillae each with one fine glassy terminal point ( Fig. 13DView FIGURE 13). Oral papillae clustered at proximal tip of jaw, absent distally ( Fig. 13CView FIGURE 13). One madreporite at a margin of an interradius.

Five branching arms, up to 120 mm long, tightly coiled ( Fig. 12A –BView FIGURE 12). First branch at disc margin, thereafter bifurcating 6–7 times. Aboral and lateral surfaces of arms out to between third and fourth branches with scattered tubercles similar to, but smaller than, those on disc ( Fig. 13D –EView FIGURE 13). Distal arm surfaces with dense covering of circular, raised granules ( Fig. 14AView FIGURE 14). Oral surfaces of arms covered with flat polygonal plates with slightly raised sutures ( Fig. 14B –DView FIGURE 14). Six arm segments to first arm fork at disc margin, five to second fork.

Arm spines present from adjacent to first arm fork ( Fig. 14BView FIGURE 14), four spines throughout arm ( Fig. 14B –EView FIGURE 14), spines initially simple on proximal segments, developing a glassy terminal point and three secondary points distally. Discontinuous girdle bands on arms from disc margin, becoming continuous before the third fork ( Figs. 13D –EView FIGURE 13, 14AView FIGURE 14).

Ossicle morphology. All vertebrae with hourglass-shaped streptospondylous articulations ( Fig. 15C –D, F –GView FIGURE 15). Surfaces of lateral and aboral furrows with tubercles on basal portion of the arm ( Fig. 15BView FIGURE 15). Depressions for tube feet openings in the distal part of oral-lateral side of vertebrae ( Fig. 15A, EView FIGURE 15). A pair of radial water canals opening on the oral groove of vertebrae, near depression of the tube feet ( Fig. 15A, EView FIGURE 15). Tubercles on basal portion of the arms with smooth rounded apices ( Fig. 15IView FIGURE 15). Hooklet-bearing plates possessing approximately 12 tubercle-shaped articulations for hooklets in the basal portion of the arm ( Fig. 15JView FIGURE 15), approximately 8 articulations in the distal portion ( Fig. 15KView FIGURE 15). The articulations form two parallel rows ( Fig. 15J, KView FIGURE 15). Each hooklet bearing one inner tooth ( Fig. 16A –BView FIGURE 16). Lateral arm plates long, bar like, with straight distal edge and concave basal edge ( Fig. 16C, EView FIGURE 16). On basal portion of arm, lateral arm plates with a perforation on inner side and pairs of simple nerve and muscle openings on oral-external side ( Fig. 16C –EView FIGURE 16) and on distal portion of the arms, two nerve openings besides the dorsal lobe and three articulations for hooklets on oral surfaces ( Fig. 16FView FIGURE 16). No perforation visible on inner side ( Fig. 16GView FIGURE 16). Arm spines on the basal portion of the arm ovoid and having three secondary points, approximately one-seventh length of the height of the spine ( Fig. 16HView FIGURE 16). Distally, the arm spines transformed into hooks with two or three inner teeth, respectively ( Fig. 16IView FIGURE 16). The hook-shaped arm spines distinguished from hooklets on aboral and lateral surface of the arm by lacking reticular structure ( Fig. 16A –B, IView FIGURE 16).

Remarks. Astrocladus socotrana  sp. nov. is remarkable for its dense covering of prominent, smooth disc and arm tubercles, which distinguish it from its closest congeners, such as A. exiguus  and A. euryale  , which have reduced tuberculation, mostly on the radial shields.

Distribution. Known only from type locality, south of Socotra, northern Indian Ocean, 62– 175 m.