Tetranemertes aurantia Abato, Hookabe & Kajihara, 2025, sp. nov.

Tetranemertes aurantia sp. nov. (Figs 2A–C, 3A–G) Material examined. Holotype, ICHUM 8719, extracted genomic DNA, anterior half preserved in Bouin’s fluid, then in 70% ethanol, unsectioned, posterior half in 99% ethanol; collected by Natsumi Hookabe on 30 June 2022, from a bycaught sample of a fishery gill net deployed around 10 m depth, off Sugashima island, Mie, Japan. Etymology. The new specific name, meaning ‘orange,’ is the Latin adjective aurantius (m.), -a (f.), -um (n.), referring to the body color of the new species. Diagnosis. A Tetranemertes with bright orange body color, without mid-dorsal stripe, with ~60 eyes in total, with longitudinally-grooved stylets, basis nearly rod in shape, bifurcated, stylet/basis ratio 0.42. Sequences. GenBank accession number PP413743, 16S (465 bp); PP413744, 18S (1778 bp); PP413745, 28S (1118 bp); PP413702, COI (604 bp). Description. External appearance of living specimen. Body long and thin, thread-like, uniformly deep orange in color (Fig. 2A–C) on both sides except semi-translucent head, colorless anterior end, and paler posterior end (Fig. 2B, C), orange appearance due to internal body structures visible through body, margin colorless, no markings, narrows from anterior region towards head (Fig. 2B, C), anterior-most body to head noticeably convoluted when contracted (Fig. 3A, B); head not demarcated from rest of body, narrow spearhead in shape (i.e., resembles a viper’s head), obviously thin when worm stretches at non-anesthetized state, broadens or bulges immediately in front of cerebral ganglia, narrows anteriorly (Fig. 2B), colorless (Fig. 3A, B); four longitudinal rows of laterally distributed reddish-purple ocelli visible at dorsal view of head, two ocelli rows on each side, 26–28 in total number, anterior-most ocelli do not reach head tip, posterior-most immediately in front of cerebral ganglia (Fig. 3A, B), one row of ocelli on each side of head visible at ventral view (10–12 in number) (Fig. 3B); cerebral ganglia visible as pair of round, pinkish structures at mid posterior head on both sides (Figs 2B, 3A, B); cerebral organ not discernable at dorsal view, slightly notable at ventral view (Fig. 3B); cephalic furrow not evident at dorsal view, apparent ventrally as shallow ‘Λ’ with corner directing anteriorly, located in front of cerebral ganglia, before posterior-most ocelli (Fig. 3B); posterior tip of body rounded. Rhynchocoel and proboscis. Rhynchocoel restricted to anterior-most region of body, approximately within 2 cm from tip of head (Fig. 2B); proboscis transparent in color, with two distinct regions separated by proboscis bulb, diaphragm dark brownish, dotted with reddish-purple pigments at high magnification (Fig. 3G), with single central stylet (Fig. 3C) and two overlapping pouches, each holding two accessory stylets (Fig. 3E, F); central stylet shaft straight, glassy, fluted, accessory stylets same (Fig. 3D–F), 115 µm in length, 19 µm in width; stylet basis rod-shaped, dark brown under light microscope, narrower anteriorly, posterior appears bifurcated (Fig. 3C, D), 273 µm in length, 53 µm in width; central stylet to basis ratio 0.42. Remarks. The new species can be distinguished from the named Tetranemertes based on morphology, DNA sequence data, and locality. The new species can be morphologically distinguished from the pale to deep pinkish T. arabica Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023, the white or yellowish and mid-dorsally striped T. rubrolineata (Kirsteuer, 1965), the pale yellowish orange to pale pink and mid-dorsally striped T. unistriata, from the dark pink (wine) colored T. antonina, from the pale yellow to orangish yellow T. pastafariensis Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023, from the uniformly intense pink T. majinbuui Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023, from the pale yellow or deeppinkish orange T. ocelata Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023, from the dusky pink T. hermaphroditica (Gibson, 1982), and from the multi-colored and stripped and spotted T. bifrost Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023. Tetranemertes aurantia sp. nov. exhibits color resemblance with the orange-colored T. paulayi Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023 but both species differ in the number of ocelli and the morphology of the stylet basis. Phylogeny and genetic distance. Tetranemertes aurantia sp. nov. forms a highly supported clade along with other members of Tetranemertes and is sister to a clade consisting of T. arabica, T. rubrolineata, and T. unistriata (Fig. 4) with uncorrected pairwise COI genetic distances of 11.3%, 11.6%, and 10.6%–12.3%, respectively (Table 2).