Atlantapseudes tridens, Matsushima & Bâlcu & Kakui, 2025

Matsushima, Yoshinobu, Bâlcu, Maxim-Jean & Kakui, Keiichi, 2025, Atlantapseudes tridens sp. nov. (Tanaidacea: Apseudidae) from the Okinawa Trough, Japan, with a note on the taxonomic status of Atlantapseudes curvatus, Zootaxa 5679 (4), pp. 484-500 : 492-498

Atlantapseudes tridens sp. nov.

[New Japanese name: Sanbonduno-apuseudesu]

( Figs 5–8 )

Atlantapseudes sp. : Kakui et al. 2011, table 1, figs 2–5; Matsushima & Kakui 2024, fig. 7.

Type material. Holotype: preparatory female ( ICHUM4002 , TBL = 7.10 mm, CW = 1.39 mm, 11 slides and one vial), R / V Soyo-maru , Stn Kago-7, Okinawa Trough , East China Sea (28°29'24.0"N, 127°59'30.0"E to 28°29'42.0"N, 128°00'48.0"E), 1076–1079 m depth, plankton net attached inside beam trawl, 29 July 2010, collected by K. Kakui. Paratype: one preparatory female ( ICHUM8985 , TBL = 5.66 mm, CW = 1.05 mm, 11 slides and one vial, R / V Soyo-maru , Stn Kago-9, Okinawa Trough , East China Sea (27°30'27.5"N, 126°58'10.9"E to 27°30'47.8"N, 126°58'04.0"E), 1595– 1557 m depth, plankton net attached inside beam trawl, 26 July 2014, collected by K. Kakui.

Representative DNA Sequences. One 18S ( INSD accession number AB618176 ; 2189 bp long; Kakui et al. 2011) and one COI ( LC860315 ; 658 bp long, encoding 219 amino acids; this study) sequences were determined from the holotype .

Etymology. The specific name tridens (Latin: trident) is a noun in the nominative case, referring to the shape of the tridentate rostrum.

Diagnosis. Rostrum tridentate. Epistome strongly curved ventrally. Pereonites 1 and 2 without lateral spines. Lateral margin of pereonites 3–6 with anterior spine (tiny in pereonite-6) but without posterior spines. Squama more than twice longer than antennal article-3. Labial palp with two distal spiniform setae. Maxillipedal palp article-2 with outer distal seta. Chelipedal carpus with two ventral long setae.

Description of female (based on holotype unless noted otherwise). Body ( Figs 5 , 6A 1 , 2 , B [holotype ICHUM4002]; 6C1–4, D [paratype ICHUM 8985]) well calcified, dorsoventrally flattened, 5.1 times longer than CW, whitish in specimens before fixation ( Fig. 5 ). Cephalothorax length 0.2 times TBL, 1.1 times as long as wide; rostrum tridentate; eye lobes distinct, with anterior spine; mid-lateral margin with spine; acute ventral keel (hyposphenium) present, posterior to maxillipeds. Epistome ( Fig. 6E ) strongly curved ventrally. Pereonites 1–6 with length ratio 1.00: 1.16: 1.59: 1.84: 1.79: 1.25; with width/length ratio 3.09: 2.31: 1.93: 1.89: 1.59: 1.70 and with acute hyposphenium; lateral margin of pereonites 3–6 with anterior spine, with the spine of pereonite-6 tiny. Pleon length 0.2 times TBL. Pleonites distinctly narrower than pereonites; each pleonite with width/length ratio 3.60: 3.01: 3.05: 3.06: 2.48, similar in shape, with hyposphenium (acute in holotype; obtuse and rounded on pleonites 1–4 but acute on pleonite- 5 in paratype; Fig. 6B, D ); epimera ( Fig. 6A 2 , B, C 4 , D) projecting ventroposteriorly, with pointed apex. Pleotelson ( Fig. 6A 2 , B, C 4 , D) 1.1 times as long as wide, narrower than pleonites, with posterior projection.

Antennule ( Fig. 7A ) with eight-articled outer flagellum and three-articled inner flagellum. Combined length of peduncle and outer flagellum 1.26 times longer than cephalothorax. Peduncular articles 1–4 with length ratio 1.00: 0.41: 0.11: 0.11; article-1 with two mid-outer, one mid-inner, and one inner subdistal simple setae and ten outer (four broken) PSS; article-2 with one mid-outer, two outer distal (one long), one inner subproximal, and one inner subdistal simple setae and three outer distal (one broken) and two inner distal PSS; article-3 with one outer distal and three inner distal simple setae; article-4 with two PSS at insertion of outer flagellum. Outer flagellum 0.5 times as long as peduncle; articles 1 and 2 naked; articles 3–8 with three distal, one distal, three distal (one long), one distal, two distal (one long), and two middle and four distal simple setae, respectively; article-8 with one PSS (broken); article-7 with distal long aesthetasc. Inner flagellum 0.5 as long as outer flagellum; articles 1–3 with two, one (broken), and three (one broken) distal simple setae, respectively; articles 1–3 each with distal PSS.

Antenna ( Fig. 7B ) with 11 articles, 0.9 times as long as antennule; articles 1–11 with length ratio 1.00: 1.80: 0.67: 1.56: 2.43: 0.91: 0.64: 0.63: 0.46: 0.47: 0.45. Article-1 naked. Article-2 with one outer subproximal, one outer distal, and one inner subproximal simple setae. Article-3 with inner distal simple seta. Article-4 with inner subproximal simple seta and two inner subdistal PSS (one broken). Article-5 with one inner subproximal and one inner subdistal simple setae and one outer subproximal, one mid-outer, two inner subdistal, and two dorsal subdistal PSS. Article-6 with one mid-outer, one distal long, and one inner subdistal simple setae. Articles 7–11 with three distal (one long), one inner subdistal and three distal (one long, one broken), two distal (one long, one broken), and five distal simple setae, respectively; article-9 with inner distal PSS. Squama 0.2 as long as antenna, 2.4 times longer than antennal article-3, and 1.0 times as long as antennal article-4; narrow, with two simple setae (one long).

Labrum ( Fig. 7C ) bipartite with numerous fine setae.

Mandibles ( Fig. 7D–G ) with well-developed molar process bearing six tufts of fine setae ( Fig. 7D ). Left mandibular incisor ( Fig. 7E ) with five rounded processes; setiferous lobe with two setulate and serrate setae, two setulate bifurcate setae, two spiniform setae, and fine setae; lacinia mobilis with five rounded processes. Right mandibular incisor ( Fig. 7F ) with four rounded processes; setiferous lobe with five round-tipped setae and fine setae. Palp ( Fig. 7G ) with three articles; articles 1–3 with length ratio 1.00: 2.67: 0.79; article-1 with middle simple seta; article-2 with seven bipinnate setae in distal region; article-3 with 10 distal bipinnate setae.

Labium ( Fig. 7H ) with lobe bearing several outer simple setae and inner setation. Palp with several short and long simple setae on lateral margins, and two distal bifurcate spiniform setae.

Maxillule ( Fig. 7I ) with biarticulate palp; palp article-1 with three fine setae; palp article-2 with five pinnatetipped setae (one broken) in distal half. Outer endite with numerous simple setae in middle region, outer proximal and inner fine setation, 10 distal spiniform setae, and two subdistal plumose setae. Inner endite with outer and inner distal fine setation and three distal setulate spiniform setae.

Maxilla ( Fig. 7J ) with body bearing inner distal fine setae, short simple setae in inner distal region, 13 outer distal simple setae, and outer proximal serration. Inner lobe of fixed endite with several short simple setae, three biserrate setae, and 17 basally swollen setae. Outer lobe of fixed endite with outer fine setae, eight inner short simple setae, six distal simple setae, one distal and one subdistal plumose setae, and three distal trifurcate setae. Inner lobe of movable endite with inner fine setae, nine ventral short simple setae, three plumose setae (one broken), furcate seta, and four distal simple setae. Outer lobe of movable endite with two ventral short simple setae and seven setulate serrate setae.

Maxilliped ( Fig. 7L, l 1 , K) with naked coxa. Basis with outer fine setae and two mid-inner simple setae. Endite ( Fig. 7K , paratype) with several fine setae in middle region; inner region with four setulate serrate setae and two coupling hooks; distal region with fine setae, two plumose setae, and eight setulate serrate setae. Palp article-1 with one outer distal seta and one inner distal simple seta; article-2 with one outer distal and eleven inner simple setae; article-3 with eleven inner simple setae; article-4 with nine inner simple setae and two inner serrate setae (broken) ( Fig. 7l 1 ). Epignath ( Fig. 7M , paratype) cup-shaped, with setulate terminal seta.

Cheliped ( Fig. 8A ) without exopod. Basis 2.6 times as long as wide, with one mid-dorsal and four ventral simple setae and one dorsoproximal and one ventro-subdistal spines; inner surface smooth. Merus with ventrodistal simple seta and ventrodistal spine. Carpus 0.64 times as long as basis, 2.5 times as long as wide, with one dorso-subproximal, one dorsodistal, two ventro-subdistal short simple setae and two ventral long simple setae. Propodus with one mid-outer and four ventral simple setae; dorsal region of palm with two distal simple setae; cutting surface with one outer proximal, two inner proximal, and two inner subdistal simple setae, eleven outer round-tipped setae, and several denticles; unguis curved and pointed. Dactylus longer than fixed finger, with three outer and three inner simple setae and several denticles; unguis curved and pointed, longer than fixed-finger unguis.

Pereopods 1–6 with length ratio 1.00: 0.94: 0.82: 0.84: 0.88: 0.82. Pereopod-1 ( Fig. 8B ) 0.5 times as long as TBL, with length ratio of basis, ischium, merus, carpus, propodus, and dactylus–unguis 1.00: 0.07: 0.43: 0.39: 0.28: 0.27. Coxa with anterior spine and six plumose setae. Basis 4.83 times as long as wide, with one dorso-subdistal, one mid-ventral, one ventro-subdistal, and two ventrodistal (one broken) simple setae and two mid-dorsal (one long) PSS. Ischium with two ventrodistal simple setae. Merus with ventro-subproximal row of five simple setae, four dorsodistal and three ventrodistal simple setae, and ventrodistal spiniform seta. Carpus with two mid-dorsal, six dorsodistal, four ventral, and three ventrodistal simple setae and one dorsodistal and two ventrodistal spiniform setae. Propodus with one mid-dorsal, one dorsodistal, and two ventrodistal simple setae, two dorsal and five ventral spiniform setae, inner dorso-subdistal PSS, and inner ventrodistal pinnate seta. Dactylus with two dorso-subproximal simple setae and three ventral spines; unguis half as long as dactylus, naked.

Pereopod-2 ( Figs. 6A 1, B, C1 , D, 8C ) with length ratio of articles from basis to dactylus–unguis 1.00: 0.13: 0.36: 0.38: 0.32: 0.34. Coxa with spine (broken in left pereopod-2 of holotype, Figs. 6A 1, B, C1 , D) and three small simple setae. Basis 6.67 times as long as wide, with four setae, one mid-dorsal, one ventro-subproximal, one ventro-subdistal, and one ventrodistal simple, and one dorso-subproximal and one mid-dorsal PSS. Ischium with ventrodistal simple seta. Merus with three dorsodistal, one ventro-subproximal (broken), one mid-ventral, and three ventrodistal simple setae. Carpus with seven dorsal and nine ventral (four broken) simple setae. Propodus with six dorsal and ten ventral simple setae, dorso-subdistal PSS, and inner distal pinnate seta. Dactylus with two dorso-subproximal and two ventrodistal simple setae and ventral spine; unguis more than half as long as dactylus, naked ( Fig. 8C ).

Pereopod-3 ( Fig. 8D ) with length ratio of articles from basis to dactylus–unguis 1.00: 0.10: 0.31: 0.33: 0.27: 0.15: 0.14; coxa without spine; other characters similar to pereopod-2 except in number of setae.

Pereopod-4 ( Fig. 8E ) with length ratio of articles from basis to dactylus–unguis 1.00: 0.09: 0.25: 0.38: 0.30: 0.23. Coxa naked. Basis 4.7 times as long as wide, with one ventro-subdistal seta and one ventrodistal simple seta, and three dorso-subproximal (one broken), one ventroproximal, and one mid-ventral (long) PSS. Ischium with ventrodistal simple seta. Merus with one dorsodistal, one mid-ventral, and two ventrodistal (one broken) simple setae. Carpus with one mid-ventral, one ventro-subdistal, and seven distal simple setae. Propodus with mid-ventral simple seta, mid-dorsal PSS, nine distal pinnate setae, and two distal long serrate setae. Dactylus with ventrodistal simple seta; unguis half as long as dactylus, naked.

Pereopod-5 ( Fig. 8F ) with length ratio of articles from basis to dactylus–unguis 1.00: 0.07: 0.22: 0.37: 0.26: 0.18. Coxa with simple seta. Basis 7.1 times as long as wide, with one dorsoproximal, one dorso-subproximal, and one ventrodistal (broken) small simple, setae, and two dorso-subproximal (broken) and one mid-ventral (broken) PSS. Ischium with ventrodistal simple seta. Merus with one dorso-subdistal and two ventrodistal simple setae. Carpus with one dorso-subdistal, two dorsodistal, one mid-ventral, and three ventro-subdistal simple setae. Propodus with one dorso-subdistal, one dorsodistal, one mid-ventral, three ventro-subdistal, and one ventrodistal simple, setae, dorso-subdistal PSS, and two distal pinnate setae (one broken). Dactylus with three ventrodistal simple setae; unguis half as long as dactylus, naked.

Pereopod-6 ( Fig. 8G ) with length ratio of articles from basis to dactylus–unguis 1.00: 0.09: 0.22: 0.37: 0.26: 0.29. Basis 6.8 times as long as wide, with one ventroproximal, one mid-ventral, and one ventrodistal small simple, setae, and three dorso-subproximal (two broken) and one mid-ventral (long) PSS. Ischium with ventrodistal simple seta. Merus with one dorsodistal seta and one ventrodistal (broken) simple seta. Carpus with three dorsodistal, one ventro-subdistal, and two ventrodistal simple setae. Propodus with one ventro-subproximal, three mid-outer (one broken), and four distal (one broken) simple setae and mid-dorsal PSS. Dactylus with two mid-dorsal setae and one ventrodistal simple seta; unguis half as long as dactylus, naked.

Pleopods absent.

Uropod ( Fig. 8H , paratype) 4.21 times longer than pleotelson ( Fig. 6D ), with protopod bearing two outer distal, one mid-inner, one inner subproximal (long), and inner distal simple setae. Exopod with four articles; article-3 naked; articles 1, 2 and 4 with one subproximal, one distal, and four distal (one broken) simple setae, respectively. Left endopod with 18 articles; articles 1, 3, 5, 7, 9, 14, 16, 17 naked; articles 4, 6, 8, 10–13, 15, 18 with two, one (broken), two, one (broken), one, two, one, two, and four distal simple setae, respectively; articles 2, 4, 8, 12, 15, 18 with one, three, three (two long), two (one long), two (one long), and two (one long) distal PSS, respectively. Right endopod with 19 articles (for setation see Supplementary Table S2).

Variation and stability. In addition to the holotype, one paratype female (ICHUM8985) was dissected and selected characters were observed (see Supplementary Table S2). Both specimens shared the same state for the characters listed in the Diagnosis section for this species (for the other shared characters, see Supplementary Table S2).

Remarks. Atlantapseudes tridens sp. nov. resembles A. nigrichela in having a tridentate rostrum but differs from the latter in having (1) the posterolateral margin of pereonites 1–3 without spines (one spine in A. nigrichela ); (2) a tiny anterolateral spine on pereonite-6 (large in A. nigrichela ); (3) two distal spiniform setae on labial palp (three in A. nigrichela ); and (4) one outer distal seta on maxillipedal palp article-2 (two in A. nigrichela ).

A partial COI sequence (658 bp, encoding 219 amino acids; INSD accession number LC860315 ) was determined from the holotype specimen. The sequence in the INSD most similar to our sequence, determined by a BLAST search, was from the insect Forcipomyia sp. (MG475921.1; identity score 76.02%, query cover 100%; deWaard et al. 2019). This was likely because our COI sequence was the first from any Atlantapseudes species in the INSD; a similar result has been reported for several other crustaceans (e.g., Tanabe & Kakui 2019; Kakui & Hiruta 2022; Shiraki & Kakui 2024; Kakui 2024).

Figures

FIGURE 1. Map showing the distributions of Atlantapseudes species. A. Global distributions of Atlantapseudes species. B. Atlantapseudes records in the northeastern Atlantic. C. Sampling sites for A. tridens sp. nov. off the Ryukyu Islands, Japan. Circles with black outline indicate type localities. The maps were generated with GMT6 software (Wessel et al. 2019).

FIGURE 2. Atlantapseudes nigrichela, holotype ovigerous female 393/30686, Muzeul Național de Istorie Naturală “Grigore Antipa,” photomicrographs, dorsal view. A. Anterior region of cephalothorax (black arrowheads = spines of rostrum; white arrowheads = anterior spines on eye lobes). B. Cephalothorax (left) and left cheliped (grey arrowhead = mid-lateral spine on cephalothorax; black arrows = ventral spines on chelipedal basis and merus; white arrows = ventral simple setae on chelipedal carpus). C. Cephalothorax (posterior), pereonites 1–4, and pereonite-5 (anterior). D. Left region of pereonite-2. E. Pereonite- 2 (right) and pereonite-3 (right anterior). F. Pereonite-2 (left posterior), pereonite-3 (left), and pereonite-4 (left anterior). G. Pereonite-4 (posterior), pereonite-5, and pereonite-6 (anterior). H. Pereonite-5 (posterior), pereonite-6, and pleon (anterior). C–H: black circles = anterolateral spines on pereonites; white circles = posterolateral spines on pereonites. I. Right antenna. Scales not available.

FIGURE 4. Atlantapseudes sp., male 394/30687, Muzeul Național de Istorie Naturală “Grigore Antipa,” photomicrographs. A. Cephalothorax, dorsal view (black arrowhead = spine of rostrum; white arrowheads = anterior spines on eye lobes; grey arrowheads = mid-lateral spines on cephalothorax). B. Cephalothorax (posterior), pereonites 1 and 2, and pereonite-3 (anterior), dorsal view. C. Pereonites 1–3 and pereonite-4 (anterior), lateral view. D. Cephalothorax (right posterior), pereonite-1 (right), and pereonite-2 (right anterior), dorsal view. E. Pereonite-1 (right posterior), pereonite-2 (right), and pereonite-3 (right anterior), dorsal view. F. Pereonite-2 (posterior), pereonites 3 and 4, and pereonite-5 (anterior), dorsal view.G. Pereonite-2 (right posterior), pereonite-3 (right), and pereonite-4 (right anterior), dorsal view. B–G: black circles = anterolateral spines on pereonites; grey circles = spines on pereopodal coxae. H. Right antenna. Scales not available.

FIGURE 5. Atlantapseudes tridens sp. nov., holotype, preparatory female, fresh individual, dorsal view. Scale, 1 mm.

FIGURE 6. Atlantapseudes tridens sp. nov., preparatory female;A1, 2, B, holotype; C1–4, D, E, paratype.A1, A2. Body, dorsal view from different angles (at the front of pereonite-3 and pereonite-6, respectively). B. Body, left view. C1–4. Body, dorsal view from different angles (at the front of pereonite-1, pereonite-3, pereonite-5, and pleonite-3, respectively). D. Body, left view. E. Cephalothorax in glycerin, ventral view, with an arrow indicating the epistome.

FIGURE 7. Atlantapseudes tridens sp. nov., preparatory female;A–J, L, l1, holotype; K, M, paratype.A. Left antennule, ventral view. B. Left antenna, ventral view. C. Labrum. D. Left mandible, molar process. E. Left mandible, incisor and setiferous lobe. F. Right mandible, incisor and setiferous lobe. G. Left mandible, palp. H. Labium. I. Outer and inner endites and palp of left maxillule. J. Left maxilla. K. Left maxillipedal endite. L. Right maxilliped (endite omitted). l1. Serrate seta. M. Left epignath.

FIGURE 8. Atlantapseudes tridens sp. nov., preparatory female; A–G, holotype; H, paratype. A. Left cheliped. B. Right pereopod-1. C–G. Left pereopods 2–6 (spine on coxa broken in C, coxa broken in G). H. Left uropod.