Basoniscus hikurangi, Wilson, 2024

Wilson, George D. F., 2024, A new family of Asellota (Crustacea, Isopoda) from the deep sea of Zealandia, European Journal of Taxonomy 917 (1), pp. 122-151 : 134-147

Basoniscus hikurangi gen. et sp. nov.

urn:lsid:zoobank.org:act:4BE8DF59-B79A-4381-A26B-DEE543C35BC1

Figs 5–12

Etymology

The species name ʻhikurangiʼ is taken from the sacred Mountain Hikurangi given to it by the Māori, the indigenous people of New Zealand. Several offshore undersea features, a subduction zone and the plateau on which the Palmer Seamount resides where this species was collected also bear the same name. The name is treated as noun in apposition to avoid changing the name.

Holotype

NEW ZEALAND • ♂, 3.1 mm; Hikurangi Plateau , Palmer ridge tip, west side of small twin cone south of Palmer Seamount; 39°32.39′–32.291′ S, 178°30.747′–30.291′ W; depth 3183– 2974 m; 21 Dec. 2002; B. Neuhaus, B. Berning, J. Hoffmann and C. Lüter leg.; from 0.5 m manganese boulder with corroded cores of volcanic breccia; R/V Sonne cruise 168, dredge station DR23; NIWA162662 .

Paratypes

NEW ZEALAND • 1 ♀, 3.4 mm, dissected, (slides No. 5131–34); same collection data as for holotype; NIWA162663 • 2 ♂♂, 3.3 mm and 3.1 mm; same collection data as for holotype; ZMB 34579 .

Description

BODY ( Figs 5 , 6A , 8A, C ). Broad, length 1.7–1.8 (F, H) width, wide, depth 0.39 (F) width; body smoothly arched dorsomedially with thin flat cuticular margin on all body segments ( Fig. 5 ). Cuticle color white, well calcified. Margins with evenly spaced short straight sharp setae, setal length less than width of thin cuticular margin, and scattered longer simple setae; each seta with cuticular channel from body to setal base (similar but larger channels seen in Mastigoniscus pistus , described in Lincoln (1985: fig. 21j). Head and pleotelson with pairs, rarely 3, of robust squat distally sensillate setae curved in opposing directions, both male and female with 3 groups on each lateral margin of head and 4 groups on each lateral margin of pleotelson, posterolateral corner of pereionites 6–7 with single squat sensillate seta. Margins without marginal denticles or spines.

HEAD ( Figs 5A , 7A , 8B–C ). Length 0.41–0.42 (H, F) width. Vertex and frons in dorsal view not projecting anteriorly, without pseudorostrum or rostral projections. Clypeus projecting anterodorsally from mandibular articulations, anteriorly flattened and dorsally acute and triangular between antennulae in anterior view. Labrum narrower than clypeus, medially shorter than clypeus in anterior view.Anterolateral antennal processes absent. Eyes absent, eye region without structures. Head margins widening posteriorly, aligned with anterolateral margins of pereion, width subequal to pereionite 1 anterolateral margin. Head articulation with pereionite 1 fully expressed dorsally and ventrally. Head depth subsimilar to anterior pereion depth. Antennula and antenna with recessed insertions under anterior margin of head, sides separated with gap for clypeus.

PEREION ( Figs 5E , 6A , 8A, C ). Pereiopodal insertions midway between lateral margin and midline of pereion. Pereion somites free, articulations fully expressed. Pereion without dorsal midline spines. Tergites projecting laterally on all pereionites, extending over coxae. Coxae placed on medioventral surface, not projecting laterally from pereionite, without spines or setation. Pereionites 1–7 not substantially differentiated, mainly eXpressed by orientation and position of legs on pereion; dorsum and sternite articulations fully expressed. Sternites without sternal keels, without ventral midline spines or anterolateral robust setae on margin.

PEREIONITE 1. Lateral length in male similar to female. Pereionites 2–7 tergites projecting, lateral margins confluent, serially homologous, without integrated subgroups. Pereionite 4 length subequal to pereionite 3, without collum (apomorphy of Macrostylidae Hansen, 1916 ). Pereionite 5 distinctly longer than pereionites 3–4, longer in female than in male, length ratios 0.73–0.83, 0.64–0.83 (H, F), respectively. Pereionite 7 width slightly narrower than pereionite 6 width, ratio 0.93–0.95 (H, F); lateral margins not enclosed by preceding segment.

FEMALE GENITALIA ( Fig. 8C–E ). Cuticular organ (spermathecal duct) separate from oviduct, emerging anterodorsally under articular margin of pereionite 5, roughly halfway between midline and tergite margin; dorsal orifice opening to shallow funnel. Oopore ventral, adjacent medially to coXa V. Both ducts meeting in a vestibule (spermatheca) on posterior part of ovary in pereionite 4.

PENES ( Fig. 7A ). Positioned medially, emerging internally, covered by posteromedial margin of pereionite 7 and anteromedial margin of pleopods I. Penes shorter than coxa VII diameter, width tapering medially, tip distally rounded.

PLEOTELSON ( Figs 5C , 6A , 7A , 8A, C ). Pleonites not expressed, merged into pleotelson. Pleotelson broad, length 0.42–0.53 (F, H) width, much broader than depth, deepest anteriorly, smoothly sloping to posterior margin; anterior margin adjacent and confluent with to pereionite 7 in frontal plane with pereion. Dorsal surface with paired sensory organs presenting as 2 epicuticular penicillate setae (see Riehl et al. 2014: 252), placed midway between anterior and posterior margin, each approXimately ⅓ pleotelson width from lateral margin; dorsal surface sparsely setose, without spines. Posterior margin apex without spine-like projections, with shallow concavities behind positions of ventral uropods. Posterolateral margin without spines or denticles. Ventrolateral surface without ridges or setal rows. Ventral surface without preanal ridge; pleopodal cavity closed posteriorly, completely enclosed by opercular pleopods. Anus ventral, subterminal, outside pleopodal cavity, covered by uropodal protopods, with postanal margin projecting to broadly rounded apex between shallow concavities.

ANTENNULA ( Fig. 6B , 9A ). Length less than head width female shorter than male, ratios 0.48–0.65 (F, H), shorter than antenna length, ratios 0.42–0.55 (H, F). Positioned anteriorly on head; basal articles orientated anteriorly. Antennula with 7 articles in female, 9 in male (H). First article wider than second article, length:width ratio 1.8–2.0 (F, H), flattened in cross-section, broader basally. Article 4 shorter than article 3, length ratio 0.5, shorter than article 5, length ratio 0.83 (H). Flagellum in adult male not hypertrophied but with more articles than in female, 4 articles after article 5 in male (H), 2 in female, with only one aesthetasc per article, aesthetasc length much longer than flagellar segment. Terminal article cylindrical and distally rounded, shorter than penultimate article.

ANTENNA ( Fig. 6C ). Directed anteriorly; podomere axis with only moderate bends at articles 3–5, distal podomeres not folding against proXimal podomeres; all articles cylindrical, straight, with fleXible obtuse bend between articles 5 and 6. Article 1 fully articulated. Article 2 squat, wider than long. Article 3 longer and wider than articles 1–2 or 4, widest midlength, margins rounded, length less than twice width, not distally embedded in article 4; exopod cone-like, distally rounded, basally articulated, not scale-like or spinose, length greater than width. Articles 4–5 approximately linear with article 3. Article 5 not distally geniculate, rounded in cross section, shorter than combined length of articles 1–4, shorter than article 6. Article 6 longer than articles 1–4 together. Flagellum length subequal to (H) or shorter than (F) podomeres length, with 20–23 articles in male (H), not hypertrophied, 7 articles in female; flagellum basal article length longer than but less than twice neXt article length; flagellum basal article derived from single article (not conjoint), not inflated relative to distal articles.

MANDIBLE ( Fig. 9A–G ). Molar process subcylindrical, approximately parallel sided to distal margin; gnathal surface ridged and concave, with 2 rows of robust sharp spines on projecting margin, posterior row with more elongate spines and subdistal fine spines. Incisor process multidentate, with 5 cusps, approXimately in transverse linear row. Left lacinia mobilis differentiated from other members of spine row, with 4 cusps. Right lacinia mobilis indistinguishable from spine row. Spine rows well-developed, with 3–2 single shaft (not bifurcate) distally dentate spines and 3–2 (right, left) distally spinulate spines, spines as long as or longer than incisor process, spine row basal length less than 50% molar–incisor distance. Palp subequal to mandible body length, insertion on lateral margin without lateral seta; article 1 without setae, shorter than article 2, length ratio 0.66; elongate article 2 with 2 subdistal bispinulate setae; article 3 laterally curved through approximate right angle, with dorsal margin with approximately 20 monospinulate setae increasing in length proximally to distally.

PARAGNATHS ( Figs 9H ). Broad, length 0.74 width, strongly curved medially, with reduced proximomedial lobes. Medial margin v-shaped, with many fine cuticular hairs. Mediodistal margins with medially projecting group of fine spinules. Distolateral margins with scattered cuticular hairs.

MAXILLULA ( Fig. 10A ). Lateral lobe with 11 robust setae of which 3 lateral setae lack denticles or spinules, 1 medial seta distally spinulate, and remaining setae distally denticulate; margin adjacent to medial lobe with row of fine setae. Medial lobe distal tip with single elongate distally spinulate seta and 3 small setae; medial margin with 3 elongate fine setae.

MAXILLA ( Fig. 10B ). Medial lobe distally with 6 setae in 2 overlapping layers; medial margin distally with one elongate monospinulate seta, centrally with 5 pairs of fine setae, proXimally with 6 elongate curved setae. Lateral lobes distally with 2 (inner), 3 (outer) long setae; inner lobe mediobasal margin with cluster of setae; outer lobe medial margin with 2 groups of setae, distal group longer than proximal group, lateral margin with elongate fine setae grading proXimally to fine setae. Lateral base of lobes with small group of cuticular hairs.

MAXILLIPED ( Figs 9A , 10C ). Coxa laterally narrowing, longest medially, extending beyond lateral margin of basis, without oöstegite. Endite length 0.39 basis total length; medial margin with 2 coupling hooks, with group of approximately 7 setae; distolateral margin with 4 bifurcate sinuate robust setae, one branch distally spinulate, sizes decreasing proximally; distal margin with distomedial concavity, 2 fan setae lateral to concavity, one narrow and tapering, one distally truncate and broad; distolateral margin with scattered fine setae and 3 closely-spaced thick tubular setae. Epipod well developed, narrow, length 3.2 width, epipod projecting beyond palp insertion, length 1.1 basis length, lateral margin with obtuse angle and shallow concave margin distally to narrow rounded apex. Palp with 5 free articles, width distinctly less than endite width, ratio 0.62; male articles 3–5 similar to female; article 1 length less than width, without distolateral process; article 2 width near to article 1 width, lateral length greater than width, ratio 1.5, lateral length subequal to medial length, without distomedial projection, width 2.2 article 4 width; article 3 without distomedial projection, medial length 1.2 width, 1.1 lateral length; article 4 length 2.9 width, without distomedial projection; article 5 length greater than width.

PEREIOPODS ( Figs 6D–E , 10D–F , 11 , 12A–B ). Pereiopods I–VII similar in length and shape, similar in male and female, without broadened podomeres, long setae or spination. Pereiopod I shortest; pereiopods IV–V shorter than pereiopods II–III and VI–VII, length:body length ratios, respectively: 0.45, 0.52, 0.54, 0.43, 0.49, 0.57, 0.58. Coxae lacking anterolateral robust setae; oöstegites on coxae I–IV only, developing internally, buds or ridges not present externally in preparatory female, until deployed at maturation. Basis lengths longer than ischium lengths, ratios 2.2 (I–V), 1.8 (VI–VII). Pereiopod ventral margins without robust setae, with fine setae, more frequent on carpus and propodus where setae may be elongate, straight sided and thin on pereiopods V–VII. Pereiopods II–VII dorsal margins of merus and carpus with groups of robust bifid setae, each merus with group of 3 each short bifid setae on distodorsal margin, each carpus with group of 5 bifid setae on distal third of dorsal margin, one distal branch of each seta finely spinulate, central seta being approximately twice as long as outer setae. Pereiopod I–VII dactylus with ventral claw, positioned distally on limb, shorter than dorsal claw; distal sensillae not enclosed by claws; medial sensillae thin, not projecting beyond claws; dactylus without third claw (accessory robust seta). Pereiopod I ventral dactylar claw basally slender and shorter than dorsal claw, length half dorsal claw length; pereiopods II–VII ventral claw length 0.8 dorsal claw length, basally wider and more robust than dorsal claw, basal width more than half dactylus distal width.

PEREIOPOD I. Projecting anteriorly and ventrally; length 0.44 body length (H, F), similarly robust as pereiopod II, shorter, length 0.80 pereiopod II length. Ischium dorsal margin weakly convex; merus dorsal length exceeding width and lacking distodorsal projection. Carpus and propodus with only ventral robust setae and subparallel margins, length 0.83 propodus length, elongate, dorsal and lateral margins subparallel, of subequal length; carpus ventral margin without expansion or projections; carpus and propodus not prehensile, not dimorphic, females and males similar. Propodus without distal elongate robust seta, ventral margin elongate, margins subparallel, ventral margin without comb of spinules; dactylus short, length (including claws) 0.09 pereiopod length.

PEREIOPOD II. Projecting anteriorly and ventrally, shorter or near body length; merus width near length; carpus and propodus without opposing margins, without multiple dense rows of setae, margins subparallel, near length of basal segments.

PEREIOPOD III. Projecting anteriorly and ventrally; carpus and propodus without opposing margins; ambulatory; carpo-propodal joint rotation absent; ischium dorsal margin with only simple setae; limb shorter or near body length, carpus and propodus near length of basal segments.

PEREIOPOD IV. Projecting anteriorly and ventrally, shorter than pereiopods II–III and VI–VII; carpus and propodus without opposing margins, in male similar to female, not prehensile; carpus longer than merus, length ratio 1.9, shorter than propodus, length ratio 0.9, width near thickness.

PEREIOPOD V–VII. Carpus and propodus elongate with subparallel margins, sparsely setose along margins.

PEREIOPOD VII. length subequal to pereiopod VI length; male similar to female, not sexually dimorphic; basis posterior margin paucisetose.

PLEOPODS ( Figs 7B–D , 12C–F ). Pleopod I of female absent. Female pleopod II opercular; male pleopods I–II opercular.

PLEOPOD I OF MALE. Length more than half pleon length, ratio 0.57. Medial margins fused, with medial sperm tube; broadest proximally, narrowest midlength, widening distally, with dorsolateral stylet guides, orientation transverse to body axis, opening subdistally and laterally. Medial and lateral lobes in single plane. Lateral lobes reduced, set proximally from distal margin, expressed as small lobe, distance 0.93 pleopod length from proximal margin. Medial lobes medially truncate with small distolaterally projecting horn, distally acutely pointed, with 3 medially placed stiff setae and several shorter setae on distolateral margin.

PLEOPOD II OF FEMALE. Medially fused into operculum without endopods or exopods, ovoid, without rounded ridge or keel, without ventral spine-like projections; completely enclosing pleopodal cavity, not covering anus, length 0.86 width, 0.6 pleotelson length; lateral and distal margins with thin flange, with evenly placed short straight sharp setae, setal length less than width of thin cuticular margin.

PLEOPOD II OF MALE. Protopod elongate and robust, longer than exopod; total length less than half pleon length. Endopod appendix masculina geniculate, articulation expressed, position on protopod margin medial; appendix masculina stylet shaped, with closed tube, opening only on distal tip, unidirectionally curved, short, length 0.74 protopod length, not projecting beyond protopod distal margin, proximal opening medial. EXopod medial position on protopod, single article, thick, not flattened; stout, length 0.6 width; without posterior appendage, with rounded hook on distal article.

PLEOPOD III. Length 0.38 pleotelson length. Endopod monoarticulate, distal margin with 3 plumose setae unevenly distributed, middle and lateral setae close together, medial seta subdistally. Exopod monoarticulate, distal tip not extending beyond endopod, length 0.7 endopod length, width 0.4 endopod width; distal margin acute, fine setae on medial and lateral margins, with one distal simple seta.

PLEOPOD IV. Biramous, length 0.44 pleotelson length. Exopod with only one segment, central articulation absent; narrow, width 0.4 length, with narrowing margins.

PLEOPOD V. Monoarticulate, uniramous, length 0.38 pleotelson length.

UROPOD ( Figs 7A, E , 12G ). Shorter than pleotelson, length ratio 0.31–0.34 (H, F); inserting on posteroventral surface, emerging from ventral margin, adjacent to anus, covering anus ventrally. Protopod external to articular socket, axis of rotation suppressed, length 0.52–0.53 (F, H) pleopod length, distal width 1.3 proXimal width, flattened, medial margin with lobe projecting over and covering anus. Rami biramous, positioned distally, exopod adjacent to endopod on distomedial margin. Exopod lateral to endopod, minute, length 0.03–0.05 (H, F) uropod length, proximal articulation expressed. Endopod ramus rod-like, distally rounded, proximal articulation free, length distinctly longer than protopod, ratio 1.1 (H, F).

Remarks

The ordinary isopodan appearance of Basoniscus hikurangi gen. et sp. nov. is unusual for janiroideans, which typically have many specializations of both limbs and body. No pereion segments of Basoniscus hikurangi are specialized compared to any other. Instead of the typical asellotan 4:3 tagmosis, this species has no obvious differentiation of the pereionites other than lateral curvature depending on their position on the body. The broad habitus with flanged lateral margins and short hidden uropods is remarkably like a fully terrestrial oniscidean isopod. If it had been collected on land in Australia or New Zealand, it might have been mistaken for a species of Armadillidae Brandt, 1831 .

The broad body with thin flat marginal flanges and minute curved robust setae plus robust pereiopodal claws, especially the ventral claw, suggest that these isopods may have the ability to cling closely in scale-like fashion to surfaces, such as the hard volcanic surfaces from which the specimens were collected. The middle two pereiopods (45.75% body length) are shorter than those anteriorly (pereiopods II–III, 53.1% body length) or posteriorly (pereiopods VI–VII, 57.4% body length). This may also be a feature of a scale-like existence, which would require that the legs are held under the broad tergites. Asellotans that have a plesiomorphic ambulatory habitus typically have limbs that increase in length posteriorly. Unusual squat and curved robust setae on head and pleotelson margins may also be part of the scale-like adaptation, in that they might assist in maintaining contact with a surface. These robust setae have obvious subdistal sensillae so sensing the surface may also be important.

This species has attenuated sexual dimorphism, which is unusual for many isopods where the male is variably distinct from the female. The only apparent differences of males from the female specimen appears to be a longer antenna, more articles in both antenna and antennula, and perhaps the female being slightly larger than the males. Only one female was collected but was slightly larger (3.4 mm) than the largest male (3.3 mm), one of the paratypes. The holotype male was clearly adult because it had fully developed testis ( Fig. 6A ) as well as a pleopod II appendix masculina with a cuticular duct that was open at the end of the stylet ( Fig. 7D ). The single female was also fully mature with well developed ovaries containing ovae. All four specimens were approximately the same size, supporting the assumption that they were all adult.

The opening for cuticular organ (spermathecal duct) is found dorsally as in the Joeropsididae (see Just 2001: fig. 1) and is similar to that of the Haploniscidae (see Wolff 1962: 227, duct to “receptaculum seminis”). In Basoniscus gen. nov. ( Fig. 8D ), Joeropsididae and other janiroideans, it is positioned under the anterior margin of pereionite 5, either dorsally or laterally, whereas in Haploniscidae , similar to other deep-sea janiroideans (e.g., Ischnomesidae Hansen, 1916 ; see Cunha & Wilson 2006: fig. 10), it has moved to an external location posterior to the anterodorsal margin of the pereionite. The janiroideans have a range of positions for the opening on pereionite 5, ranging from ventral for Munnidae G.O. Sars, 1897 and Santiidae Kussakin, 1988 to fully eXposed dorsally on Ischnomesidae . Whether this is an adaptation to deep-sea conditions (a parallelism) or an ancestral feature is not clear because the basal arrangements between multi-familial clades in the phylogeny (e.g., see unweighted or implied weighted jackknife trees in supplementary data) remain equivocal owing to low character support and absence of information on related taxa in other superfamilies.

Figures

Fig. 5. Basoniscus hikurangi gen. et sp. nov. A–C. Holotype ♂, NIWA162662. A. Habitus, dorsal oblique. B. Head, ventral view. C. Pleotelson, ventral view. D–E. Paratype ♂♂, ZMB34579. D. Both paratypes, dorsal (left) and lateral oblique (right) views. E. Left paratype, ventral view. Scale bars = 1 mm.

Fig. 6. Basoniscus hikurangi gen. et sp. nov., holotype ♂, NIWA162662. A. Dorsal habitus with enlargment of head right lateral margin, shaded regions indicate position of vas deferens. B. Antennula, dorsal view. C. Antenna, dorsal view. D. Pereiopod I, with enlargment of dactylus and propodus. E. Pereiopod II, with enlargements of setal groups on merus and carpus. Scale bars: A = 1 mm; B–C = 0.5 mm; D–E = 0.5 mm.

Fig. 7. Basoniscus hikurangi gen. et sp. nov., holotype ♂, NIWA162662. A. Pleotelson and posterior part of pereionite 7, ventral view. B–C. Pleopod I and enlargement, dorsal right side of distal lobes. D. Pleopod II, dorsal side. E. Uropod, right side. Scale bars: A = 0.5 mm; B–D = 0.1 mm; E = 0.1 mm.

Fig. 8. Basoniscus hikurangi gen. et sp. nov., paratype ♀, NIWA162663. A. Lateral habitus.B. Enlargement of head, lateral view. C. Dorsal habitus. D. Enlargement of area in box on C, showing internal reproductive organs in pereionites 3–5, ovary and cuticular organ. E. Enlargement of internal reproductive organs. Scale bar: A, C = 1 mm. Indications: per = pereionite.

Fig. 9. Basoniscus hikurangi gen. et sp. nov., paratype ♀, NIWA162663. A. Head, ventral view. B. Right mandible, dorsal view. C. Right incisor process and spine row, anterior view. D. Right molar process, anterior view.E. Left mandible, dorsal view. F. Incisor process and lacinia mobilis, anterior and posterior views. G. Palp. H. Paragnaths, ventral view. Scale bars: A = 0.5 mm; B = 0.1 mm; E–F = 0.1 mm; H = 0.1 mm. Indications: L = labrum; m = mandible; mxp = maxilliped.

Fig. 10. Basoniscus hikurangi gen. et sp. nov., paratype ♀, NIWA162663. A. Maxillula, ventral view. B. Maxilla, ventral view. C. Maxilliped with enlargement of endite distal tip. D. Pereiopod I, lateral view. E. Pereiopod I propodus and dactylus with enlargment of ventral margin seta. F. Pereiopod I dactylus. Scale bars: A–B = 0.1 mm; C = 0.1 mm; D = 0.2 mm.

Fig. 11. Basoniscus hikurangi gen. et sp. nov., paratype ♀, NIWA162663. A. Pereiopod II, left, with enlargements of setal groups on merus and carpus. B. Pereiopod II, right, dactylus and distal part of propodus showing articular plate.C. Pereiopod III, left. D. Pereiopod II, enlargements of propodus distal setal group. E. Pereiopod IV, left. F. Pereiopod V, left. Scale bars: A, C = 0.1 mm; E–F = 0.2 mm.

Fig. 12. Basoniscus hikurangi gen. et sp. nov., paratype ♀, NIWA162663. A. Pereiopod VI, right. B. Pereiopod VII, left, with enlargement of propodal marginal seta. C. Pleopod II, ventral view. D. Pleopod III, ventral view. E. Pleopod IV, ventral view. F. Pleopod V, ventral view. G. Uropod, left, ventral view. Scale bars: A–B = 0.2 mm; D–F = 0.2 mm; G = 0.2 mm.