Bathyferdina caelator Arai & Fujita 2021, sp. nov.

Bathyferdina caelator sp. nov.

[New Japanese name: Koyo-akamon-hitode] (Figs 2A, 3)

Bathyferdina sp.: Arai et al. 2018: 194–196.

Material examined. Holotype: NSMT E-8265, KY-09-21, Northwest of Ototo-jima Island, 135.5–135.8m. Paratypes: NSMT E-8266, 1 individual, KT-09-2-TW1-1, West of Chichi-jima Island, 138.6–145.2 m; NSMT E-8267, 1 individual, dry, locality unknown.

Diagnosis. A species of Bathyferdina with glassy bosses on abactinal, actinal and adambulacral plates. Disc broad with R/r 2.0–2.3. Abactinal plates are homogeneous in size and shape. Superomarginal plates rectangular to barrel-like in outline, and regularly decreasing in size from the interradius to the tip of arms. Glassy boss/ridge present on abactinal and actinal plates. One or two glassy bosses on the center of each adambulacral plate. Three furrow spines on each adambulacral plate.

Description of holotype. R= 23.2 mm, r= 9.9 mm, R/r= 2.3, width of arm is 11.1 mm at base, 6.6 mm at half of R, and 3.1 mm at 1/10 R from the tip. Body is flat on both abactinal and actinal sides. Arms are five and tapering more greatly near the disc than near the arm tip (Fig. 3A).

Abactinal plates are polygonal to elliptical in shape, not lobed, variable in size, and tessellated so that only small spaces are left for papulae around the corners of these plates (Fig. 3B). Those on the disc are about 1.5–2.0 times larger than those on the arms. The arrangement of abactinal plates is regular, and median rows of plates are conspicuous. Between two upper interradial corners of first superomarginal plates on a ray, there are seven to nine abactinal plates. The entire abactinal surface of abactinal plates is rough with glassy bosses which are circular around the center of the plate and elongated around the periphery (Fig. 3D). Madreporite is single, 1 mm in diameter, flat and pentagonal with rounded corners; gyri extend radially from the central area where nine discontinuous, short gyri lie in parallel to one another. Terminal plates are conical with a broad base and a narrow rounded apex, smaller than the distalmost marginal plates, smooth and bare without skin or granules (Fig. 3C).

Superomarginal and inferomarginal plates correspond in number and size. There are six (seven on one arm) plates to each side of an arm. Superomarginal plates are rectangular in outline, longer than width, rounded at corners, and conspicuous on the abactinal side; first superomarginal plates are about 3 mm in length and 2 mm in width on the abactinal surface. They gradually decrease in size toward the arm tip where the distalmost superomarginal plates are more squarish, measuring about 1–1.5 mm in both length and width. No glassy boss was observed on any marginal plates.

Actinal plates are quadrangular, regularly arranged, and normally flat except several small plates slightly convex between larger actinal plates (Fig. 3F) and inferomarginal plates are slightly convex. The surface of actinal plates is rough with glassy bosses when denuded of granular skin.

Adambulacral armature is composed of three (distally two) furrow spines on each adambulacral plate (Fig. 3F). Furrow spines are truncated at the tip, prismatic, and quadrangular or triangular in a cross section. The adambulacral plates are rectangular with slightly rounded corners. On the proximal first to tenth adambulacral plates, there are one or two glassy bosses at the center of the plates.

Abactinal, actinal and adambulacral plates are covered with a thin skin concealed by fine granules. On the marginal plates, the skin and granules are limited at the periphery, leaving the rest of the plate surface exposed (Fig. 3B). Papulae are isolated, confined around the abactinal plates except in the abactinal interradial areas where they are absent. There are no papulae on the actinal surface. Most papulae occur in spaces where corners of three abactinal plates meet. There are pedicellariae on some of the first or second superomarginal plate and actinal interradial plates (Fig. 3B). Each pedicellaria with a single fan-shaped valve rather than two or more valves normally observed in Ferdininae. Some alveoli lacking a fan-shaped valve are present on some marginal plates.

Color in life is brownish on the abactinal surface and white on the actinal. Bare areas on marginal plates are darker brown (Fig. 2A).

Notes on paratypes. The body dimensions in the paratypes are: R =10.0 mm, r=5.0 mm, and R /r=2.0 (NSMT E-8266); R = 20.4 mm, r= 9.7 mm, and R /r=2.1 (NSMT E-8267). The other characters are consistent with the description of the holotype.

DNA sequence. A partial sequence of COI (655 bp) was obtained from NSMT E-8265 and deposited in DDBJ (Acc. No. LC427072).

Remarks. Mah (2017) established Bathyferdina with a description of B. aireyae. Bathyferdina caelator sp. nov. is devoid of any subambulacral spines or enlarged granules while it shows small R/r, a uniform granular skin covering abactinal and actinal surface, marginal plates continuous in size, large quadrate bare area on marginal plates, homogenous abactinal plates completely covered with skin, and glassy bosses on abactinal plates. These characters support placement of the present species in Bathyferdina. Mah (2017) mentioned that Bathyferdina has no glassy bosses on marginal plates in his key of Ferdininae but he also stated that it has glassy bosses on both abactinal and marginal plates. According to Mah (personal communication), B. aireyae in fact has glassy bosses on the abactinal and marginal surfaces but not on the actinal surface. We have emended the diagnosis of Bathyferdina accordingly.

Bathyferdina caelator is distinguished from the single congeneric species, B. aireyae, in having glassy bosses on not only abactinal but actinal and adambulacral plates while lacking them on marginal plates. Bathyferdina caelator lacks granules or tubercles on the inferomarginal plates, while they are occasionally present in B. aireyae. Mah (2017) described B. aireyae as lacking any pedicellariae but one of the paratypes reexamined in this study (CASIZ 219696) possesses a large number of pedicellariae on marginal and actinal plates. Although these pedicellariae varied in number of valves from one to five, the shape of the valves closely resembled those in B. caelator.

Distribution. Ogasawara Islands, 135.5–145.2 m (this study).

Etymology. The specific epithet, caelator, is a Latin masculine noun in apposition meaning sculptor, and alluding to the complex patterns of glassy bosses on abactinal, actinal and adambulacral plates.

Japanese name. Koyo is taken from R/V Koyo, the vessel which collected the holotype, and akamon-hitode comes from the Japanese name of Neoferdina cumingi (Gray, 1840), another species of Ferdininae which occurs commonly in Japanese waters.