Paragonaster stenostichus Fisher 1913

[Japanese name: Minami-gokaku-hitode]

(Figure 3)

Paragonaster stenostichus (Fisher 1913), p. 627; Fisher 1919: 232–235, pl. 60, fig. 2, pl. 70, fig. 2, pl. 71, fig. 1, pl. 91 figs. 10, 10a

Paragonaster ctenipes – Oguro 1987: 46–51, figs. 1–2; Oguro 1990: 47

Material examined. OMNH-Iv 10662, 9 July 2020, north-west of Yamaguchi Prefecture, Sea of Japan, 34.9717°N, 130.8150°E, 130 m in depth, collected by the R / V Kaisei, fixed in 99% ethanol, R = 42.9 mm, r = 12.0 mm .

Diagnosis. R/r> 3; most rectangular carinal abactinal plates as narrow as or narrower than length, except several first plates wider than length; lateral abactinal series extends to less than half of R; superomarginal plates widest in 3rd–4th plates; inferomarginal plates bear granules and 1–5 conical spines; actinal plates bear granules, spines and no pedicellariae; each adambulacral plate bears 6–7 furrow spines.

Description. Body stellate, flat (Figure 3A). Arms five, broad proximally, gradually tapering to arm tips. R/r 3.6. Abactinal surface densely covered with numerous abactinal paxillae comprising abactinal plates and granules (Figure 3B, C). Abactinal plates in two types, larger carinal and smaller lateral abactinal plates (Figure 3C). Lateral abactinal plates mostly hexalobate, with cylindrical to prismatic paxillar columns, arranged in 5–6 regular longitudinal series on each side of arm. Paxillar columns wider than spaces between neighbouring paxillar columns. Adradial-most lateral abactinal series longest, extending to 29% of R. Carinal abactinal plates on proximal 30% of R, hexagonal with cylindrical paxillar columns and those on distal 70%, rectangular with prismatic paxillar columns. Carinal abactinal plates arranged in one regular longitudinal series extending to arm tip, wedged between two superomarginal series in distal 71% of R. Hexagonal carinal abactinal plates as narrow as length and gradually increase in width towards first rectangular plates. Rectangular carinal abactinal plates wider than length at first several plates but mostly narrower than length, gradually decreasing in width distally. Lateral and carinal abactinal plates bear mushroom-shaped granules without spines (Figure 3B). Papular areas mostly contain one papula, every six surrounding one lateral abactinal plate (Figure 3C). Madreporite single, round, convex, with radiated tortuous grooves, located at interradial disc.

Supero- and inferomarginal plates wide rectangular with oblong to oval ridges and arranged in longitudinal series along body margin (Figure 3A, C–E). Superomarginal plates gradually increase in width up to 3rd–4th plates, and beyond these plates, gradually decrease in width distally (Figure 3A, C). Inferomarginal plates gradually decrease in width distally (Figure 3A, D). Supero- and inferomarginal series comprise 54 plates per interradius, without odd plates. Fascicular grooves narrow, shorter than ridges of neighbouring supero- and inferomarginal plates (Figure 3C, D). Supero- and inferomarginal plates covered with mushroom-shaped granules and proximal to middle inferomarginal plates bear 1–5 conical spines arranged in one zigzag to two transverse series (Figure 3E).

Actinal plates oval to rectangular, convex, and arranged in three longitudinal series (Figure 3D). Actinal series confined within 26% of R. Each actinal plate bears mushroom-shaped to clavate granules and typically one major conical spine (Figure 3E). Pedicellariae absent.

Adambulacral plates oval and arranged in one longitudinal series along the ambulacral furrow (Figure 3D). Adambulacral series comprises 44 plates, extending to arm tips. Each adambulacral plate bears 6–7 furrow spines arranged in longitudinal curved series, one major subambulacral spine on plate centre and 12–15 minor subambulacral spines on plate margin (Figure 3D, E).

Each pair of oral plates, forming jaw, bears one unpaired oral spine on its proximal edge (Figure 3F). Each oral plate bears 25–28 oral spines arranged in three longitudinal series.

Tube feet biserial, with suckered discs.

Distribution. Waters adjacent to the Philippine Islands (Fisher 1919); south and southeast off Shikoku Island, Japan (Oguro 1987, 1990); north-west of Yamaguchi Prefecture, Sea of Japan (this study). Bathymetric range is 110– 314 m.

Remarks. The observed specimen aligns with the morphological characteristics of P. stenostichus described by Fisher (1913, 1919) (see Diagnosis and Key). Paragonaster ctenipes Sladen 1889, reported by Oguro (1987, 1990) most probably belong to P. stenostichus, because most rectangular carinal abactinal plates are narrower than the length of his specimens, unlike those of P. ctenipes, which are wider than the length (Sladen 1889; Fisher 1919). This is the first report of P. stenostichus from Japan, extending its northern distributional limit.