Sphenoderes japonicus, Adrianov & Maiorova, 2024

S. japonicus View in CoL sp. nov ( Figs. 2 View Fig –12).

3.1.1. Diagnosis

Adult with trunk Segment 1 with smooth cuticle; trunk segments 2–11 with minute scale-like cuticular elements in between anterior margin and free flap; free flap wide, with distinct longitudinal lines; acicular middorsal spines on trunk segments 1–11; acicular lateroventral spines on trunk segments 3–9; additional pair of cuspidate spines in lateroventral position on trunk segment 5; trunk segment 9 with a pair of large ventromedial papillae in females; trunk segment 10 with a pair of acicular (in female) or crenulated (in male) spines in laterodorsal/ midlateral position; trunk segment 11 with very long midterminal spine (93–95 % of trunk length), pair of lateral terminal spines and additional pair of long acicular spines in laterodorsal position, with three pairs of large sensory spots type 3.

3.1.2. Etymology

The species name refers to the Russian-speaking name of the Sea of Japan (“Japanese Sea”) where this species is discovered.

3.1.3. Material examined

Type material. Holotype, adult female, collected in the Sea of Japan (44 ◦ 47.6533’ N, 137 ◦ 15.3019 E) at 1530 m depth in August 14, 2010 by the RV “ Akademik M.A. Lavrentyev ”; sample of oxidized brown clay; mounted in VECTASHIELD® on Higgins-Shirayama plastic slide frames deposited at the Museum of NSCMB FEB RAS under accession number: MIMB 49932 View Materials GoogleMaps . Paratypes, adult female and two pre-adult stages with same sampling data as for the holotype; mounted in VECTASHIELD® on Higgins-Shirayama plastic slide frames; deposited at the Museum of NSCMB FEB RAS under accession numbers: MIMB 49933 View Materials , 49934 View Materials and 49935 GoogleMaps .

Other material. One adult male and one adult female with same sampling data as for the holotype, mounted for SEM GoogleMaps ; two adult females, collected from brown clay in the Sea of Japan (44 ◦ 56.6397′ N, 137 ◦ 12.1063’ E) at 515 m depth in August 13 in the same cruise, also mounted for SEM. This material is also deposited at the Museum of NSCMB IMB GoogleMaps FER RAS ( MIMB 50052 View Materials ) .

3.1.4. Description

The body in adults is stout, triangular in cross-section, with very long midterminal spine (about 95 % of trunk length in average). Primary scalids consist of a basal sheath and a long, flexible distal portion with a blunt tip, without external pseudosegmentation and any internal septa ( Fig. 5A–B View Fig ). Number and arrangement of scalids, at least in the well visible dorsal and ventral sectors ( Fig. 5A–C View Fig ), seem to be congruent with those in Sphenoderes aspidochelone Sørensen et Landers, 2018 and Sphenoderes neptunus Sørensen et Herranz, 2022 . Nevertheless, complete information about the introvert morphology could not be obtained from the only specimen with not completely everted head. The neck has 16 placids; midventral and middorsal ones, located in deep midventral and middorsal incisions of first trunk segment, are largest and conical or nearly triangular in shape ( Figs. 2 View Fig and 3 A, C View Fig ; 5C View Fig ). Paraventral and paradorsal placids are also elongated ( Fig. 2 View Fig ). Trunk segment 1 consists of a complete cuticular ring, but with intracuticular fissures, thus giving impression of trapesoid midventral plate, which is also externally marked by short grooves/splits at its anterior portion ( Fig. 3A View Fig ; 11B View Fig ). Trunk segments 2–11 consist of a single tergal plate and two sternal plates; tergosternal junctions are marked by intracuticular fissures only (visible with DIC), without any external cuticular split.

Sexual dimorphism is expressed in the presence of ventromedial female papillae on trunk segment 9 and large round gonopores on trunk segment 11 in female; in the presence of gantlet-like appendages on trunk segment 11 in male ( Fig. 3B View Fig ; 4 View Fig ; 11 View Fig C-D).

Trunk segment 1 is clamshell-like in frontal view, consists of a complete cuticular ring with deep and wide middorsal and midventral incisions, with two paraventral fissures thus giving appearance of incomplete midsternal plate ( Figs. 2 View Fig , 3A View Fig and 9A View Fig , 10 View Fig A-B). Middorsal acicular spine is stout, with conspicuous longitudinal row of minute cuticular hairs on the exterior surface. Four pairs of sensory spots are present in paradorsal, subdorsal, laterodorsal and ventromedial position ( Fig. 2 View Fig ; 3A, C View Fig ; 9A View Fig ; 10B View Fig ). All sensory spots on trunk segments 1–10 and ventromedial sensory spots on trunk segment 11 are rounded in shape and belong to type 2 (sensu Nebelsick, 1992). Cuticle is completely smooth, free flap with distinct longitudinal lines ( Fig. 9A View Fig ). Long and wide midventral cuticular apron covers midventral area of following segment ( Fig. 10B View Fig ). Posterior segment margin with middorsal notch posterior to middorsal spine ( Fig. 2B View Fig ).

Trunk segment 2 with middorsal acicular spine. Sensory spots are present in paradorsal, subdorsal, laterodorsal and ventromedial position ( Figs. 2 View Fig , 9A View Fig and 11A View Fig ). A pair of glandular cell outlets in paradorsal position ( Fig. 2B View Fig ). Secondary fringe is present on the anterior half of the segment. The segment is covered with minute scale-like hairs between anterior margin and the free flap. The posterior margin with the same substructure forming longitudinal lines as on preceding segment and with middorsal notch ( Fig. 2B View Fig and 11A View Fig ).

Trunk segment 3 with middorsal and lateroventral acicular spines ( Figs. 2 View Fig and 9A View Fig ). Basements of lateroventral spines split longitudinally, with hirsute distal portions. Sensory spots are present in paradorsal, laterodorsal, midlateral and ventromedial position ( Figs. 2 View Fig , 9A View Fig and 10C View Fig , 11 View Fig A-B). Posterior segment margin with notches posterior to middorsal and lateroventral spine attachments ( Fig. 10B–C View Fig , 11B View Fig , 12). Other characters as on preceding segment.

Trunk segment 4 similar to preceding segment, except for the absence of paradorsal outlets of glandular cells.

Trunk segment 5 with middorsal and lateroventral acicular spines; with a pair of cuspidate spines also in lateroventral position, but slightly more ventral than acicular ones ( Figs. 2 View Fig , 3A View Fig and 5C View Fig , 9A View Fig ). Sensory spots are present in paradorsal, laterodorsal and ventromedial position ( Figs. 2 View Fig , 3A View Fig and 9A View Fig , 10C View Fig ). A pair of glandular cell outlets in ventromedial position, shifted to the anterior border of sternal plates ( Fig. 2 View Fig ). Secondary fringe, cuticle hairs, and shape of posterior segment margin as on trunk segment 2.

Trunk segment 6 similar to trunk segment 3 except for the glandular cell outlets absent in paradorsal but present in ventromedial position.

Trunk segment 7 similar to preceding segment.

Trunk segment 8 with paradorsal, laterodorsal and midlateral sensory spots on the tergal plate ( Figs. 2 View Fig , 9B View Fig and 10 View Fig C-D). Additional pair of sensory spots are present in ventromedial position in female and in ventrolateral position in male ( Fig. 2A–D View Fig ). Other characters as on preceding segment.

Trunk segment 9 with a pair of large papillae in ventromedial position in female ( Fig. 2A View Fig and 3B View Fig ). Other characters as on preceding segment.

Trunk segment 10 with middorsal and laterodorsal/midlateral spines ( Fig. 2B–C View Fig , 3E View Fig , 4 View Fig , 8 View Fig , 9B View Fig , 11C View Fig ). Spines in females are acicular, but more slender than those on preceding segment ( Fig. 8 View Fig ). Spines in males are not acicular, crenulated and flexible ( Fig. 9B View Fig and 11A View Fig ). Both sexes are without lateroventral acicular spines and corresponding notches on the posteror margin. Sensory spots are present in paradorsal, subdorsal, laterodorsal, midlateral/sublateral and ventrolateral position ( Figs. 2 View Fig , 3B View Fig and 9B View Fig , 11C View Fig ).

Trunk segment 11 with middorsal, midterminal, laterodorsal and lateral terminal spines ( Fig. 2 View Fig ). One pair of conical-shaped extensions with terminal sensory spots type 3 is present in paradorsal position (flanking the midterminal spine) ( Fig. 5E View Fig , 9B View Fig and 11C View Fig ). Two additional pairs of sensory spots type 3 are present in subdorsal and ventrolateral position. These sensory spots are associated with very large hypodermal glands ( Fig. 5E View Fig ). A pair of sensory spots is also present in ventromedial position ( Fig. 2A–D View Fig ). A pair of glandular cell outlets is present in ventromedial position, shifted to the anterior border of sternal plates ( Fig. 2A–D View Fig ). Large paired gonopores are located in ventrolateral position in females ( Fig. 3B View Fig ). Males with a pair of gantlet-like appendages (presumably, copulatory structures) in laterodorsal/midlateral position ( Fig. 11C–D View Fig ). Tergal plate bears prominent pectinate fringe with dagger-like cuticular tips, more prominent in males; also with secondary fringe ( Fig. 11C–D View Fig ). Free flap is present only on sternal plates.

Both, holotype and paratype adult females, possess well sclerotized cuticle and well developed cuspidate spines, thus being tentatively identified as stage-2 of adults (sensu Sørensen et Herranz, 2022).

Juvenile stage (TL 371 μm), collected at the type locality, is also assigned to the new species ( Fig. 6 View Fig ). This stage is characterized by a clamshell-like closing apparatus with enlarged middorsal and midventral placids, and by nearly the same set of spines as it was noted for the adults of S. japonicus sp. nov. Midventral articulations are still underdeveloped. All spines seem to be acicular. Dorsal spines are elongated, dagger-like, with slightly alternating arrangement. Lateroventral spines are relatively short and stout, including additional pair on trunk segment 5.

Young specimen (TL 423 μm), also collected at the type locality, has well distinguished closing apparatus, including distinct intracuticular fissures of midventral plate, with elongated midventral and middorsal placids ( Fig. 7 View Fig ). Set of spines is nearly congruent to this of adult stages. All spines are acicular, including additional lateroventral spines on trunk segment 5. Dorsal spines did not show any alternating in their arrangement. Cuticle seems to be very thin, parchment-like. Gonads are not visible. Probably, this stage could be assigned to stage 1 of adults (sensu Sørensen et Herranz, 2022).