Paragonaster obtusus ( Hayashi 1973a )

Kobayashi, Itaru, Sonoyama, Takayuki, Hibino, Mai, Kawano, Mitsuhisa & Kohtsuka, Hisanori, 2024, Pseudarchasteridae (Asteroidea: Paxillosida) in Japanese waters, with description of a new species and range extension of three species, Journal of Natural History 58 (25 - 28), pp. 949-964 : 953-955

Paragonaster obtusus ( Hayashi 1973a)

[Japanese name: Momiji-hitode]

( Figure 2 )

Pseudarchaster obtusus Hayashi (1973a) , pp. 4–6, pl. 8, figs. 1–3; Hayashi 1973b: 54–56, pl. 8, figs. 1–3

Paragonaster obtusus – Saba and Fujita 2006: 272–273, fig. 16

Material examined. OMNH-Iv 10661, 27 July 2021, north-west of Yamaguchi Prefecture, Sea of Japan, 35.0650°N, 130.3167°E, 129 m in depth, collected by the R / V Kaisei, fixed in 99% ethanol, R = 61.2 mm, r = 25.2 mm .

Diagnosis. R/r <3; rectangular carinal abactinal plates as narrow as or narrower than length; lateral abactinal series extends to more than one-half of R; interradial superomarginals widest, decreasing in size distally; inferomarginal plates bear granules and 1–4 leaf-shaped spines; actinal plates bear granules, spines, and no pedicellariae; each adambulacral plate bears 4–6 furrow spines.

Description. Body stellate, flat ( Figure 2A ). Arms five, broad proximally, gradually tapering to arm tips. R/r 2.4. Abactinal surface densely covered with numerous abactinal paxillae comprising abactinal plates, granules, and spines. Abactinal plates of two types: larger carinal and slightly smaller lateral abactinal. Lateral abactinal plates mostly hexalobate, with tabulate paxillar columns, arranged in 9–10 regular longitudinal series on each side of arm ( Figure 2A, B ). Paxillar columns are narrower than spaces between neighbouring columns. Adradial-most lateral abactinal series longest, extending to 56% of R. Carinal abactinal plates on proximal 58% of R, hexagonal with tabulate paxillar columns and those on distal 42% rectangular with cylindrical to prismatic paxillar columns. Carinal abactinal plates are arranged in one regular longitudinal series extending to arm tip, wedged between two superomarginal series in distal 44% of R. Hexagonal carinal abactinal plates as narrow as length and gradually increase in width towards first rectangular plates. Rectangular carinal abactinal plates narrower than length, gradually decreasing in width distally. Lateral and carinal abactinal plates bear mushroom-shaped granules on paxillae centre and conical spines on paxillae margin ( Figure 2C ). Papular areas mostly contain one papula, every six surrounding one lateral abactinal plate. Madreporite single, round, convex, with radiated tortuous grooves, located at interradial disc.

Supero- and inferomarginal plates transversely elongated, rectangular with oblong to oval ridges and arranged in longitudinal series along body margin ( Figure 2A, B, D, E ). Superomarginal- and inferomarginal series comprises 88 plates per interradius, no odd plates, gradually decreasing width distally ( Figure 2A, B ). Fascicular grooves narrow, shorter than ridges of neighbouring supero- and inferomarginal plates ( Figure 2B, D ). Supero- and inferomarginal plates covered with mushroom-shaped to clavate granules and proximal to distal inferomarginal plates also bear 1–4 leaf-shaped spines ( Figure 2E ).

Actinal plates oval, convex and arranged in 5–6 longitudinal series ( Figure 2D ). Actinal series are confined within 53% of R. Each actinal plate bears mushroom-shaped to clavate granules; most bear one major leaf-shaped spine ( Figure 2E ). Pedicellariae absent.

Adambulacral plates rhomboid and arranged in one longitudinal series along ambulacral furrow ( Figure 2D, E ). Adambulacral series comprises 42 plates, extending to arm tips. Each adambulacral plate bears 4–5 furrow spines, arranged in a longitudinal curved series and 10–14 minor subambulacral spines.

Each pair of oral plates, forming the jaw, bears one unpaired oral spine on its proximal edge ( Figure 2F ). Each oral plate bears 14–17 oral spines arranged in two longitudinal series.

Tube feet biserial, ending with suckered discs.

Distribution. Paragonaster obtusus is only recorded from Japanese waters: Pacific side, Sagami Bay ( Hayashi 1973a, 1973b); Uraga Strait ( Saba and Fujita 2006); off Choshi, Chiba Prefecture ( Saba 2000); and Sea of Japan side, Toyama Bay ( Hayashi 1973a, 1973b); off Togi, Ishikawa Prefecture ( Kohtsuka and Ogiso 2018); off Niigata Prefecture ( Saba 2000); off Maizuru, Kyoto ( Saba 2000); north-west of Yamaguchi Prefecture (this study). Bathymetric range is 129– 230 m.

Remarks. This specimen is identified as P. obtusus based on the following morphological characteristics: R/r is less than 3; lateral abactinal series extends to more than one-half of R ( Figure 2C ); superomarginal plates gradually decrease in width towards arm tips. This species was originally described as the genus Pseudarchaster ; however, Saba and Fujita (2006) considered the genus Paragonaster based on no lateral abactinal series reaching arm tips. This paper extends the distributional range for P. obtusus to western Honshu, Japan.

Figures

Figure 2. Paragonaster obutsus, OMNH-Iv 10661. A, Whole body, abactinal (top) and actinal (bottom) side; B, denuded abactinal surface of proximal portion of arm; C, abactinal surface of proximal portion of disc; D, denuded actinal surface of interradii to proximal portion of arm; E, actinal surface of interradii to proximal portion of arm; F, denuded actinal surface of a jaw. Abbreviations: ad, adambulacral series; c, carinal abactinal series; i, inferomarginal series; o, oral plate; s, superomarginal series; u, unpaired oral spine. Scale bars: A = 10 mm; B–F = 1 mm.