Mysidella antarctica, 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 : 124-130

Mysidella antarctica sp. nov.

urn:lsid:zoobank.org:act:40D908D9-D8C6-479B-BA31-7A6289440CBB

Figs 62–65

Diagnosis

Based on adult females only. All female features as in generic diagnosis. Carapace with broadly rounded anterior margin; rostrum forming a very short, wide-angled, apically rounded prolongation of carapace. Large, broadly rounded (shoulder-like) disto-lateral lobes anteriorly extending to basis of antennal scale. Eyes set apart, cushion-like, only slightly dorsoventrally flattened, with slightly produced rostral margin; presence of a few strongly reduced, completely internal ommatidia not forming a compound organ, no ocular papilla. Second and third segments of antennular trunk separated by, in frontal plane, oblique articulation, resulting in lateral bending of entire trunk. Antennal scale length about 5/2 of maximum width. Scale extending slightly less than half its length beyond antennular trunk. Only basis of thoracic endopod 1 mesially with setose endite (lobe); enlarged propodus with distal margin armed with about seven spiniform modified setae, laterally increasing in size; dactylus slender, about half as long as propodus; dactylus with slender nail about as long as propodus. Endopod of uropods with ≈32 spines ventrally along mesial margin between statocyst and tip; proximal fifth of spine series with small spines discontinuously increasing in size distally, followed by medium-sized subequal spines up to a single subapical spine measuring more than twice that size. Lateral margins of telson straight, slowly converging, proximal ⅖ bare; distally remaining stretch with uninterrupted series of 19–22 spines organized in groups of medium-sized spines with smaller spines in between, average spine size of groups increasing in length distally. Terminal margin with triangular, somewhat ogival incision at 11–12% of telson length. Incision armed with ≈ 12 laminae, no spine, no seta. Each latero-terminal lobe of telson apically with large spine (precise length unknown) flanked by pair of spines with length subequal to that of distal-most lateral spines.

Etymology

The species name is a New Latin adjective with feminine ending referring to the distribution of the species in Antarctic marine waters.

Material examined

Holotype SOUTHERN OCEAN • 1 ♀ ad. (BL = 7.5 mm); eastern Weddell Slope , Kapp Norvegia, ANDEEP-III station 080-9; 70°39.07ʹ S, 14°43.36ʹ W to 70°39.22ʹ S, 14°43.39ʹ W; depth 3103– 3102 m; 23 Feb. 2005; EBS supranet; ZMH 64682 .

Paratype SOUTHERN OCEAN • 1 ♀ ad. (BL = 7.9 mm, on slides); same collection data as for holotype except for occurrence in epinet; ZMH 64683 .

Type locality

Type locality is ANDEEP III station 080-9: eastern Weddell Slope, Kapp Norvegia, SW of Wegener Canyon, 70°39.07ʹ S, 14°43.36ʹ W to 70°39.22ʹ S, 14°43.39ʹ W, depth 3103– 3102 m. Both type specimens from the same haul, the holotype from the supranet and the paratype from the epinet.

Description

Holotype (♀)

Adult female with BL 7.5 mm, brood pouch empty, not dissected. All features visible without dissection are within limits of specific diagnosis. Rostrum contributes 1%, cephalothorax 31%, pleon (without telson) 54% and telson 16% to total BL.

CARAPACE. Normal, length 28% of BL (including rostrum), surface smooth ( Fig. 62D ), no sulci visible. Concave posterior margin leaving ultimate 1½ thoracomeres dorsally exposed.

EYES ( Fig. 62C–D ). Antero-posterior extension 0.8 times maximum width (= transverse extension) and 1.3 times length of terminal segment of antennular trunk. Disto-lateral margin of eyes with spiniform scales (as in Fig. 63D ). Additional details given below for the dissected paratype.

ANTENNULA ( Fig. 62C–D ). Trunk length 9% of BL, extending half its length beyond eyes. Measured along dorsal midline, basal segment 38% trunk length, median 27% and terminal 35%. Length of basal segment 0.8 times width, a setose dorsal apophysis shortly extending beyond its rostral margin. Length of terminal segment 0.7 times width. Width of outer antennular flagellum near basis 1.3 times width of inner flagellum.

ANTENNA ( Fig. 62C ). Sympod 2-segmented, caudally in addition with large end sac of antennal gland. Segments 1–3 of peduncle contribute 18%, 40% and 42% to total length in dorsal view.

CEPHALOTHORAX ( Fig. 62 ). Clypeus with short triangular rostral process reaching to basal third of basal segment of antennular trunk; acute apex of process visible in dorsal view ( Fig. 62D ). Thoracic endopod 1 with six segments. Endopods 2–8 broken.

PLEON AND TAIL FAN ( Fig. 62A–B ). Length of pleomeres 1–5 is 0.8, 0.8, 0.8, 0.7 and 0.7 times length of pleomere 6, respectively. Scutellum paracaudale convex, broadly rounded. Uropods with smooth cuticle, not considering setae and spines. Exopod 1.1 times as long as endopod and 1.2 times as long as telson; endopod 1.1 times as long as telson. Endopod with straight margins weakly converging to blunt terminus. Telson length 1.9 times maximum width and 1.5 times length of ultimate pleomere; width at terminus (measured between lateral margins of latero-terminal spines) 0.4 times maximum width near basis and 0.2 times telson length.

Paratype (♀)

Covers features requiring dissection. No pores or scales detected on carapace.

EYES ( Fig. 63A–E ). Spiniform scales loosely scattered over dorsal face ( Fig. 63E ) and more densely in series along disto-lateral margin ( Fig. 63D ). About twenty completely internal, large, seemingly empty bodies present in lateral third of eye, here interpreted as residuals of ommatidia ( Fig. 63C ). Organ of Bellonci located mesially from these residuals; a completely internal sphere containing more than fifty small black bodies ( Fig. 63B ). No pores detected.

ANTENNULA ( Fig. 64A ). Basal segment of trunk without antennular bursa, no ventral carina. Terminal segment with disto-median lobe armed with three small teeth increasing in size laterally, lobe disto-laterally with three barbed setae. This segment without callynophore, no female lobe.

PRIMARY MOUTHPARTS ( Fig. 64D–G ). Both lobes of labrum with longitudinally narrow, rugged margin ( Fig. 64D ). Left mandibular palp ( Fig. 64E ) comparatively small, 3-segmented; basal segment contributes 11%, median segment 56% and apical segment 33% (not including laterally bent tip) to total palp length (right palp complete though artificially distorted). Median segment 3.1 times as long as wide, both margins slightly convex, lateral margin almost straight. Length of apical segment 2.7 times maximum width. Each palp with a few setae, not hispid, its basal segment without setae, median segment with 3–4 smooth setae on distal third of lateral margin and only one smooth seta on mesial margin at ⅔ of length from basis; apical segment with only 1–2 small setae on proximal half of lateral margin; its distal half rostrally with 10–14 setae (rugged due to stiff barbs) along lateral margin; distal half of caudal face with longitudinal series of 6–8 barbed setae along ¼ of segment width from mesial margin, these setae with soft barbs. In addition, mandibular palp bears three setae at laterally bent tip, namely two setae with stiff barbs and a shorter seta without barbs, only largest barbed seta at tip extending beyond any other seta of apical segment ( Fig. 64F ). Cutting edge of mandibles narrowly rugged along mesial margins ( Fig. 64E ). Labium bilobate, no seta detected ( Fig. 64G ).

GUT ( Fig. 63F ). Foregut with gross structure as in other Mysidae examined here. As a novelty within this family, foregut contains only smooth setae and smooth spines, no modified spines. No foreguts of any other species of Mysidellinae have been studied so far. Storage volume filled to 80% (partly removed in Fig. 63F ) with unidentifiable masticated material and abundant mineral particles plus a few diatoms. Midgut content with essentially same material, though with smaller maximum size of particles. Anal lobe weakly cuticularized ( Fig. 65I ).

MAXILLULA ( Fig. 64H ). Distal segment with distally increasing width, length 2.3 times maximum width; with 14 smooth spines on transverse distal margin, no seta, no pores; spines in continuous series strongly decreasing in size mesially. Endite terminally with three large, barbed setae plus one tiny smooth seta.

MAXILLA ( Fig. 64I ). Without pores. Sympod with three mesial lobes which are densely setose along disto-mesial margins. Exopod reaches to terminal margin of basal segment of maxillary palp. Exopod with plumose setae all along lateral margin; largest seta at tip; mesial margin bare. Distal segment of palp contributing ⅔ to total length; palp length 3.2 times maximum width. Distal segment densely setose on transverse terminal margin and on distal ⅔ of mesial margin; lateral margin bare, no spines. Terminal margin of proximal segment ventrally with two large barbed setae, lateral seta partly covered by distal segment in Fig. 64I .

THORACOPODS ( Fig. 65A–D ). Basal plate of thoracic exopod 1 with smooth cuticle, length 1.4 times maximum width; plate with bluntly tipped rectangular disto-lateral edge; flagellum 7-segmented ( Fig. 65A ); exopods 6–8 with length of basal plate 1.7–2.2 times maximum width, flagellum 8-segmented. Epipod 1 leaf-like, 1.2 times as long as merus 1 in frontal view, no seta ( Fig. 65A ). Endopods 2–8 broken. Outer face of marsupium without setae; inside proximally with setae microserrated by minute stiff cils, ventrally and distally with barbed (plumose) setae (many setae broken in Fig. 65D ).

PLEOPODS AND TAIL FAN ( Fig. 65E–J ). Pleopods 1–2 ( Fig. 65E–F ) of about same size, pleopods 2–5 increase in length caudally, no modified setae. Statoliths composed of fluorite, diameter 0.08–0.11 mm (n = 2). Telson with spines and laminae only, no setae, scales or pores.

Figures

Fig. 62. Mysidella antarctica sp. nov., holotype, adult female with BL of 7.5 mm (ZMH 64682). A–B. Holotype in toto, lateral (A) and dorsal (B) views (thoracic endopods 2–8 and most exopods broken). C–D. Cephalon, lateral (C) and dorsal (D) views. Objects artificially separated from background.

Fig. 63. Eyes and foregut in Mysidella antarctica sp. nov., paratype, adult female with BL of 7.9 mm (ZMH 64683). A. Right eye rudiment, dorsal view. B–D. Details of A, showing inclusions in organ of Bellonci (B), rudimentary ommatidia (C) and scales on anterior margin (D). E. Scales in respective position on dorsal face of left eye. F. Foregut, dorsal view, caudal fifth not shown. Abbreviations: di = dorsolateral infolding; la = lateralia; ob = organ of Bellonci; om = rudimentary ommatidia; ps = pyloric secondary filter; sm = superomedianum; sp = storage space. A, F. Objects artificially separated from background.

Fig. 64. Mysidella antarctica sp. nov., paratype, adult female with BL of 7.9 mm (ZMH 64683). A. Antennula, dorsal view. B. Antenna with antennal gland, dorsal view; setae of antennal scale omitted. C. Carapace and eyes expanded on slide, dorsal view. D. Labrum. E. Left mandible, rostral view. F. Detail of E, showing distal segment of palpus with only three distalmost setae depicted. G. Labium. H. Maxillula, rostral view. I. Maxilla, caudal view.

Fig. 65. Mysidella antarctica sp. nov., paratype, adult female with BL of 7.9 mm (ZMH 64683). A. Thoracopod 1 with epipod, frontal view. B. Detail of A, showing propodus with modified setae plus dactylus with nail. C. Detail of B, showing modified seta of propodus. D. Oostegite 2, inner face. E. Pleopod 1, mesial = caudal. F. Pleopod 2, lateral = rostral. G. Pleopod 5, lateral view. H. Uropods, ventral view, setae omitted. I. Telson, ventral view. J. Detail of I, showing terminus of telson. D–G. Many setae broken.