Tetrastemma album sp. nov.

Fig. 2A-C [New Japanese name: misaki-oshiroi-himomushi]

Etymology.

The species name is derived from the Latin Tetrastemma album (white), referring to pure white body of the new species. The Japanese name is named after the white powder foundation traditionally used by Maiko, Geisha, Kabuki actors in Japan.

Material examined.

Holotype: NMST-NE-H-06, unsectioned complete specimen except for the posterior tip, fixed in Bouin’s fluid and later preserved in 70% ethanol, posterior tip preserved in 99% ethanol, collected on March 12, 2021 by NH, biological dredge (R/ V Rinkai-maru) at depths of 144-200 m, off Jogshima (35°07.41'N, 139°34.11'E - 35°07.32'N, 139°33.572'E), Miura, Kanagawa, Japan, NW Pacific.

Description.

Head spatulate to rounded in profile (Fig. 2A-C), demarcated by posterior cephalic furrows from body (Fig. 2A). Before anesthetization, body of a live specimen 17 mm long and 1.0-1.2 mm wide. Body uniformly pale colored, without longitudinal or transverse stripe markings (Fig. 2A). Pure white transverse cephalic patch present between anterior and posterior pairs of eyes (Fig. 2B). Head not wider than maximum body width (Fig. 2A-C). A pair of cephalic furrows present; anterior pair not meeting mid-dorsally and ventrally curving anteriorly but not reaching to proboscis pore; posterior pair V-shaped and barely meeting mid-dorsally (Fig. 2B) and running transversely on ventral surface (Fig. 2C). Cerebral ganglia and blood not red and probably uncolored. Internal organs (proboscis, foregut, and intestine) visible as pale regions. Four reddish brown eyes regular in size (Fig. 2B).

Type locality and distribution.

The species is only known from the type locality, Sagami Bay, Kanagawa Prefecture, Japan, at depths of 144-200 m (Fig. 1).

Remarks.

Having a pure white cephalic patch on a uniformly pale body, T. album sp. nov. differs from all the described species. Tetrastemma coronatum (Quatrefages, 1846), T. diadema Hubrecht, 1879, T. olgarum Chernyshev 1998, and T. pseudocoronatum Chernyshev 1998 have white cephalic patches but are distinguished from T. album sp. nov. in possessing a light brown to dark transverse band on the head. Tetrastemma albomaculatum Chernyshev, 2016 also possesses a white cephalic patch but differs from the new species in having a pale-ochre body dorsally spotted with small white dots (Chernyshev 2016).