Mimastrella komkom Kobayashi sp. n.

[New Japanese name: Houyuu-shiawase-momijigai]

(Figures 2, 3)

Description

Arms five, broadening proximally, gradually tapering to arm tips (Figure 2 (A)). R/r 2.8–2.9. Abactinal surface slightly inflated, covered by numerous penicillate paxillae comprising abactinal plates and spines (Figure 2 (B)). Abactinal plates mostly quadrilobate, possessing paxillar columns crowned with a tuft of abactinal spines, arranged in oblique transverse series only at arm sides and interradial disc (Figures 2 (B, C), 3(A)). Paxillar columns slender, cylindrical and slightly capitate. Heights of these paxillar columns five or more times greater than their widths and two or more times greater than the length of abactinal plates. Abactinal spines triangular prism-shaped with concave side, straight, smooth laterally, slightly flaring distally, and sometimes with three or four hyaline points (Figure 3 (B)). Papular areas contain one papula. Madreporite exposed, 4.0– 4.1 mm in diameter.

Superomarginal plates trilobate, paxilliform, longitudinally elongated, two times longer than adjacent abactinal plates, and arranged in one longitudinal series at abactinolateral portion of arm (Figures 2 (D), (3C)). Inferomarginal plates rectangular, paxilliform, longitudinally elongated, as long as adjacent superomarginal plates, and arranged in one longitudinal series at lateral portion of arms (Figures 2 (D), 3(D)). Each supero- and inferomarginal series composed of 42–43 plates. Paxillar columns slender, cylindrical, slightly capitate on superomarginal plates, and broad, flat, wedge-shaped on inferomarginal plates. Heights of these paxillar columns greater than length and width of superoand inferomarginal plates, respectively. Marginal fasciolar grooves distinct between neighbouring inferomarginal paxillar columns, and longer than longitudinal length of inferomarginal paxillar columns (Figure 2 (D)). Supero- and inferomarginal plates densely covered by numerous spines like abactinal spines on paxillar columns (Figures 2 (E), 3(E)).

Actinal plates paxilliform, transversely elongated, and arranged in oblique transverse series at 85–89% of R (Figure 2 (F)). Actinal plates contact each other in each series, but apart from longitudinally neighbouring actinal plates. Paxillar columns of actinal plates broad, flat, and wedge-shaped, height of which as great as or greater than length of actinal plates but smaller than width of actinal plates. Actinal plates densely covered with numerous actinal spines (Figure 2 (G)). Actinal spines spatulate, straight, smooth laterally, or triangular prism-shaped with concave side like abactinal spines (Figure 3 (F)).

Adambulacral plates transversely elongated rectangular and arranged in one longitudinal series along ambulacral furrow (Figure 2 (F)). Each adambulacral plate bears 5–13 furrow spines (Figure 2 (G)). Adambulacral spines conical, straight, and smooth laterally (Figure 3 (G)).

Each pair of oral plates forms a rhomboid jaw, lacking an unpaired oral spine (Figure 2 (A,H)). Each oral plate bears at least 38 oral spines arranged in 2–3 irregular longitudinal series; however, actual number of spines unknown due to damage.

Pedicellariae absent.

Tube feet biserial, ending with suckered discs (Figure 2 (I)).

Material examined

Holotype: JAPAN: Sea of Okhotsk, Nemuro Strait, 44.2390°N, 145.4008E, depth 930– 1170m; 15 May 2023; collected with a commercial gill net by the fishing boat Houyuu-maru, frozen and later fixed in 99% ethanol, R = 53.6 mm, r = 19.0 mm (NSMT E-14848); Paratypes (70): same data as holotype, except: R = 66.3 mm, r = 22.7 mm, 44.3185°N, 145.4102°E (NSMT E-14849).

Distribution

Mimastrella komkom sp. n. is only known from the type locality, Nemuro Strait, Sea of Okhotsk, Japan, at a depth of 930–1170 m.

Etymology

The specific name, komkom, is derived from the Indigenous language of Hokkaido, Ainu, meaning down feathers of little birds. This new species has delicate abactinal spines and paxillar columns, providing a hairy appearance. The Japanese name ‘houyuu’ is derived from the fishing vessel Houyuu-maru which collected the specimens, and ‘shiawase’ means happiness in Japanese, alluding to the quadrilobate abactinal plates with high and slender paxillar columns that resemble four-leaf clovers. ‘Momijigai’ comes from the Japanese name for the family Astropectinidae.

Remarks

This new species resembles three Paxillosida genera, Gephyreaster, Mimastrella and Radiaster, in possessing the following diagnostic characteristics (Fisher 1916; Oguro 1991; Kobayashi et al. 2024), paxilliform supero- and inferomarginal plates (Figures 2 (D,E) 3(C,D)) and tube feet with sucking discs (Figure 2 (I)). Among these genera, this new species can clearly be assigned to the genus Mimastrella since it possesses the cruciform abactinal plates regularly arranged on the abactinolateral side of the disc and arms (Figure 2 (C)) and lacking the unpaired oral spines (Figure 2 (H)). Gephyreaster has hexalobate abactinal plates and unpaired oral spines (Kobayashi et al. 2024), and Radiaster has non-cruciform abactinal plates irregularly arranged on the abactinolateral side of the disc (Fisher 1916).

Mimastrella komkom sp. n. represents the first discovery of the genus Mimastrella from the Northern Hemisphere, significantly extending the distributional range of this genus. Despite the far-distant distribution between this new species and the Antarctic-to-Chilean M. cognata (Mutschke and Mah 2009), these two species share many morphological features. However, M. komkom sp. n. can be distinguished from M. cognata by the heights of the abactinal, superomarginal, and inferomarginal paxillar columns: the abactinal paxillar columns are five or more times taller than their width (Figure 3 (A)) [vs one to two times taller than their widths in M. cognata (Sladen 1889: pl. 62; fig. 3)]; the superomarginal paxillar columns are taller than the plate length (Figure 3 (C)) [vs as high as the plate length in M. cognata (Ludwig 1903, pl. 2, fig. 15)]; and the inferomarginal paxillar columns are taller than the plate width (Figure 3 (D)) [vs shorter than the plate width in M. cognata (Ludwig 1903, pl. 2, fig. 15)]. Abactinal spines of type specimens often lack several hyaline points at their apex, unlike M. cognata (Ludwig 1903; Clark 1962); however, these spines are possibly damaged by freezing since they mostly have traces of pronging at their apex.

The ecology of the genus Mimastrella has not been known to date (Mutschke and Mah 2009). However, our specimens were collected using fishery cages baited with fish fillets, suggesting this species is a scavenger or predator. Further deep-sea surveys may unravel the potential distributional range and ecology of this new species.