Dougaloplus echinatus

Dougaloplus echinatus (Ljungman, 1867)

[New Japanese name: Toge-ana-suna-kumohitode]

(Figs 3–7)

Ophiophragmus echinatus Ljungman 1867: 316.

Ophiocnida echinata ? (sic)— Lyman 1874: 230 –231, pl. 4, figs 22, 23.

Ophiocnida echinata. Lyman 1882: 155.— Sluiter 1898: 306.— Koehler 1898: 68, pl. 5, fig. 47; 1900: pl. 17, fig. 25; 1905: 32; 1930: 113.— Verrill 1899: 317.— H. L. Clark 1915: 250; 1932: 204; 1946, 203.

Amphioplus luctator Koehler 1922: 178 –179, pl. 68, figs. 1–3; 1930: 106.

Dougaloplus echinatus. A. M. Clark 1970: 35 –36, fig. 7h–k.— A. M. Clark & Rowe 1971: 80 –81,100.— Gibbs et al. 1976: 123.— Kingston 1981: 129.— Liao & A. M. Clark 1995: 200 –202, fig. 97.— Rowe & Gates 1995: 353.

Ophiostigma formosa Lütken 1872: 77, pls 1–2, fig. 5.— Koehler 1898: 77, 1905: 24, pl. 2, fig. 1.

Material examined. Ie Island (RUMF-ZE-02040[1], RUMF-ZE-02041[1]): “entrance” of the “Unnamed cave”, under coral rubble, approximately 20 m depth, 24 June 2017. Discs of all specimens are separated from the bodies by being cast off upon collection.

Description of external morphology (RUMF-ZE-02040). Disc. Five lobed, 10 mm in diameter (Figs 3; 4A), covered by scales, approximately 100–200 µm in length (Fig. 4B). Some disc scales bear spines, approximately 70–100 µm in height (Fig. 4B, C). Exposed part of radial shields narrow and long, approximately 1 mm long, 150 µm wide distally, and gradually tapering towards disc center (Fig. 4B), slightly in contact distally but separated towards disc center. On oral surface, adoral shields triangular, wider than long, approximately 650 µm in length, 200 µm in width on inner side, 350 µm in width on outer side, in contact with the first ventral arm plates (Fig. 4E). Oral plates invisible, obscured by skin (Fig. 4E). Oral shields oblong, longer than wide, approximately 650 µm in length and 430 µm in width, and slightly pointed on proximal side (Fig. 4E). All oral shields similar in size and shape, oral shield serving as madreporite unrecognizable in external view (Fig. 4E). Interradial oral surface also covered by imbricating scales, approximately 160–200 µm in length (Fig. 4F). Some scales on peripheral area bearing spines, approximately 150 µm in length (Fig. 4F). Shapes and length of genital slits unknown because discs of all examined specimens detached (Fig. 4D). Two or three oral papillae at each side of jaw (Fig. 4E), proximal two papillae square, the distal one 1.5 times as wide as the proximal papillae, and the distal most papillae smaller and flat (Fig. 4E). Two infradental papillae at the apex of each jaw, trapezoidal, slightly larger than middle larger oral papillae (Fig. 4E). Teeth forming vertical series l row aboral to the infradental papillae, large, wider than long (Fig. 4E). Second tentacle pore inside mouth angle (Fig. 4E).

Arms. Five, approximately 135–180 mm long, 1.1 mm wide and 0.7 mm high, with an arched aboral surface and flattened oral surface at proximal portion. Arms tapering gradually toward distal tip and aboral surface is flattened (Fig. 3).

On proximal to middle portion of the arm, ventral arm plates pentagonal with slightly convex edges, of equal length and width, in contact with each other (Fig. 5A). On distal portion of the arm, ventral arm plates becoming triangular and separated from each other (Fig. 5C). Dorsal arm plate fan-shaped, slightly larger than wide, contacted with each other on proximal portion (Fig. 5D). On middle portion of arm, dorsal arm plates becoming pentagonal, almost twice wider than long (Fig. 5D); toward the distal tip, the plates becoming triangular, almost as long as wide, separated from each other (Fig. 5E). Lateral arm plates small, widely separated by dorsal and ventral arm plates on proximal portion (Fig. 5A, B, D, F), in contact with each other on distal portion (Fig. 5C, E) on both oral and aboral sides. Three conical arm spines, of almost the same length as the corresponding arm segment on proximal to middle portion of arms (Fig. 5A, B, D, F), though few segments have four arm spines. On distal portion, the number of spines decreasing to two and becoming acute, spiniform (Fig. 5C, E). Two oval tentacle scales, almost the same sizes at each tentacle pore (Fig. 5A).

Ossicle morphology (RUMF-ZE-02040). Dorsal arm plates trapezoid, wider than long on distal portion (Fig. 6A), and triangular with convex distal edge on proximal portion (Fig. 6B). Ventral arm plates pentagonal with slightly convex lateral side on proximal portion (Fig. 6C), and pentagonal with straight lateral side on distal portion (Fig. 6D). Lateral arm plates longer than wide, aboral edge straight and oral-distal edge concave (Fig. 6E, F). On inner side, 2 defined horizontally elevated structures on center part (Fig. 6E), proximal structure slightly longer than wide on proximal portion of arm. On distal portion, the structures almost of the same size (Fig. 6I). On external side, proximal edge convex with three equal-sized spine articulations on distal edge (Fig. 6F, G), composed of parallel, horizontal dorsal and ventral lobes (Fig. 6H). The lobes almost equal-sized on proximal portion of arm but on distal portion of arm, ventral lobe slightly shorter than dorsal lobe (Fig. 6K). Large tentacle notches opening on vantro-distal edge and end in center of lateral arm plates, where a single perforation is opening (Fig. 6E, I). Arm spines conical, thicker at oral side and thinner at aboral side the arms (Fig. 6L, M), and smaller, pointed on distal portion (Fig. 6N). The spines with serrate minute spinelets on lateral surfaces (Fig. 6L, M, N).

Vertebrae with zygospondylus articulation (Fig. 7A, B, E, F), longitudinal keel on aboral side (Fig. 7C, G) and longitudinal oral groove on oral side (Fig. 7D, H). Two continuous holes on the aboral keel (Fig. 7C, G). Ambulacral groove open (Fig. 7D, H). Depression for tentacles located on lateral distal side of the vertebra (Fig. 7D, H).

Color. In living, disc dull yellow, arms with red and white bands (Fig. 3).

Distribution. Ie Island, Okinawa Island Group, southwestern Japan (this study); Orissa, India (Koehler, 1898); Gaspar Strait, Indonesia (Ljungman, 1867; Koehler, 1905, 1930, type locality); Singpore (Ljungman, 1867); Philippines (Lyman, 1874; H. L. Clark, 1915); Gulf of Thailand (Koehler, 1930); Pernambuco, Brazil (Sluiter, 1898). Great Barrier reef, Gillet Cay, northeastern Australia (H. L. Clark, 1932; Gibbs et al., 1976; Kingston, 1981); Southern China to Taiwan Strait (Lütken, 1872; Liao & A. M. Clark, 1995). Depth range 11– 118 m.

Remarks. This species falls within the genus Dougaloplus by virtue of having disc scales that bear spines and the presence of tentacle scales, and identified as D. echinatus in having numerous disc spines on the aboral disc surface and on the interradial oral peripheral surface of the disc; three oral papillae on each jaw; and narrow, mostly separated radial shield (A. M. Clark, 1970). This is the first record of Dougaloplus echinatus and the second record of species of Dougaloplus from Japanese waters.

The two specimens were collected at entrances of caves, not an anchialine environment. Therefore, this discovery may not indicate an anchialine environment affinity of D. echinatus.