Gephyreaster swifti (Fisher 1905)

[New Japanese name: Daiou-momiji-hitode] (Figure 1)

Mimaster swifti Fisher 1905, p. 301

Gephyreaster swifti – Fisher 1910: 171; 1911: 175; Djakonov 1950: 43; Baranova, 1957: 159

Material examined. NSMT E-14747, 25 July 2022, Nemuro Strait, Sea of Okhotsk, Japan, 44.0695°N, 145.3202°E, 570–750 m in depth, collected by the fishing boat Houyuu-maru, frozen and later fixed in 99% ethanol, R = 152.0 mm, r = 58.8 mm .

Diagnosis. As for genus.

Description. Body stellate, flat. Arms five, broad proximally, gradually tapering to arm tips (Figure 1A). R/r 2.6. Abactinal surface covered with numerous abactinal paxillae comprising abactinal plates and spines. Carinal abactinal plates indiscernible from lateral abactinal plates. Abactinal plates mostly hexalobate, with tabulate paxillar columns narrower than spaces between neighbouring ridges, arranged in 15–16 regular longitudinal series (Figure 1A, B). Abactinal series gradually irregular towards arm tip, but traces of two or three midradial series extend to arm tip (Figure 1C). Abactinal plates bear hourglass-shaped spines on paxillae centre and conical spines on paxillae margin (Figure 1D). Papular areas mostly contain 1–4 papulae, every six surrounding one abactinal plate. Madreporite single, round, convex, with radiated tortuous grooves, located at interradial disc.

Supero- and inferomarginal plates paxilliform, longitudinally and transversely elongated, respectively and arranged in longitudinal series (Figure 1E). Supero- and inferomarginal series comprise 108 plates per interradius, no odd plates, gradually decreasing width distally. Inferomarginal plates projected farther laterally than superomarginal plates, fringing body margin. Marginal fascicular grooves as long as or longer than ridges of neighbouring supero- and inferomarginal plates. Supero- and inferomarginal plates bear conical to clavate spines on ridges, without major spines (Figure 1F).

Actinal plates rectangular, flat and arranged in oblique transverse series between each pair of inferomarginal and adambulacral plates (Figure 1G). The actinal series extends 93% of R. Each actinal plate bears clavate spines (Figure 1H). Pedicellariae absent.

Adambulacral plates transversely elongated rectangular and arranged in one longitudinal series along ambulacral furrow (Figure 1G). Each adambulacral plate bears 2–3 furrow spines arranged in oblique longitudinal series and 4–9 subambulacral spines.

Each pair of oral plates, forming the jaw, bears one unpaired oral spine on its proximal edge (Figure 1I). Each oral plate bears 48–66 oral spines arranged in irregular or 3–4 longitudinal series.

Tube feet biserial, ending with suckered discs.

Distribution. Off Washington, USA (Fisher 1911); British Columbia, Canada (Lambert 2000); Stephens Passage, Alaska, USA (Fisher 1905); adjacent waters of Attu Island, Aleutian Island, USA (Fisher 1911); Olyutorskii Bay, Bering Sea, Russia (Djakonov 1950); Nemuro Strait, Sea of Okhotsk, Japan (this study). Bathymetric range is 11– 750 m.

Etymology of Japanese name. The Japanese name follows the generic name of the genus Gephyreaster proposed in this study.

Remarks

The genus Gephyreaster shows close morphological affinities with two Paxillosida genera, Mimastrella ( Astropectinidae) and Radiaster ( Radiasteridae), and was considered an intermediate genus between these genera and the confamilial Pseudarchaster (Fisher 1911) . However, our specimen distinctly differs from these genera by the following combination of morphological characters: suckered tube feet (Figure 1A, H), flat actinal plates (Figure 1G), and paxilliform marginal plates (Figure 1E).

Notably, our specimen has inferomarginal plates that encroach upon the abactinal side (Figure 1A, E), unlike the holotype of Gephyreaster swifti, which has inferomarginal plates restricted to the actinal side (Fisher 1905). However, the remaining morphological characters of our specimen aligned with those of the holotype. It is uncertain whether this morphological difference is an artefact because the specimen had a weakened and somewhat depressed abactinal body wall. This is the first occurrence record of the genus Gephyreaster from Japanese waters, extending the southern- and western-most distributional range.