Desmocornea subchelata, Wittmann, 2024

Wittmann, Karl J., 2024, The Mysidae (Crustacea, Mysida) of the ANDEEP I-III expeditions to the Antarctic deep sea with the description of twelve new species, establishment of four new genera and with world-wide keys to the species of Erythropinae and Mysidellinae, European Journal of Taxonomy 940, pp. 1-180 : 49-59

Desmocornea subchelata gen. et sp. nov.

urn:lsid:zoobank.org:act:3B26DF92-6C03-4CC6-A5FE-0097382D8D57

Figs 25–30

Diagnosis

Based on adult female. All features as in generic diagnosis. Eyes bilobate, differentiated in a bulbous, dorsoventrally flattened mesial lobe and a smaller, roughly calotte-shaped lateral lobe. Eyestalk fused only with mesial lobe. Reduced ommatidia of mesial lobe face centripetally; modified, in some way functional ommatidia of lateral lobe face laterally, tightly set in parallel orientation, together forming self-contained ribbon. One large tooth close to acute disto-lateral edge of antennal sympod. Antennal scale extends 0.6 times its length beyond antennular trunk and 0.3–0.4 times beyond antennal peduncle. Total scale length three times maximum width; its disto-lateral tooth mesially closely accompanied by two short spines. Thoracic exopod 1 with 9-segmented flagellum, exopods 2, 5–6 and 8 with 10-segmented flagellum (exopods 3–4 and 7 broken). Carpopropodus 2 weakly elongate, longer than merus; reflexed dactylus strongly elongate,> 4 times as long as wide, claw stout and short, ⅖ as long as dactylus. Endopod of uropods with one small slender spine close to mesial margin below statocyst. Telson trapeziform, length 1.7 times maximum width; distal half with continuously converging lateral margins. Proximal 45% of lateral margins without spines, distally remaining portion with about 13–14 normal-shaped spines increasing in length caudally. Transversely truncate terminal margin with very shallow median indentation lined with pair of setae flanked by pair of minute spines; indentation in turn flanked by two pairs of large latero-terminal spines, submedian spines largest, measuring about ¼ of telson length. Telson with total of ≈33 spines and two setae.

Etymology

The species name is a Latinized adjective with feminine ending, referring to the subchelate second thoracic endopod.

Material examined

Holotype SOUTHERN OCEAN • 1 ♀ ad. (BL = 18.5 mm, on slides); Bellingshausen Sea , NW of Anvers Island, ANDEEP-III station 153-7; 63°19.31ʹ S, 64°36.94ʹ W to 63°19.15ʹ S, 64°37.18ʹ W; depth 2092–2118 m; 29 Mar. 2005; EBS supranet; ZMH 64679 .

Paratypes SOUTHERN OCEAN • 1 ♂ imm. (most thoracopods and pleopods broken, BL ≈ 9.7 mm, on slides); Powell Basin, SW continental slope of South Orkney Islands, ANDEEP-III station 151-7; 61°45.52ʹ S, 47°07.68ʹ W to 61°45.42ʹ S, 47°08.04ʹ W; depth 1182–1185 m; 21 Mar. 2005; EBS supranet; ZMH 64681 • 1 juv. (damaged, BL = 5.3 mm); South Sandwich Trench , E of Montagu Island, ANDEEP-II station 140-8; 58°15.98ʹ S, 24°53.72ʹ W to 58°16.13ʹ S, 24°53.87ʹ W; depth 2947–2970 m; 22 Mar. 2002; EBS supranet; ZMH 64680 .

Type locality and distribution

The type locality is ANDEEP III station 153-7: Bellingshausen Sea, NW of Anvers Island, 63°19.31ʹ S, 64°36.94ʹ W to 63°19.15ʹ S, 64°37.18ʹ W, depth 2092–2118 m. Also recorded from South Sandwich Trench, E of Montagu Island and from the Powell Basin. Total ranges 58– 63° S, 25– 65° W, depth 1182–2970 m.

Description

Holotype (♀)

All features as in specific diagnosis. Adult female with empty marsupium, BL = 18.5 mm, completely dissected. Carapace measures 28% of BL, cephalothorax 34%, pleon without telson 54% and telson 12%. Clypeus with mid-ventral carina rostrally passing into distally rounded median projection reaching to median segment of antennular trunk.

CARAPACE ( Figs 25E , 28A ). With short, broadly rounded, uptilted anterior margin, rostrum indistinct (though distinct in juvenile). Uptilted portion of carapace as long as 9% of terminal segment of antennular trunk. Disto-lateral edges well-rounded; concave posterior margin leaving ultimate 1.5

thoracic segments dorsally exposed. Eight pores flanking a larger donut-like structure ( Fig. 25E ) in mid-sub-caudal position (arrows in Fig. 28A ) on carapace.

EYES ( Figs 25B , 26 ). Both eyes strictly symmetrical. Bilobate structure evident in loco ( Fig. 26A ), but lobate shape accidentally disappeared upon expansion on slides ( Figs 26B , 27C ). Lateral eye lobe with self-contained ribbon formed by parallel-oriented, laterally directed ommatidia ( Fig. 26B ) without rhabdom-like striation. Mesial eye lobe with few, throughout vestigial, mostly inconspicuous ommatidia ( Fig. 26B ) not reaching surface, no rhabdom detected. Antero-posterior extension of mesial eye lobe about equal to its maximum width and 0.8 times as long as terminal segment of antennular trunk; lobe dorsoventrally flattened by a factor of about 1.4 ( Fig. 25B ). Distally rounded subtriangular papilla positioned dorsally close to mesial margin of mesial eye lobe ( Fig. 26B ). Papilla measuring ⅕ of antero-posterior extension of mesial eye lobe. Papilla with terminal pore, no scales. Organ of Bellonci spherical, near papilla. Tooth-like scales, mostly 15–25 µm long, along anterior margin of mesial eye lobe (as in Fig. 27C ) and along low ridge (emphasized by long arrow in Fig. 26B–C ) associated with ocular papilla.

ANTENNULA ( Fig. 28B–C ). Trunk measures 9% of BL. It extends ⅔ of its length beyond eyes. Transverse articulation between three trunk segments. Measured along dorsal midline, basal segment with 0.4 of trunk length, median 0.2 and terminal 0.4. Length of basal segment 0.7 width in dorsal view. This segment mid-dorsally with antennular bursa, no ventral carina. Segment produced into short lobe at disto-lateral edge, lobe distally with four setae. Basal segment with two dorsal setose apophyses in paramedian position near rostral margin; median segment with one setose, approximately mid-dorsal apophysis also near rostral margin. Length of terminal segment 0.8 times width in dorsal view, 0.9 in ventral view; difference due to lengthwise weakly oblique segmental border between median and terminal segments. Terminal segment with disto-median lobe armed with four barbed setae and large subterminal tooth mesially accompanied by two minute teeth and laterally by one intermediate-sized tooth ( Fig. 28C ). Terminal segment without female lobe, no callynophore. Width of lateral antennular flagellum measured near basis 1.3–1.5 times width of mesial flagellum. Basal 6–7 segments of mesial flagellum ( Fig. 28B ) separated by transverse sutures, followed by consecutive clusters of oblique sutures with 1–3 transverse sutures in between; series of clusters demonstrable up to segment 20, more distal segments broken.

ANTENNA ( Fig. 28D ). Sympod 2-segmented, caudally in addition with large end sac of antennal gland. Segments 1–4 contribute 22%, 9%, 33% and 36% to total length of peduncle in dorsal view, vs 30%, 20%, 25% and 25% in ventral view, respectively. Difference between these views mainly reflects strongly oblique border between second segment and dorsally overlapping third segment. Two spine-like structures mesially accompanying large distal tooth of antennal scale show basal articulations ( Fig. 25G ), confirming that these structures are spines and not teeth.

MANDIBLES ( Fig. 28E–G ). Palp accidentally turned to show its rostral face, whereas the masticatory part shows its caudal face in Fig. 28E . Basal segment contributes 8–10%, median segment 61–62% and apical segment 29–30% to total palp length. Median segment 2.8–2.9 times as long as wide, mesial margin slightly sigmoid; lateral margin convex, bent mesially. Length of apical segment 3.1–3.4 times maximum width. Palp not hispid, its basal segment without setae, median segment strongly setose along mesial margin and less strongly along mesially bent lateral fold, only few setae on lateral margin, apical segment densely setose along mesial margin and with only few setae along lateral margin. Masticatory part of right mandible ( Fig. 28F ): pars incisiva with three large plus one medium-sized teeth; digitus mobilis with two large and eight small teeth, only largest tooth with two small humps on its concave face. Pars centralis with nine acute spines armed with a few stiff bristles, three distal spines basally thick and decreasing in length proximally, dorsally (= orally) followed by six more slender spines in dense series increasing in length proximally. Processus molaris with strong grinding lamellae distally ending in teeth, proximally with dense series of bristles. Masticatory part of left mandible ( Fig. 28G ): pars incisiva with three large teeth; left digitus mobilis more than twice size of right digitus, with five large and several small teeth. Pars centralis with ten slender spines armed with stiff bristles, spine length decreasing proximally; processus molaris with strong grinding lamellae not ending in teeth and with dense cover of stiff bristles on oral margin.

GUT ( Figs 29B–E , 30F ). Foregut similar to that of Amblyops arianii sp. nov. Both species particularly showing an unusually great diversity of (in detail differing) spines on anterior part of lateralia, in D. subchelata gen. et sp. nov. with various transitions from apically coronate ( Fig. 29B 1 ) to apically microserrated (cactus-like) spines ( Fig. 29B 2–B 3 ). These spines smooth along their basal to subapical portions. Central part of lateralia with slender, apically pronged, short spines ( Fig. 29C ), sparsely coated with minute teeth along about distal half of shaft. Posterior part of lateralia on each side with dense cluster of seven spines with dentation (unilateral serration) increasing with spine size ( Fig. 29D ). Dorsolateral infoldings on each side with cluster of four spines increasing in length laterally; these spines unilaterally dentate, three lateral spines with only few denticles; thicker, most mesial spine serrated by many more denticles ( Fig. 29E ). Setae of superomedianum as in A. arianii sp. nov. Storage volume of foregut almost empty in the two dissected specimens. Sparse presence of small unidentifiable organic particles together with two small fragments (0.2–0.3 mm) of euphausiacean larvae. Anal lobe distinct, weakly cuticularized (dashed line in Fig. 30F ).

MAXILLULA ( Fig. 28H ). Distal segment with eleven strong spines on transverse terminal margin, these spines unilaterally serrated mostly along their subapical portions. This segment subterminally with three setae bearing long stiff barbs; no nearby pores detected. Endite of maxillula terminally with three large, distally spiny setae accompanied by several less strong barbed setae. Endite more proximally with five smooth setae. Most proximal seta backward curved. Proximal segment with low ridge bilaterally furnished with long, dense series of fine hairs (again denser than visualized in Fig. 28H ), in contrast to most mysids, which exhibit only unilateral series.

MAXILLA ( Fig. 28I ). Sympod with three mesial lobes, densely setose along distal margin. Field of small triangular scales on sympod sub-proximally from median lobe. Mesial circumference of sympod with large fan of setae; thickest seta distally with stiff barbs; this seta, when outstretched, only marginally extending beyond fan of setae (unlike in most mysids). Exopod of maxilla closely approaches but does not reach terminal margin of basal palp segment. Exopod with dense series of plumose setae all along lateral margin; largest seta at tip (dashed line in Fig. 28I ). Mesial margin without setae. Exopod bare not considering setae. Maxillary palp with distal segment contributing 4/7 of palp length. This segment three times as long as maximum width, densely setose along distal and most of mesial margin, only few setae on lateral margin, no spines. Mesial margin of proximal segment with three normal-shaped barbed setae (on rostral face, therefore visualized by dashed lines in Fig. 28I ).

THORAX ( Fig. 29G–J ). Sternite 1 with distally rounded median lobe as in most mysids. One or two large, basally thick barbed seta closely accompanied by 0–4 smaller such setae on intersegmental joint between sternites 2 ( Fig. 29G ), 3 and 5–8, and respective sympods, no such setae on sternite 1 (respective parts of sympod and sternite 4 damaged). Exopods 1–2, 5–6 and 8 available, remaining exopods broken. Available basal plates with smooth cuticle, length 1.5–1.7 times maximum width ( Fig. 29G ), plates with minute tooth-like distal projection on disto-lateral edge. Epipod 1 leaf-like, about as long as combined ischium and merus of endopod 1; no seta ( Fig. 29G ). Only endopods 1 and 2 available, remaining endopods broken. Available endopods with smooth cuticle, not considering setae; no pores detected. Endopods 1–2 with six segments ( Fig. 29G, I ) counting from basis to dactylus. Coxa of endopod 1 ( Fig. 29G ) with small mesial lobe apically bearing one small barbed seta; basis with large, setose endite, remaining segments without endite. Dactylus 1 normal, with basally wide, smooth, weakly curved, acute nail ( Fig. 29G–H ). Dactylus 2 elongate, reflexed, bearing basally wide, smooth, acute claw facing propodus ( Figs 25B , 29I–J ).

MARSUPIUM ( Figs 25A , 29K ). Empty in holotype. Length increases by a factor of three from oostegite 1 to oostegite 3. Basal to median portions of dorsal margin without setae in oostegites 1–2, and from basal to subapical portions in oostegite 3. All oostegites with smooth cuticle, not considering setae, ventral and anterior margins plus part of posterior margin with dense series of barbed setae. Posterior part of oostegites 1–3 on inner face with comparatively long setae microserrated on their distal half. No setae on outer face of marsupium, except for 0–1 barbed seta near rostral edge of oostegite 2 ( Fig. 29K ).

PLEON ( Figs 25A, C–D , 30A–D ). Length of pleomeres 1–5 is 0.6, 0.4, 0.5, 0.4 and 0.5 times length of pleomere 6, respectively. Pleomere 6 about as long as combined pleomeres 4–5 ( Fig. 25D ). Female pleopods increasing in length caudally. Scutellum paracaudale subtriangular with convex margins and blunt apex.

TAIL FAN ( Figs 25A, C–D , 30E–H ). Uropods with smooth cuticle, not considering setae and single spine. Exopod extends 0.3–0.4 times its length beyond endopod and half its length beyond more rostrally inserting telson. Exopod measures 1.4 times endopod length and 1.7 times that of telson; endopod 1.2 telson length. Exopod with almost straight lateral margin and convex mesial margin. Margins of endopod converge in V-shaped manner up to blunt apex. Telson subbasally with a pair of transverse pore fields flanking midline ( Fig. 30F ); fields with 22 and 23 pores ( Fig. 30G ), respectively, with diameter <2 µm.

Paratypes Juvenile paratype with BL 5.3 mm, not dissected, with distally rounded rostral plate extending along midline to ⅔ of antero-posterior extension of eyes ( Fig. 27F ), covering part of mesial portions of eyes though leaving lateral portions dorsally exposed. Rostrum shorter in immature specimen, indistinct in adult holotype. Damaged immature male with BL ≈ 9.7 mm, completely dissected ( Fig. 27A–E ).

Carapace with mid-sub-caudal pore group comprising nine pores. Clypeus with acute, triangular, mid-anterior extension reaching to half-length of basal segment of antennular trunk. Shape of clypeus resembles a heart in upside-down orientation.

Juvenile with essentially well-developed, probably functional eyes, though no pigment detected. Eyestalk almost all around covered by cornea, except for ocular papilla and its close surroundings and insertion of eye on frons. Ommatidia reaching surface, giving mesial eye lobe a moruloid shape ( Fig. 27F ). Ocular papilla measuring ⅖ of antero-posterior extension of mesial eye lobe. Left and right eyes of immature male symmetrical, showing near-complete as well as clearly incomplete ommatidia ( Fig. 27C ). Distal sixth of mesial eye lobe with only few incomplete ommatidia. Remaining portion with most ommatidia positioned centripetally reaching surface ( Fig. 27C ); major part of these ommatidia proximally with rhabdom-like striation ( Fig. 27E ). Lateral eye lobe with self-contained ribbon ( Fig. 27B ) formed by oblong ommatidia, most oriented parallel, directed laterally and containing banded rhabdom ( Fig. 27D ). Ocular papilla measuring ¼ of antero-posterior extension of mesial eye lobe. Papilla positioned dorsally at about one-sixth lobe width from mesial margin of mesial eye lobe. Organ of Bellonci ellipsoidal. Without inspection of non-adults, one could erroneously interpret mesial eye lobe as an eyestalk laterally bearing a rudimentary cornea, and thus entire eye as non-lobate.

Figures

Fig. 25. Desmocornea subchelata gen. et sp. nov., holotype, adult female with BL of 18.5 mm (ZMH 64679). A. Habitus, right lateral aspect. B. Anterior half of cephalothorax, left lateral aspect. C–D. Dorsal (C) and ventral (D) aspects of body. E. Pore group near caudal margin of carapace, dorsal view. F. Distal edge of antennal scale, dorsal view. G. Detail of F, showing accessory spines. Abbreviations: mp = mandibular palp;sc = subchelate second thoracic endopod.A–D. Objects artificially separated from background.

Fig. 26. Eyes in adult Desmocornea subchelata gen. et sp. nov., holotype, adult female with BL of 18.5 mm (ZMH 64679). A. Left eye in loco, lateral view. B. Right eye expanded on slide, dorsal view. C. Ocular papilla of right eye; arrows from panel (B) point to tip of papilla and to scales along ridge running close to papilla.D. Tip of ocular papilla of left eye. Abbreviations: ca = carapace; ll = lateral eye lobe; ml = mesial eye lobe; ob = organ of Bellonci; om = ommatidia; op = ocular papilla; tb = toroidal bulge.

Fig. 27. Eyes in non-adult Desmocornea subchelata gen. et sp. nov., paratypes, immature male with BL of 9.7 mm (A–E: ZMH 64681) and juvenile 5.3 mm (F: ZMH 64680). A–B. Left eye of immature in loco, lateral (A) and dorsal (B) view, prior to dissection. C. Same left eye as above, on slide, ventral view, dashed line enhances contour of self-contained ribbon formed by ommatidia. D–E. Details of panel C, with focus on rhabdom. F. Left eye and rostrum of juvenile, lateral view. Abbreviations: ca = carapace; ll = lateral eye lobe;ml = mesial eye lobe; op = ocular papilla; rh = rhabdom; ro = rostrum.

Fig. 28. Desmocornea subchelata gen. et sp. nov., holotype, adult female with BL of 18.5 mm (ZMH 64679). A. Carapace and eyes expanded on slide, dorsal view. B. Right antennula, dorsal view. C. Detail of B, showing disto-median lobe of antennular trunk, dorsal view. D. Antenna, its sympod with end sac of antennal gland, ventral view; setae of antennal scale omitted. E. Left mandible, caudal view; palpus accidentally turned to show rostral face. F–G. Masticatory part of right (F) and left (G) mandibles, caudal view. H. Maxillula, caudal view. I. Maxilla, caudal view.

Fig. 29. Desmocornea subchelata gen. et sp. nov., holotype, adult female with BL of 18.5 mm (ZMH 64679). A. Labrum, ventral = aboral view. B–E. Modified spines of foregut, namely from anterior (B, variants labeled B1–B3), median (C) and posterior (D) parts of lateralia and from dorsolateral infolding (E). F. Labium, obliquely caudal view. G. Thoracopod 1 with epipod and part of sternites 1–2, caudal view. H. Detail of G, showing dactylus with nail, setae omitted. I. Thoracic endopod 2, rostral view. J. Detail of I, showing dactylus with nail, setae omitted. K. Oostegite 2, outer face.

Fig. 30. Pleopods and tail fan in Desmocornea subchelata gen. et sp. nov., holotype, adult female with BL of 18.5 mm (ZMH 64679).A. Pleopod 1, lateral = rostral.B. Pleopod 3, mesial = caudal.C. Pleopod 4, mesial view, many setae broken. D. Pleopod 5, lateral view, many setae broken. E. Uropods, dorsal view, setae omitted. F. Telson, dorsal view. G. Detail of F, showing left subbasal pore field. H. Detail of F, showing part of mid-terminal armature.

Fig. 1. Dactylamblyops murrayi W.M. Tattersall, 1939, adult female with BL of 15.6 mm. A. Distal ⅔ of right eye, dorsal view. B. Ommatidia, ventral view. C. Tip of ocular papilla, dorsal view. D. Right eye, lateral view. E. Antennal peduncle, ventral view. F. Telson, dorsal view.

Fig. 2. Dactylamblyops murrayi W.M. Tattersall, 1939, adult female with BL of 15.6 mm. A. Oostegite 1, inner face. B. Detail of A, showing flagellate seta. C. Oostegite 2, inner face. D. Uropods, ventral view, setae omitted.

Fig. 3. Dactylamblyops benthophilus sp. nov. A–B. Cephalon of paratype, adult male with BL of 19.9 mm, dorsal (A) and ventral (B) views (ZMH 64673). C. Paratype, adult female 25.4 mm long, lateral view (ZMH 64672). D. Holotype, adult male 22.6 mm long, lateral view (ZMH 64669). A–D. Objects artificially separated from background.