Tsuranarthrura shinsei Kakui & Tomioka 2018, sp. nov.

Tsuranarthrura shinsei sp. nov. (Figs 1–5)

Diagnosis. Same as for the genus.

Etymology. The specific name (a noun in apposition) is derived from R/V Shinsei-maru, the vessel from which the type specimens were collected.

Material examined. Holotype. Sex indeterminate, NSMT-Cr 25816 (BL 1.99 mm, CW 0.38 mm), dissected, six slides and one vial; INSD accession number LC326400; R / V Shinsei-maru, Off Miyagi, Japan, Northwestern Pacific Ocean (38°44.29′N, 143°10.06′E), 1890 m depth, box corer, mud bottom, 11.viii.2017. Paratypes: one sex indeterminate, NSMT-Cr 25817 (CW 0.36 mm), dissected, three slides; INSD accession number LC326401; one sex indeterminate, NSMT-Cr 25818 (CW 0.38 mm), dissected, one slide and two SEM stubs. Paratypes with same collection data as for holotype.

Other material. One sex indeterminate, NSMT-Cr 25819 (BL 1.79 mm, CW 0.37 mm), one vial, with same collection data as for holotype.

Description. Based on holotype; sex indeterminate.

Body (Figs 1, 2 a–c, 4) slightly dorsoventrally flattened, 5.24 times as long as CW, BL 1.99 mm; body wall not heavily calcified. Cephalothorax 0.16 times as long as BL, 0.86 times as long as wide, naked; anterior edge rounded in dorsal view; eye lobe absent. Pereonites 1–5 with length ratio of 1.00: 1.23: 1.37: 1.51:1.52; all wider than long; pereonites 1–3 naked; pereonites 4 and 5 with lateral simple seta. Pereonite 6 and pleon fused (Fig. 2c), 0.32 times as long as BL, 1.91 times as long as wide, with one lateral and one distal pairs of simple setae; lateral margins almost parallel; posterior edge round in dorsal view; vestigial articulations between segments evident on ventral surface (Fig. 4 e–g).

Antennule (Fig. 2d) 0.96 times as long as cephalothorax; articles 1–4 with length ratio of 1.00: 0.43: 0.37: 0.60. Article 1 with distal simple seta and several PSS. Article 2 with distal simple seta and two PSS. Article 3 with two distal simple setae. Article 4 with five simple setae, PSS, and aesthetasc in distal region. Antenna (Figs 2e, 4a) with five articles, 0.72 times as long as antennule; articles 1–5 with length ratio of 1.00: 0.67: 1.63: 1.20:0.20. Articles 1 and 2 naked. Article 3 with two distal simple setae and two PSS (arrowhead, Fig. 2e: artifactual depression). Article 4 with distal simple seta. Article 5 with one subdistal and four distal simple setae.

Labrum (Figs 2f, g, 4a, b) conical, not depressed laterally, naked. Mandibles (Fig. 3a, b) without molar process and lacinia mobilis; incisor narrow and distally multifurcate. Labium not observed. Maxillule (Fig. 3c) with endite bearing eight distal spines; palp not observed. Maxilla (Figs 3d, 4b) rectangular, naked. Maxilliped (Figs 3e, 4b) with naked basis; endite slightly widening distally, with ventrosubdistal simple seta. Palp with article 1 naked; article 2 with three inner simple setae; article 3 with three long and one short inner simple setae; article 4 with five simple setae. Epignath (Fig. 3f) falciform, with minute distal setae.

Cheliped (Figs 3g, h, 4c, d) attached ventromedially to cephalothorax. Basis shorter than wide, with outer simple seta. Merus with ventral simple seta. Carpus 1.67 times as long as wide, with one dorsal, one dorsodistal, and two ventral simple setae. Chela slightly longer than carpus. Propodal palm with one inner and one outer simple setae at insertion of dactylus. Fixed finger slightly longer than palm, with simple seta on ventral margin, three mid-dorsal simple setae and four dorsal processes (Fig. 3h) on cutting surface, and triangular claw. Dactylus-unguis slightly longer than fixed finger, with inner simple seta and row of dorsal ridges. Unguis triangular.

Pereopods 1–5 cylindrical, with length ratio of 1.00: 0.94: 0.85: 0.84: 0.86 (distal portion of pereopod-3 unguis broken). Pereopod 1 (Fig. 3i) 0.19 times as long as BL, with length ratio of basis, ischium, merus, carpus, propodus, and dactylus-unguis 1.00:0.07:0.34:0.36:0.61:0.83. Coxa with simple seta. Basis cylindrical, narrow, 4.36 times as long as wide, naked. Ischium with ventral simple seta. Merus naked. Carpus with one dorsal, one inner, and one ventral simple setae in distal region. Propodus with ventrodistal simple seta and dorsodistal microtrichia. Dactylus with proximal seta. Unguis 1.22 times as long as dactylus, naked. Pereopod 2 (Fig. 3j) with length ratio of articles from basis to dactylus-unguis 1.00: 0.10: 0.29: 0.37: 0.52: 0.76. Coxa, basis, ischium, merus, and dactylus-unguis similar to those of pereopod 1. Carpus with one dorsodistal and one ventrodistal simple setae and ventrodistal spiniform seta. Propodus with ventrodistal simple seta and dorsal serration on edge in distal half. Pereopod 3 (Fig. 3k) with length ratio of articles from basis to dactylus-unguis 1.00: 0.08:0.25:0.33:0.48:0.69; similar to pereopod 2. Pereopod 4 (Figs 3l, 5) without coxa. Length ratio of articles from basis to dactylus-unguis 1.00:0.08: 0.25:0.39: 0.44:0.38. Basis cylindrical, 3.38 times as long as wide, with two ventral PSS. Ischium with ventral simple seta. Merus with two ventrodistal serrate spiniform setae. Carpus with dorsodistal simple seta and two distal serrate spiniform setae (cf. Fig. 5). Propodus with one dorsal and two ventral serrate spiniform setae and dorsal microtrichia in distal region. Dactylus naked. Unguis 0.48 times as long as dactylus, with ventral setation. Pereopod 5 (Fig. 3m) with length ratio of articles from basis to dactylus-unguis 1.00: 0.09: 0.27: 0.35: 0.43: 0.38; similar to pereopod 4, except carpus with three distal serrate spiniform setae.

Pleopods absent.

Uropod (Fig. 3n) with exopod fused to basal article bearing two simple setae. Endopod biarticulate; article 1 with two distal PSS; article 2 0.78 times as long as article 1, with one middle and four distal simple setae and PSS.

Variation. In addition to the holotype (NSMT-Cr 25816), two paratype specimens (NSMT-Cr 25817, 25818) were observed. All specimens had a fused segment consisting of pereonite 6 and the pleon. The numbers of simple setae, spiniform setae, serrate spiniform setae and aesthetascs on appendages, and of processes on the chelipedal fixed finger, were identical among the three specimens, with the following exceptions. 1) There were five (NSMT-Cr 25817, 25818) or four (NSMT-Cr 25816) distal simple setae on antennular article 4. 2) There were two (left in holotype; Fig. 3l), three (right in holotype, left in NSMT-Cr 25817, and right in NSMT-Cr 25818; Fig. 5), or four (right in NSMT-Cr 25817) distal serrate spiniform setae on the pereopod-4 carpus.

Genetic information. Partial COI sequences (651 bp, encoding 216 amino acids) were determined from the holotype specimen NSMT-Cr 25816 and the paratype specimen NSMT-Cr 25817; INSD accession numbers LC326400 and LC326401; the two sequences were identical. The sequence in the INSD most similar to our COI sequence, as determined by BLAST searches (Altschul et al. 1990), was from the tanaidacean Typhlotanais variabilis Hansen, 1913 (Typhlotanaidae) (identity score 74%, query cover 70%, Błażewicz-Paszkowycz et al. 2014). To date, no other anarthrurid nucleotide sequences have been deposited in public databases (DDBJ 2017).

Distribution. So far known only from the type locality.

Remarks. Although Anarthrura simplex does not have pereonite 6 fused to the pleon, this is the only confamilial species having a fused segment (consisting of the pleonites and pleon). Here we present additional differences between T. shinsei sp. nov. and A. simplex not mentioned in the Remarks section for Tsuranarthrura gen. nov. 1) All pereonites are wider than long in T. shinsei sp. nov., whereas pereonites 2–5 are longer than wide in A. simplex. 2) The mandibular incisor is narrow and multifurcate in T. shinsei sp. nov. but subrectangular in A. simplex. 3) The number of spines on the maxillular endite is eight in T. shinsei sp. nov., but four in A. simplex. 4) The length of dactylus-unguis is about fourfifths that of the propodus in pereopods 4 and 5 in T. shinsei sp. nov., but about two-thirds in A. simplex.

So far four anarthrurid species were reported from around Japan: Anarthruropsis langi Kudinova-Pasternak, 1976, Anarthruropsis longa Kudinova-Pasternak, 1984, Siphonolabrum tenebrosus Bird, 2007, and Keska sei Błażewicz-Paszkowycz et al. 2013 (Bird 2007; Błażewicz-Paszkowycz et al. 2013). Tsuranarthrura shinsei sp. nov. is the fifth anarthrurid species around Japan.