Holophryxus citriformis Huys & Savchenko & Kosobokova 2023, SP. NOV.

HOLOPHRYXUS CITRIFORMIS SP. NOV.

(FIGS 1 A-E, 2–14)

Z o o b a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:act: 7F5CBC97-E358-4F5B-9B7B-43944023C4CE.

Representative DNA sequences: GenBank accession numbers OK489803 (COI) and OL323109 (18S).

Type locality: Central Arctic, 89°0.62’N, 61°26.15’E, sample from side net attached to multinet taken from 0–4300 m depth.

Type host: Hymenodora glacialis (Buchholz, 1874) (Pleocyemata, Caridea, Acanthephyridae) (Fig. 1F).

Type material: (1) Holotype: adult gravid female, preserved in formalin; pereopods 1–2 from one side mounted on slide; right mandible mounted on second slide; (2) paratype: adult male recovered from marsupial cavity of holotype, preserved in formalin. Collected on 6 September 2015. Type material deposited in Zoological Museum of Moscow State University under registration numbers Mc-1450 and Mc-1451, respectively.

Etymology: The specific epithet is derived from the Latin citrus, the name for a lemon [Citrus lemon (L.)], and forma, meaning shape, and alludes to the shape of the female.

Differential diagnosis: Holophryxus. Body length 13.0 mm (♀; N = 1), 2.75 mm (♂; N = 1). Colour (based on observation of live specimens) bright orange with darker red area dorsally (indicating position of ovary) in ♀; yellowish-amber in ♂.

FEMALE: Body citriform (lemon shaped), about 1.85 times as long as maximum width. Cephalon clearly demarcated from pereon in dorsal aspect by constrictions bilaterally and by weak transverse surface fold dorsally; dorsal prominence with slightly rounded anterior margin, not discernible in ventral aspect; ventral cephalic ridge well developed, distinctly crescentiform, embracing oral area, only small section of frontal part discernible in dorsal aspect, about 0.7 times as long as wide, anterior margin rounded, without paired pit organs; posterolateral corners of cephalic ridge with rounded lobes, representing coxal plates of incorporated first pereonite. Maxillipeds absent. Median sternal plate without paired posteriorly directed appendices. Pereon with five pairs of incubatory plates; oostegite 1 slightly bilobate, without medial projection on proximal lobe; oostegite 5 unilobate, covering 63% of ventral side of body, posterior margin with 11–12 minute, spinular extensions. Coxal plates of pereopods 2–4 expressed, represented by rounded processes on lateroventral margins of pereon, separated by transverse folds; coxal plate of pereopod 5 completely absent. Posterior part of pereon without ventrolateral papillae near posterior margin of fifth incubatory plate. Pleon clearly demarcated from pereon; about 13% of total body length; posterior margin rounded, without terminal processes.

MALE: Body elongate, about 3.3 times as long as maximum width. Cephalon fused dorsally to first pereonite but separated from it laterodorsally and laterally by functional articulation; posterolateral corners not produced into lobate extensions; about 1.5 times wider than long, slightly narrower than pereonite 1 and distinctly narrower than remaining pereonites; anterior margin weakly concave. Antennule trilobate; anterior surface of central lobe with numerous spinular combs and two branched sensory structures; outer lobe with spinous projection (probably homologous to flagellum) and spinular combs; inner lobe without ornamentation. Antenna 2-segmented; with broad basal peduncle and elongate, posteriorly directed flagellate part showing traces of segmentation and with eight setiform elements; extending beyond posterior margin of first pereonite. Pereonites 2–7 completely separated; ventrolateral margins forming rounded pleurotergites. Pleon not fused to pereonite 7, forming broad pyriform cone with maximum width measured in anterior third and approaching width of pereonite 7; representing 35% of body length; posterior margin rounded; anal region located ventrally, with paired raised pores, branched sensory elements and numerous spinular combs.

Description of female: Body (Figs 1A, B, 3A) comprising cephalon, pereon and pleon but without functional articulations between tagmata; broadly oval, symmetrical and lemon shaped (citriform) in outline; tapering abruptly at both anterior and posterior ends with cephalon and pleon offset by distinct bilateral constrictions; pleon with strongly convex lateral margins; dorsal curvature markedly convex (Fig. 1C); ventral surface swollen medially at level of fifth incubatory plates (Fig. 1A). Total body length 13.0 mm; maximum width 7.0 mm, measured at around halfway body length; maximum height 5.9 mm. Colour prior to fixation bright orange with darker red area dorsally indicating branching ovary (Fig. 1B, E).

Cephalon (Figs 2A, 3A, 4A) bell-shaped in ventral aspect; clearly demarcated from pereon in dorsal aspect by lateral constrictions and weak transverse surface fold dorsally. Dorsal prominence broad and short, with slightly rounded anterior margin; not discernible in ventral aspect. Ventral cephalic ridge well developed, about 0.7 times as long as wide, 1.6 mm long; distinctly crescentiform, embracing oral area; anterior margin rounded, without paired pit organs; only frontal rim discernible in dorsal aspect; posterolateral corners of cephalic ridge with rounded lobes, representing incorporated coxal plates of first pair of pereopods. Cephalic ridge forming continuous sucking cup with remaining coxal plates of pereopods 2–4 (Figs 2A, 4A).

Antennule (Figs 2 A-C, 3A-C, 4A) unsegmented, flattened, thin-walled; largely overlying labrum, labium and anterior half of antenna; distal half forming large, medially directed lobe; anterior surface with dense pattern of posteriorly directed spinular combs (Fig. 3B). Medial margins of antennules forming diamond-shaped gap (superimposed on oral opening and protruding mandibles) (Fig. 2B, C).

Antennae (Figs 2A, B, 3C, 4A) unsegmented, flattened, thin-walled and bilobate; outer lobe small, inner lobe large with posterior part fitting into anterolateral elliptical concavity of median sternal process (Fig. 3C); without surface ornamentation.

Oral cone (Fig. 3C) formed by wide, bilobate, unarmed labrum (with medially concave distal margin) anteriorly, and large labium posteriorly. Labium without ornamentation and largely concealed beneath antennules and antennae; free distal margin with pair of flimsy lobate extensions surrounding small oral opening and overlying distal margin of labrum and gnathobases of mandibles (Fig. 3D).

Mandible (Figs 2 A-C, 3C-E, 4A) with broad base, forming large round protrusion medially (acting as attachment for mandibular muscles) and spinous outer extension (acting as condyle in articulation with cephalic endoskeleton); tapering distally into elongate gnathobase with concave apical margin; anterior part of apex forming grinding process covered with numerous close-set denticles. Grinding processes protruding through small oral opening (being the only external part of mandibles that is discernible in situ; cf. Fig. 2C).

Posteroventral surface of cephalon forming median, longitudinal, sternal process with enlarged anterior end receiving inner lobes of antennae in paired concavities (Figs 3C, 4A); without paired posteriorly directed appendices. Maxillipeds absent.

Pereon with 4–5 dorsal folds, some of which V-shaped (Fig. 1B, C). Five pairs of prehensile pereopods of similar morphology located lateroventrally in anterior 15% of pereon (Figs 2A, 3A, 4A). Coxal plates of pereopods 2–4 expressed, represented by rounded processes on lateroventral margins of pereon, separated by transverse folds (Figs 2A, 4A); coxal plate of pereopod 5 completely absent (Figs 2A, 4A).

Pereopods 5-segmented, consisting of basis, ischiomerus (fused ischium and merus), carpus, propodus and dactylus (Figs 2E, 5A); basis unarmed, without noticeable ornamentation and seemingly fused at the base to lateroventral wall of pereon; ischiomerus with large membranous extension distally, overlying proximal half of carpus in anterior aspect (Fig. 5A), with numerous spinular combs on anterior surface (Fig. 5B); carpus with five sensory elements of various shapes along medial (palmar) margin (Fig. 5A, C, D), both anterior and posterior surfaces with multiple spinular combs and one raised conical pore; propodus distinctly swollen, with strongly convex outer margin (Fig. 5A), with four sensory elements of various shapes along medial margin, two of which close-set (Fig. 5D); dactylus small, recurved and claw like (Fig. 5A), medial margin bearing two stubby elements near articulation with propodus and one short and one longer element at base of unguiform, apical process (Fig. 5E), posterior surface with one setiform element.

Pereon with five pairs of incubatory plates (oostegites = IP1–5) arising from basal parts of pereopods (i.e. from the incorporated ancestral coxae); only first and fifth pairs visible, covering other three pairs (Fig. 4A). Oostegite 1 (IP1) slightly bilobate, without medial projection on proximal lobe; covering IP2 and anterior half of IP3. Oostegites 2–5 unilobate with IP3 completely covering IP4; IP2–4 gradually decreasing in size. Oostegite 5 largest, extending to posterior end of pereon covering 63% of ventral side of body and forming largest part of marsupial cavity (Fig. 3A); right and left plates overlapping medially, each with 11–12 minute spinular projections around semicircular posterior margin (Figs 2D, 4B). Posterior part of pereon without ventrolateral papillae near posterior margin of fifth incubatory plate. Marsupial cavity containing c. 15 000 eggs.

Pleon broadly conical, clearly demarcated from pereon by constrictions all around, in particular bilaterally (Figs 1 A-C, 3A, 4B); approximately 13% of total body length and about 0.6 times as long (1.9 mm) as maximum width (3.1 mm); posterior margin rounded, without terminal processes; lacking segmentation and appendages; with small, triangular anal opening subterminally on ventral surface (Figs 2D, 4B).

Description of male: Body (Figs 6A, 7A, 8) elongate, about 3.3 times as long as maximum width, slightly flattened dorsoventrally; incompletely subdivided in three tagmata (cephalon, pereon, pleon). Total body length 2.75 mm; maximum width (1.0 mm) measured at level of pereonite 6.

Cephalon (Figs 6A, 7A, 8A) subrectangular, fused dorsally to first pereonite but separated from it laterodorsally and laterally by functional articulation; posterolateral corners not produced into lobate extensions; about 1.5 times wider than long (length about 390 µm, maximum width 610 µm), slightly narrower than pereonite 1 and distinctly narrower than remaining pereonites; anterior margin concave and provided with multiple raised conical pores (spherules) (Figs 6B, 9A) and midventral crenulated tubercle; spherules also present along lateral margins and dorsal posterolateral corners of cephalon (Figs 7A, 8A; not illustrated in Fig. 8B); three pairs of sensilla discernible on dorsal surface (Fig. 7A). Eyes absent.

Antennule (Figs 6B, C, 10A) unsegmented, flat, trilobate; central lobe largest, anterior surface with numerous spinular combs (but pattern less dense than in female) and two branched sensory elements (indicated by arrows in Fig. 10A, detailed in Fig. 10B, C), proximal one of which accompanied by small pore (Fig. 10B); outer lobe with spinous projection (probably homologous to flagellum) and few spinular combs; inner lobe without ornamentation.

Antenna (Fig. 10D) 2-segmented, consisting of broad basal peduncle and elongate, posteriorly directed flagellate part showing traces of segmentation and tapering towards pointed apex; extending beyond posterior margin of first pereonite. Outer margin of peduncle produced into lobate expansion; anterior surface with several minute spinular rows. Segment 2 with transverse surface folds separating proximal quarter from remainder of segment; proximal quarter with flaskshaped element near outer margin, distal part with six setiform elements arranged in pairs (one dorsal, one ventral), and minute, ventral, spiniform element subapically (indicated by arrow in insert of Fig. 10D).

Oral cone (Figs 6B, C, 9B) formed by anterior labrum and posterior labium. Labrum well developed, without ornamentation; produced into thin-walled distal lobe with concave free margin. Labium with deeply concave free margin (Fig. 6C) and two pairs of secretory pores on anterior surface (Fig. 9B).

Mandible (Fig. 9B) with robust, proximal part tapering to slender, ventrally directed, distal part with terminal gnathobase. Gnathobases protruding through small oral opening (Fig. 6B, C) and bearing numerous spiniform denticles (but not as numerous and densely packed as in female) (Fig. 9C).

Pereon 7-segmented (Figs 7A, 8B); pereonite 1 fused dorsally to cephalon, pereonites 2–7 completely separated; ventrolateral margins inflated, forming rounded pleurotergites (Figs 6A, 7A, 8). Pereonite 1 only slightly wider than maximum width of cephalon, much narrower than remaining pereonites; pleurotergites not discernible in dorsal aspect. Pereonites 2–5 and 7 about equally wide, pereonite 6 widest. All pleurotergites with characteristic ornamentation elements on outer surface (Figs 7 B-E, 9D, F, H): pleurotergites 1–3 with two raised pores and one bipartite sensillate tubercle (indicated by arrow in Fig. 9D, F, H) with pore exiting on apical part (Fig. 9E, G, I); pleurotergites 4–6 with similar arrangement but with additional pore (on 4–5) and (on 6) or bipartite tubercle (6) (Fig. 7 B-D); pleurotergite 7 with single bipartite tubercle (Fig. 7E). Pleurotergites 1–5 with multiple spinular combs anteriorly (Figs 7B, C, 9D, F, H). Pereonites 2–7 with paired sensillate tubercle ventrolaterally (Fig. 8B), 1–5 with pair of dorsal sensilla (Fig. 7A).

Pereopods 1–7 prehensile (Figs 6A, D, 8A, 11–13, 14 A-C) and similar in structure; 5-segmented, comprising basis, ischiomerus (fused ischium and merus), carpus, propodus and dactylus; pereopod 1 the most robust and compact, pereopods 2–7 slightly increasing in length. Basis longest, without surface ornamentation; outer distal margin with large membranous area allowing for extensive movement around the basisischiomerus joint. Ischiomerus directed at almost right angle to basis; shorter than basis and without ornamentation. Carpus a short triangular segment (e.g. Fig. 11A, C, F); with four sensory elements and one pore on the palmar (inner) surface (labelled a–e in Figs 11C, F, 12C, 14C): two bifid or apically serrated elements (a, b), two cone-like projections with apical pore (c, d), and one (occasionally raised) pore (e) located in between both pairs of sensory structures. Propodus (Fig. 11A, D) distinctly swollen and with strongly convex outer margin in pereopods 1–6, much longer, slenderer and with weakly convex margin in pereopod 7; palmar side typically with three (often bifid or trifid) spiniform elements on anterior surface (e.g. Fig. 11B) and one larger, distinctly bifurcate element posteriorly near articulation with dactylus. Dactylus partly withdrawn into terminal socket of propodus; drawn out into pointed terminal claw (Fig. 11B, E); in pereopods 1–6 with two slender subterminal setae, located either side of claw, two spiniform elements along inner margin and one typically bifurcate spine on anterior surface (Figs 11B, E, 12B, E, 13B, D); in pereopod 7 with only three elements (Fig. 14B); with anterior and posterior condyle near articulation with propodus (Figs 11B, 12B, E, 13A, D, 14B).

Pleon (Figs 6A, 7A, 8) not fused to pereonite 7, demarcated laterally and dorsally by functional articulation and membranous inserts; forming broad pyriform cone, tapering towards posterior margin in both dorsal and lateral aspects; about 0.96 mm long, with maximum width (0.80 mm) measured in anterior third and approaching width of pereonite 7; representing 35% of body length; posterior margin rounded; dorsal curvature slightly arched, ventral surface flat. Lateral ornamentation comprising five pairs of sensory spherules located laterally along pleon at subequal distance from each other (Figs 7A, 8B); each spherule consisting of a globular base bearing a sensillum with trifid tip (Fig. 14D); last pair of spherules accompanied by raised conical pore (Fig. 14D). Five pairs of serially arranged globular structures of unknown origin and function (glandular?), each about 80–90 μm in diameter, discernible internally in lateral areas of pleon (Figs 7A, 8). Ventral surface of caudal region with irregular pattern of transverse, lateroventral surface striations; ornamentation consisting of numerous spinular combs arranged around and anterior to subterminal anal slit, one pair of raised conical pores, and (presumably) two pairs of branched (bifid or trifid) sensory elements (Fig. 14D); presence of three sensory elements (proximal two atypically stubby and not branched) on right side probably aberrant.

Remarks: The female holotype was found attached to the dorsal surface of the carapace of the shrimp Hym. glacialis with the cephalon directed posteriorly towards the pleon of its host (Fig. 1E). After it became detached in the laboratory, round marks were found on the carapace, conceivably from the mandibles and pereopods of the isopod (Fig. 1F, G). The brown marks were surrounded by a bright red area which retained its colour after fixation (Fig. 1G).