Tetrastemma persona, Hookabe & Kohtsuka & Fujiwara & Tsuchida & Ueshima, 2023

Hookabe, Natsumi, Kohtsuka, Hisanori, Fujiwara, Yoshihiro, Tsuchida, Shinji & Ueshima, Rei, 2023, Three new species in Tetrastemma Ehrenberg, 1828 (Nemertea, Monostilifera) from sublittoral to upper bathyal zones of the northwestern Pacific, ZooKeys 1146, pp. 135-146 : 135

Tetrastemma persona sp. nov.

Fig. 2D [New Japanese name: misaki-kamen-himomushi]

Etymology.

The species name is derived from the Latin persōna (mask), referring to a broad cephalic patch of the new species masking eyes and internal organs in head region. The Japanese name “kamen” means a mask in Japanese.

Material examined.

Holotype: NMST-NE-H-07, 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 July 31 2020 by NH, biological dredge (R/ V Rinkai-maru) at depths of 116-211 m, off Jogshima (35°08.32'N, 139°32.857'E - 35°08.40'N, 139°32.504'E), Miura , Kanagawa, Japan, NW Pacific . Paratype: NMST-NE-P-08, unsectioned complete specimen fixed in Bouin’s fluid and later preserved in 70% ethanol, collected on the same date and locality as the holotype.

Description.

Head slightly narrower than middle part of body and weakly demarcated from trunk (Fig. 2D ). Before anesthetization, body of a live specimen 7.0-10 mm long and 0.8-1.0 mm wide. Body uniformly pale to yellow colored without longitudinal or transverse stripe markings (Fig. 2D ). Vermilion-red cephalic patch spade-shaped (Fig. 2D ), covering both anterior and posterior pairs of eyes (Fig. 2D ) but not posteriorly reaching to anterior pair of cephalic furrows; eyes regular in sizes. A posterior pair of cephalic furrows not well distinguished probably due to the small body size. Cerebral ganglia and blood not red and probably uncolored. Internal organs (proboscis, foregut, and intestine) not well visible through body wall. Rhynchocoel visible as whitish region through body wall, extending about 1/2-2/3 of the body length.

Type locality and distribution.

The species is only known from the type locality, Sagami Bay, Kanagawa Prefecture, Japan, at depths of 116-211 m (Fig. 1 , Table 1 ).

Remarks.

Tetrastemma persona sp. nov. has atypically short rhynchocoel in the genus and most resembles T. roseocephalum (Yamaoka, 1947) and T. yamaokai Iwata, 1954 in having a pale body without any markings and a red cephalic patch. Pattern variation of a cephalic patch (shield shape or horse-shoe shape) was reported in both T. roseocephalum and T. yamaokai ; referring to the original description of T. yamaokai , the name may be a junior synonym of T. roseocephalum , as suggested by Kajihara (2007). The external morphology of T. persona sp. nov. is similar to a form with a shield-shaped cephalic patch of T. roseocephalum ( Iwata 1954).

The subtle difference in the shape of cephalic patch between T. persona sp. nov. (spade-shaped) and T. roseocephalum (shield-shaped) was supported by our molecular analysis. The new species did not constitute a clade with T. roseocephalum but with T. album sp. nov. (Fig. 3 ); T. roseocephalum belongs to Clade C of Chernyshev et al. (2021).

An uncorrected genetic distance based on 657 bp of COI was 16% between T. album sp. nov. and T. persona sp. nov., comparable with interspecific values observed among Monostilifera (e.g., Sundberg et al. 2016; Hookabe et al. 2022).

Figures

Figure 1. Collection sites of the specimens examined in the present study.

Figure 2. Holotype specimens of new Tetrastemma species; photographs were taken in life by NH A-C T. album sp. nov. A complete body, dorsal view B head, dorsal view C head, ventral view D T. persona sp. nov., complete body, dorsal view E-I T. shohoense sp. nov. E complete body F head, ventral view G squeezed specimen under a cover slip, complete body, dorsal view H head, dorsal view I stylet apparatus. Abbreviations: acf, anterior cephalic furrow; pcf, posterior cephalic furrow, cg, cerebral ganglia; co, cerebral organ; pb, proboscis; pp, proboscis pore. Scale bars: 2 mm (A); 500 μm (B, C, G); 1 mm (D, E); 100 μm (F, H); 50 μm (I).

Figure 3. A maximum-likelihood (ML) tree based on concatenated sequences of two mitochondrial [16 S rRNA (16 S; 380 bp) and cytochrome c oxidase subunit I (COI; 626 bp)] and three nuclear gene markers [18 S rRNA (18 S; 1738 bp), 28 S rRNA (28 S; 505 bp), and histone H 3 (H 3; 329 bp)]. Numbers near each node are support values generated by a separate partitioned ML bootstrap analysis with 1000 replicates. Country names of each species sampling location are abbreviated as follows: CA = Canada, JP = Japan, RU = Russia, USA = United States of America, and VE = Venezuela.