Holotype spent female (♀0–) BL 26.8 mm (on slides at NHMW 27296, GenBank nos. OK351330 and OK353694), East Antarctica, Adélie Land, near Dumont d’Urville Station, NE of Claude Bernard Island, 66°39.64'S, 140°01.55'E, ice cave, dive #612, depth 10 m, diver-operated suction bottle, 15 Jan 2016, leg. P. Chevaldonné & S. Hourdez. Paratype subadult female (♀S–) BL 21.5 mm (on slides at NHMW 27297, GenBank nos. OK351329 and OK353693), dive #611, 13 Jan 2016, remaining sampling data as for holotype.

Covers females only. Species of the genus Pseudomma G.O. Sars, 1870, with cornea-like lateral portions separated by sulci from main part of eyeplate (Figs 2B, 4C, 23A), no visual elements. Disto-median fissure penetrates one third of eyeplate. Distal margin of eyeplates with series of minute teeth along sublateral sector (‘shoulders’, Fig. 23A, D). Basal segment of antennular trunk without medio-ventral carina. Antennal scale (Fig. 4B) with setose apical lobe contributing about 1/4 scale length. Mandibular palp (Fig. 4E) 3-segmented, very large, about as long as antennal scale. Three pairs of oostegites (Fig. 5I) contributing to wall of brood pouch. Pleopods (Fig. 6H–L) reduced to setose rods with residual differentiation of endopod (pseudobranchial lobes). Telson (Figs 3A, 6N) trapezoid, as long as ultimate pleonite. Its length twice maximum width at basis and four times width at apex. Lateral margins of telson without setae and spines, only minute scales present. Transversely truncate terminal margin with only two pairs of spines, both hispid due to minute scales (Fig. 3B) along more than proximal 2/3 spine length. Large latero-apical and same-sized submedio-apical spines flank median pair of closely set setae (Fig. 3C) with twice spine length. Margin with short, well-rounded indentation between each spine, median indentation largest. Disto-lateral edge without tooth, with spine only.

All features of the diagnosis. Female with body length 26.8 mm. Cephalothorax measures 39% body length, pleon without telson 48%, telson 13% and carapace 32%. Large parts of the body, particularly carapace, pleon, telson, and uropods scaly-hispid; most appendages and eyeplates only to a minor degree. However, with 30× episcopic microscopy, the entire body appears smooth (Fig. 3D–E) due to small size of scales. With 600× transmitted phase contrast microscopy, large areas of the (artificially shed) dorsal cuticle of the animal resembles fish skin (Fig. 3F) due to dense scale cover. Ventral portions of pleomeres less densely covered, thoracic sternites smooth.

Antennula (Figs 2B, 4A). Epi-antennular process triangular, projecting in median position beyond eyeplate like a small arrowhead (Fig. 2B). Antennular trunk with three sparsely setose segments, separated by transverse articulations. Basal segment 45%, median segment 16% and terminal segment 39% trunk length. Length of basal segment is only 2/3 width; mid-dorsally with deep antennular depression leading down to a striated pad at the bottom (Figs 4A, 24A, B) as described below. Basal segment not produced at outer distal corner. Terminal segment with the usual dorsal lobe on distal margin. This lobe without spiniform extension, disto-laterally with four barbed setae, mid-terminally and disto-medially with thickened, rugged margin. Flagella large, width of outer flagellum measured near basis with 1.1–1.2 times width of inner flagellum. Trunk with scales over major portions of its surface, not so the flagella.

Antenna (Fig. 4B). Antennal scale large, 1.8 times length of antennular trunk and 1.8 antennal peduncle. Scale extends 0.4 times its length beyond antennular trunk and 0.7 beyond eyeplate (taking into account that antennulae insert more rostrally). Scale unsegmented, 2.9 times longer than wide. Scale dorsally and ventrally scaly-hispid all over. The smooth portion (not considering minute cuticle scales) of its outer margin ends in a strong tooth; setose apical lobe extends 26–27% scale length beyond this tooth. Antennal peduncle three-segmented. Basal segment contributes 21%, median segment 42% and terminal segment 36% peduncle length. Sympod angular on disto-lateral edge, not forming a tooth-like projection. Sympod with hispid lateral face.

Eyes (Figs 2B, 3D, 4C, 23A, B, D). Eyeplate extending 0.9 times the length of terminal segment of antennular trunk along mid-line beyond anterior margin of carapace. Length of eyeplate, including its dorsally covered portion, 1.3 times the length of terminal segment. In dorsal view, superimposed dorsal and ventral sulci separate cornea-like lateral portions from main part of eyeplate (Figs 2B, 4C, 23A). Eyeplate containing tear-shaped cyst narrowing distally up to conjunction with eyeplate cleft (Fig. 23B). Sub-lateral portions of dorsal face with cover of minute scales (as in Fig. 3F; visualised with 400× microscopy), series of 15 minute teeth (not all in focus in Fig. 23D), closely set along anterior margin of this portion. Brilliant red cornea-like portions of eyeplate feign functional eyes in living specimens (Fig. 2B). Eyeplates become transparent (Figs 4C, 23A) after expansion on slide, embedding in Swan-medium and resultant bleaching; neither functional nor both vestigial ommatidia and neuronal structures visible.

Figure 3. 

Pseudomma kryotroglodytum sp. nov., holotype adult female BL 26.8 mm (A–E) and paratype subadult female 21.5 mm (F) A terminal portion of telson, dorsal, details show scales on left disto-mesial spine (B) and barbs on left terminal seta (C) D anterior half of cephalothorax, lateral, arrow points to distolateral edge of carapace E sixth pleomere with tail fan, lateral F example for pores (three to the left) and coat of scales on tergite of first pleomere.

Carapace (Figs 3D, 4C) with broadly rounded anterior margin, disto-lateral edges well rounded. No typical rostral plate present, but a frontal bulge dorsally covered by the carapace; bulge best seen in lateral view (Fig. 3D). Antero-lateral edge of carapace with rounded protrusion (marked by arrows in Figs 3D, 4C). By forcing the detached carapace in a plane (Fig. 4C), this protrusion becoming shifted caudally compared with its position in situ (Fig. 3D). Carapace with cervical sulcus and cardial sulcus distinct; posterior margin concave, terminal indentation widely triangular. Two submedian groups of 8–10 pores symmetrically arranged directly in front of cardial sulcus (Fig. 4D shows only seven pores in the smaller paratype). Carapace leaving posterior 1.5 thoracomeres dorsally exposed.

Mouthparts (Figs 4E, F, 5A–C). Labrum normal (Fig. 5A), rostrally forming a broad, rounded bulge; most caudal portions with strong lamellae and cover of scale-like fringes. Basal segment of mandibular palp (Fig. 4E) contributing 9–10%, median segment 55–56% and apical segment 35–37% to total palp length. Length of median segment 3.1–3.2 times maximum width; its mesial margin convex, lateral margin sigmoid. Length of apical segment 3.7–3.8 times maximum width. Palp not hispid, its basal segment without setae, remaining segments densely setose along mesial and lateral margins. Caudal face of median segment with dense field of fine hairs near basis. Masticatory part of mandibles strong, asymmetrical. Left mandible as normal in Mysidae. Pars incisiva of the new species with three large teeth and digitus mobilis with four strong teeth. Spine row with four spines ‘serrated’ by numerous stiff bristles; processus molaris with grinding lamellae not ending in teeth and with dense cover of stiff bristles. Right mandible as normal in the genus Pseudomma; in the new species with four large teeth on pars incisiva; digitus mobilis with only one large apical tooth serrated by secondary teeth. Right spine row present as series of nine medium-sized smooth teeth plus a few small ones, rather than a smaller number of ‘serrated’ spines present on the left mandible as otherwise usual for both mandibles in Mysidae. Right processus molaris with strong masticatory lamellae, each with small, tooth-like, apical projection; processus with cover of stiff bristles less dense than that of left mandible.

Paired labia (Fig. 5B) with stiff setae, lacking spines or teeth. Distal segment of maxillula (Fig. 4F) with 11–12 weakly serrated, strong spines on terminally truncate margin; subterminally with 5–6 barbed setae. Holotype with 5–9 pores on the surface between setae bases and spines; potential additional pores may be covered by these setae (no pores identified in the paratype). Endite of maxillula with numerous normal setae; distally with three large, modified setae, armed with stiff bristles near apex¸ more proximally with a shorter seta bearing an apical brush of long bristles.

Figure 4. 

Pseudomma kryotroglodytum sp. nov., holotype adult female BL 26.8 mm (A, B, E, F) and paratype subadult female 21.5 mm (C, D). A right antennula, dorsal B antenna with antennal gland, ventral, setae omitted from antennal scale C eyeplate and carapace expanded on slide, short arrow points to distolateral edge of carapace, detail (D) pore group in pre-cardial position E mandibles with left palpus, caudal aspect F maxillula, caudal. Scales omitted from objects A–C, E, F but not from eyeplate in panel C; pore diameters not to scale in C, D.

Maxilla (Fig. 5C) with well-developed exopod, two-segmented endopod (palp) and four setose endites. Exopod normal-sized, shortly extending beyond basal segment of endopod. Outer margin and apex of exopod with dense series of plumose setae distally increasing in size. Length of apical segment of endopod 1.5–1.6 times its maximum width and 1.2–1.3 times length of the basal segment. Basal segment with total of eight barbed setae, namely 3–4 on caudal and 4–5 on rostral face. Apical segment with setae all around, except for proximal third of lateral margin.

Foregut (Fig. 6A–E). Lateralia, infoldings and superomedianum of the cardiac chamber densely covered by smooth, slender setae and spines. Lateralia anteriorly with dense series of slender, apically coronate spines (Fig. 6B) of different length, more caudally with slender acute spines (Fig. 6C). The latter spines with minute teeth on and close to apex. Both coronate and acute spines hispid due to minute scales along distal 50–70% of shaft. Posterior part of lateralia with powerful complex of many blunt teeth arising from common base. Twelve teeth differentiated (Fig. 6E) with translucent microscopy, additional teeth not excluded. Dorsolateral infoldings with two smooth, bent spines (Fig. 6D, subapically slightly serrate only in paratype). About 2/3 of comparatively large storage volume of foregut contained masticated, unidentifiable organic materials and mineral particles, also observed in paratype.

Thorax (Figs 2A, B, 5D–I). At least tergites 6–8 covered by minute scales, no pores. Sternites 1–8 without pores, scales and also without ventrally projecting median processes (Fig. 5D). Sternite 1 with short anterior lobe projecting between left and right, first thoracic endopods. Basal plates of thoracic exopods (3–4) times longer than wide (Fig. 5D), not widening distally or only minimally so; lateral portions scaly-hispid, mesially smooth; disto-lateral edge unevenly rounded. Exopods 1, 8 with 15-segmented (Fig. 5D), remaining exopods with (17–20)-segmented, setose flagellum. Basis of endopod 1 with setose endite (below drawing plane in Fig. 5D), remaining segments without clear endite. Endopods 1, 2 with six segments (Fig. 5D, F), remaining endopods with eight segments counting from basis to dactylus (basis omitted in Fig. 5G). Endopods 3–8 long and slender; length and slenderness increase caudally; ischium shows the strongest relative increase (Fig. 2A). All endopods with hispid carpopropodus and dactylus, endopods 2–8 in addition with hispid merus, to a minor degree, if any, also ischium. Carpopropodus 3–8 three-segmented; more than half its length contributed by basal segment. Setae patterns might feign additional subdivision of carpopropodus (Fig. 5G). Suture between median and terminal segment transverse in carpopropodus 3, weakly (Fig. 5G), but not always distinctly oblique in carpopropodus 4, weakly and distinctly oblique in carpopropodites 5–8. Dactylus 3–8 small. Dactylus 1, 3–8 with short, smooth claw (Fig. 5E, H). No claw detected in dense jungle of setae on dactylus 2.

Marsupium (Fig. 5I) empty in this specimen. Basal to median portions of dorsal margin without setae in oostegite 1, without setae from basal to subapical portions in oostegites 2 and 3. A narrow ‘fur’ of densely set tiny hairs forming a ribbon along dorsal margin of oostegite 1 (Fig. 5I), no such hairs in oostegites 2 and 3. Scales on large portions of outer face in oostegites 2 and 3; no scales seen on oostegite 1. Ventral and anterior margins plus part of posterior margin with dense series of barbed setae, together with bilaterally opposite oostegite forming gate contributing to the ventral and caudal closure of marsupium (this configuration not impeding respiratory water flow through marsupium). Numbers of barbs per seta increase distally, albeit not reaching those of typical plumose seta. Oostegite 1 with barbed setae also on distal half of dorsal margin, suggesting pervious anterior closure of marsupium. Posterior parts of oostegites 1–3 on inner face with comparatively long setae microserrated on their distal half. Outer face of oostegites 2 and 3 with slender whip setae characterised by barbed shaft bearing longer thin flagellum. These setae implanted on distal portions of oostegites 2 and 3; additional whip setae along ventral margin only in oostegite 3. No whip setae in oostegite 1.

Figure 5. 

Pseudomma kryotroglodytum sp. nov., holotype adult female BL 26.8 mm A labrum B labium C maxilla, caudal aspect D thoracopod 1 (caudal) with thoracic sternites 1, 2 (ventral) E detail of panel (D) showing dactylus 1 with nail F thoracic endopod 2, rostral G thoracic endopod 4, rostral H detail of panel (G) showing dactylus 4 with nail I inner face of oostegite 1, drawn above part of sympod 6. Scales omitted from objects A–D, F, G; setae from E, H.

Pleon (Figs 3E, F, 6F–J). Pleonites 1–5 are each 0.6 times length of pleonite 6 measured along dorsal mid-line. Pleopod size increases distally. Pleopods 4–5 reaching only up to 2/3 length of pleonites 5 or 6, respectively. Not considering setae, pleopod 1 smooth (Fig. 6H) all around, remaining pleopods completely smooth only on caudal face, though scaly-hispid at least near basis of rostral (outer) face. Relative coverage with scales increases in series of pleopods 2–5 (Fig. 6I–L). All six pleonites dorsally densely covered by minute scales. Only pleonite 1 showing pores, namely two symmetrical transverse linear series each with 17–18 discontinuously spaced pores, in submedian position on dorsal face; and one additional, non-linear cluster of 8–9 pores on each lateral face (Fig. 6F–G shows fewer pores for the smaller paratype). Scutellum paracaudale sinusoid, well rounded.

Tail fan (Figs 3A–C, E, 6M, N). Telson, endopod and exopod of uropods 1.0, 1.1–1.3 or 1.5–1.8 times length of sixth pleonite, respectively. Uropods (Figs 3E, 6M) long, margins setose all around, dorsal and ventral faces scaly-hispid. Exopod with slightly convex, almost straight lateral margin and with more strongly convex mesial margin. Both margins diverge up to maximum width of the exopod at 1/3 length from basis and then converge up to the U-shaped, well-rounded terminus. Distal 4/5 of endopod with straight margins converging in V-shaped manner up to narrowly blunt apex. Endopod 0.7 times length of exopod, extending 1/5 of its length beyond telson, exopod 2/5 its length. Both statoliths unevenly discoid, mean diameter 0.22–0.23 mm, thickness 0.18 mm; core unevenly discoid as well, diameter 0.14 mm. Statolith formula 2 + 3 + (5–7) + 10 + (12–14) = 34. Statoliths composed of fluorite. Lateral margins and dorsal face of telson (Fig. 6N) completely covered by scales as in Fig. 3F, whereas ventral face only on its distal third.

Colour (Fig. 2A, B). Live colour was documented only in the laboratory; no difference visible between the two type specimens. Body and appendages generally whitish transparent. Lateral portions of eyeplate, foregut and part of mouthparts brilliant red, hepatic caeca yellow-green. The anterior pair of caeca covers part of the foregut dorsally, leaving a red M-like sign on the foregut in dorsal view. Posterior part of ultimate pleomere and adjoining basal portions of tail fan tinged light-red.

Figure 6. 

Pseudomma kryotroglodytum sp. nov., holotype adult female BL 26.8 mm (A–E, H–N) and paratype subadult female 21.5 mm (F, G). A cardiac portion of foregut, dorsal view, dorsal wall omitted B–D details of panel (A) showing modified spines E details of panel (A) showing teeth emerging from a common basis F pleonite 1, cuticle detached and expanded on slide, dorsal and lateral faces on top, ventral face folded in, setae omitted from pleopods G detail of panel (F) showing pore group on left lateral face H–L series of pleopods 1–5, rostral = lateral face M uropods, ventral N telson. Scales omitted from objects F, M, N pore diameters not to scale in F, G.

The species name is an adjective with Latinised neutral ending formed by adjectivation of the amalgamated Ancient Greek adjective κρύος (cold) with the noun τρωγλοδύτης (cave dweller). The adjectivation has precedence in the name of the butterfly Macroglossum troglodytus Boisduval, l875, listed by Kemal et al. (2019) as M. troglodytum.

Marine ice cave NE of Claude Bernard Island, Adélie Land, East Antarctica, 66°39.64'S, 140°01.55'E, depth 10 m.

Mysidetini with eyes well developed; cornea large, globular, with functional ommatidia; eyestalks well developed. Antennula usually without modified setae (exception: females of M. illigi Zimmer, 1914, as described below). Appendix masculina well-developed, setose. Antennal scale setose all around, no spines and no teeth. Mouthparts normal, maxilla without spines. Male thoracic endopod 2 without notches on outer margin. Thoracic endopods 3–8 normal, slender, not prehensile; with multi-segmented carpopropodus; small dactylus with weak claw. Penes long, slender, stiff, and not erectile. Pleopods non-dimorphic, reduced to bifid setose plates, no modified setae and no spines. Endopod of uropods usually with row of spines on inner margin (exception: M. hanseni Zimmer, 1914, as described below). Telson with apical cleft; cleft lined with laminae. Lateral margins of telson at least distally with spines.

Type species is Mysideis Farrani Holt & Tattersall, 1905, current name Mysidetes farrani (Holt & Tattersall, 1905). Total of 17 species including the here re-installed M. illigi Zimmer, 1914, are acknowledged as in the following key:

Holotype (by monotypy) subadult female (ZMB 18284) BL 12.7 mm, in vial with ethanol, labelled “D.-Südpol.-Exp. 31.12.1902, 200 m. Mysidetes illigi sp. nov. Typus”. Type not explicitly defined by Zimmer (1914). In accordance with the text by Zimmer (1914), the label of the jar containing this vial indicates 21.12.1902 as the date of sampling. According to Zimmer (1914), this specimen was taken on this day together with one specimen (now lectotype) of M. hanseni with a vertical haul 200–0 m at the ‘Winterstation’ (= Gauss Station), 66°02'S, 89°38'E, coast of East Antarctica, S#9.

Three samples (S#2–4) taken in austral summer 2015–2016 by P. Chevaldonné and S. Hourdez upon diving in an ice cave of Bernard Island, near Dumont d’Urville Station, Adélie Land, Antarctica, 66°39.64'S, 140°01.55'E:

One spent female (♀0–) BL 17.9 mm, 5 ♂♂S 13.2–15.0 mm, 2 ♂♂I 12.1–12.9 mm, 1 ♀S⁺ 13.9 mm, 5 ♀♀I 12.3–14.1 mm (in vials, NHMW 27298, SMF-57647, ZMB 34882, ZMH-K-60866), S#3; 1 ♀0– 18.1 mm (on slides; NHMW 27300), 5 ♀♀0– 14.2–15.9 mm, 3 ♂♂S 11.3–12.6 mm, 2 ♀♀S– 14.3–15.7 mm, 1 ♀S⁺ 15.5 mm, 7 ♀♀I (in vials, NHMW 27299, SMF-57648, ZMB 34883, ZMH-K-60867), S#4; 1 ♀0– 18.4 mm (on slides, NHMW 27301) and 1 ♀I 15.6 mm (in vial), S#2.

Covers adult females and subadults of both sexes:

Species of Mysidetes with eyes (Figs 7A, B, 9A, B) well-developed, thick. Cornea calotte-shaped, its length 0.8 times length of eyestalk, diameter 1.6–1.8 times length of terminal segment of antennular trunk. Eyestalk without ocular papilla; length 0.7–0.8 times its width at conjunction with cornea. Rostrum triangular with acute to narrowly-rounded apex and with concave, up-tilted lateral margins; rostrum 0.8–1.0 times as long as terminal segment of antennular trunk.

Antennae s.l. (Figs 8C, D, 10A–C). Terminal segment of antennular trunk with mid-ventral lobe (Figs 8D, 10B) bearing modified setae in females (Fig. 25C). Antennal sympod (Fig. 10C) with one large, acute tooth on disto-lateral edge and more caudally an additional shorter tooth. Dorsal face of sympod with triangular, apically rounded lobe. Antennal scale two-segmented, apically rounded, setose all around, with apical segment only 2% total scale length; scale 4–7 times as long as its maximum width; scale projects 0.3–0.6 times its length beyond antennular trunk.

Mouthparts (Fig. 11). Median segment of mandibular palp 2.5–3.3 times as long as maximum width (Fig. 11B), densely setose all around. Right mandible with digitus mobilis and pars centralis modified as in Fig. 11B; remaining mouthparts normal, labrum not produced rostrally, maxilla without spines.

Thorax (Figs 10D–K, 13A, B) without mid-sternal processes in females and non-adult males. Flagellum of thoracic exopods 1, 8 with eight segments, flagella 2–7 with nine segments (Fig. 13A). Carpopropodus of thoracic endopods 1–8 with 2, 2, 5–6, 6–8, 7–9, 6–9, 6–8 and 6–8 segments, respectively. Claw of endopod 1 (Fig. 10F) strong, subapically, unilaterally, weakly serrated; claws 3–8 (Fig. 10H–K) weak, slender, smooth. Marsupium formed by two pairs of large oostegites; additional rudimentary oostegite on thoracopod 6 (Fig. 13A). Subadult males with penes (Fig. 13B) stiff, slender, 1–2 times length of ischium of endopod 8.

Pleon (Figs 8A, 13B–G). Pleopods (Fig. 13C–G) reduced to unsegmented, setose plates with comparatively large endopodal portion (pseudobranchial lobe) integrated. All pleopods without spines, no modified setae. Total length increases in series of pleopods 1 to 5.

Tail fan (Figs 9E, 13H, I). Endopod of uropods (Fig. 13H) with 8–13 slender spines in series from statocyst to 25–35% endopod length from apex; proximal 2–4 spines short, in part crowded; remaining 6–9 spines longer, subequal amongst each other, about equally spaced in linear series. Telson (Fig. 13I) trapezoid, length 1.9–2.5 times maximum width near basis and 6–7 times minimum width on bifid terminus. Length 0.7–0.8 times exopod of uropod. Lateral margins of telson each with total of 35–47 spines; basal portions with 7–9 spines in continuous series followed by an unarmed stretch, median portions start with 2–6 spines increasing in length distally, followed up to tip by discontinuous series of large spines with small spines in between. Cleft U-shaped, penetrating 15–18% telson length, margin of cleft lined all along with 23–29 laminae of which proximal 3–4 laminae larger than remaining 20–25 subequal laminae. Cleft 2.0–2.5 times as deep as its width at apex. Disto-lateral lobes of telson triangular with narrowly truncate apex; each lobe armed with two spines at apex, mesial spine 0.5–0.7 times length of lateral spine.

Figure 7. 

Mysidetes illigi from ice cave of Bernard Island, Antarctica A subadult male, dorsal B subadult male, lateral C hyperbenthic association formed by mysids and early stages of nothotheniid fish D fish swarm mixed with small number of mysids several metres inside cave A, B, living specimens in laboratory.

Subadult female (Fig. 8) with 12.7 mm body length, not dissected. Status of ovarian tubes not well established. Body moderately slender, pleon (without telson) contributes 59% to total trunk length. Carapace including rostrum 32% of body length (including telson) when measured along dorsal mid-line.

Cephalothorax (Fig. 8B–D). Outer lobe from basal segment of antennular trunk extending beyond median segment. Basally wide, low lobe located mid-ventrally closely behind anterior margin of terminal segment (arrows in Fig. 8D). Antennal sympod as in diagnosis (as in Fig. 10C). Scale is 3.8–4.3 times as long as its maximum width (Fig. 8C), extending 46–59% its length beyond antennular trunk and 35–47% beyond antennal peduncle. Basal segment contributing 20–24% to length of antennal peduncle, median segment 45–46%, and distal segment 30–34%. Cornea large, bulbous (damaged in this specimen). Eyestalk smooth, no ocular papilla. Frons with five horizontal bulges vertically stacked between rostrum and antennular symphysis, these ranging from subrostral process (bulge) ventrally down to that from the antennular symphysis. Rostrum (Fig. 8A, B) large, triangular, basally broad, extending to terminal margin of eyestalks or beyond, depending on orientation of eyestalks. Carapace normal, its disto-lateral edges and its caudo-lateral lobes well-rounded. Carapace leaves ultimate 1.5 thoracomeres dorsally exposed. Median segment of mandibular palp 2.5–2.7 times as long as its maximum width. Flagellum of thoracic exopods 6–7 with nine segments, flagellum 8 with eight segments; all remaining exopods and endopods 3–8 broken.

Pleon (Fig. 8A). Pleonites 1–5 are 0.8, 0.7, 0.6, 0.6 and 0.5 times length of pleonite 6, respectively. Pleopods as in diagnosis (Fig. 13C–E). Exopod of uropods extends 31% its length beyond telson. Both endopods with broken tip. Slender, about equally-spaced spines along remainder of endopods; potential spines near statocyst not visible without dissection. Statolith diameter 0.27 mm.

Telson trapezoid, 1.2 times length of ultimate pleonite, 1.9 times as long as maximum width near basis. Right margin of cleft lined by eleven laminae, amongst which ten distal laminae short, subequal. Bottom of cleft with three larger laminae, i.e. median lamina flanked by two submedian laminae (including the proximal one on right margin). Left disto-lateral lobe of telson distally broken. Corresponding right lobe triangular with narrowly truncate apex armed with two spines, the mesial spine 2/3 length of the lateral spine. Right lateral margin of telson almost straight, with total of 35 spines. Basal portion of both margins with 7–8 spines in continuous series, followed by unarmed stretch, median portion with 3–4 spines increasing in length distally; this series distally continued by discontinuous series of large spines with small spines in between, in the right margin up to the tip, left margin distally broken.

Figure 8. 

Holotype of Mysidetes illigi, subadult female BL 12.7 mm A body, lateral, most thoracic endopods broken (specimen artificially separated from background) B anterior body region, dorsal, cuticle lifted from cornea as fixation artefact C cephalic region, ventral D detail of (C) showing distal margin of right antennular trunk, arrows point to mid-dorsal lobe (female antennular lobe = derivate of appendix masculina).

First description of adult females; all features as given in diagnosis. General appearance moderately slender, body length 14.2–18.4 mm (n = 8). Rostrum measures 3–4% body length, thorax 33–34%, pleon 48–49%, telson 14–16% and carapace, including rostrum, 29–32%. Pleon (without telson) contributes 54–59% to trunk length. Frons with 4–5 vertically stacked, horizontal bulges, these ranging from subrostral process (bulge) ventrally down to that from antennular symphysis.

Figure 9. 

Mysidetes illigi from ice cave of Bernard Island, Antarctica. Adult females BL 18.1 mm (A, C–E), 18.4 mm (B) A head, lateral B anterior body region, dorsal, dashed line enhances the anterior contour of carapace C paired circular structures symmetrically arranged in front of posterior margin of carapace D series of cuticle structures parallel to lateral margin of carapace E tail fan, lateral.

Carapace (Figs 9B–D, 10D) with normal gross structure, no apparent sexual dimorphism. Rostrum covering basal portions of normally orientated eyestalks, reaching at most to distal margin of artificially straight forward-orientated eyestalks (without cornea). Carapace with disto-lateral edges and caudo-lateral lobes well-rounded. Terminal margin leaving ultimate 1.5–2.5 thoracomeres mid-dorsally exposed. Cervical sulcus strong, cardial sulcus feebly developed. Group of about 30 pores (as in Fig. 17E) about 1/9 carapace length in front of cervical sulcus and transverse series of about 40 pores (as in Fig. 17F) along cardial sulcus (Fig. 10D shows fewer pores due to limited graphical resolution). An unusual set of cuticle structures is present: two pairs of circular structures (Fig. 9C) symmetrically arranged in front of the posterior margin (Fig. 10D). Cuticle sculptured by minute depressions with crescent-shaped margins (Fig. 9D), loosely and irregularly arranged in transverse series behind fold delimiting up-tilted anterior portion of carapace, behind cardial sulcus; series also extending short distance along posterior 2/3 of lateral margins (sculptures omitted in Fig. 10D). Outer surface of carapace smooth, except for the here-described structures.

Eyes (Fig. 9A, B). Eyestalks and cornea dorsoventrally not or only slightly compressed. In dorsal view, cornea appears calotte-shaped, in lateral view oviform to spherical. Stalk mesially near basis with hispid bulge, remaining (= major) portions with smooth cuticle.

Antennulae (Fig. 10A, B). Trunk measures 8–9% body length extending 0.3–0.5 times its length beyond eyes and is 2.2–3.2 times longer than its maximum width. Segments 1–3 measure 46–48%, 18–19% and 34–37% length of antennular trunk, respectively. Lateral lobe from basal segment extends beyond median segment. Median segment with its mesial face not inflated. Terminal segment 0.7–0.9 times as long as wide. Its mid-dorsal apophysis with 3–4 barbed setae, with small cilia lining the disto-mesial margin; no spiniform anterior projection. Lateral antennular flagellum in adult females 1.2–1.4 times width of the mesial flagellum when measured near basis of flagella.

Antennae (Fig. 10C). Sympod caudally with bulbous lobe containing end sac of antennal gland. The three-segmented peduncle with basal segment 20–23% peduncle length, second 43–46% and third 33–36%, respectively. Third segment 1.1–1.6 times as long as wide. Antennal scale with convex mesial margin; lateral margin slightly sigmoid, almost straight. Small apical segment with five plumose setae.

Mandibles (Fig. 11B). Segments 1–3 contribute 9–12%, 56–64% and 26–32%, respectively, to total length of three-segmented palp. Proximal segment of palp without setae. Median segment 2.8–3.5 times as long as maximum width, both margins setose all along. Terminal segment strongly setose along mesial margin; distal 3/4 in addition with series of shorter setae on rostral face. Left mandible essentially normal, right mandible with modified teeth on pars centralis. Pars incisiva of left mandible with 4–5 large teeth plus a few very small teeth, its digitus mobilis strong, with 3–4 teeth and its pars centralis with four separate, spiny teeth. Pars incisiva of right mandible with 4–5 large teeth, digitus mobilis small with one large and 5–6 very small teeth, pars centralis distally with two separate spiny teeth and proximally with 5–7 acute teeth projecting from a common basis. Pars molaris with well-developed grinding surface in both mandibles; part of grinding lamellae with minute teeth.

Figure 10. 

Mysidetes illigi from ice cave of Bernard Island, Antarctica. Adult females BL 17.9 mm (A) 18.1 mm (B, D, K) 17.3 mm (C), 18.4 mm (E–J). A right antennula, dorsal B distal margin of left antennular trunk, ventral, arrow points to mid-dorsal lobe (derivate of appendix masculina), flagellae omitted C antenna with antennal gland, dorsal, setae omitted from antennal scale D carapace expanded on slide E ‘tarsus’ of thoracic endopod 1, caudal, setae omitted, detail (F) shows claw G–K series of tarsus 2–5, 8, setae omitted.

Labrum and labium (Fig. 11A, C). Caudal face of labrum with field of small, stiff bristles to left and with rugged, spiny area, also with stiff bristles to right. Densely setose field on posterior half of oral (= dorsal) face. Labium normal, comprising two hairy lobes with short, dense set of stiff bristles on distal half of mesial face.

Maxillula (Fig. 11D). Distal segment of maxillula terminally with 11–15 strong spines, most of which are serrated by small denticles in median portions. No such denticles on the largest spines in innermost (mesial) position, weak or no denticles on the spine in outermost (lateral) position. Distal segment subterminally with 7–9 barbed setae, furnished with comparatively long barbs along their median third and minute barbs in comb-like arrangement along distal third; about 8–11 pores beneath basis of outermost seta. Endite of maxillula terminally with three distally-spiny setae, flanked by 2–4 proximally thick barbed setae; mesial and lateral margins of endite with numerous less thick setae; innermost (mesial) seta longest, projecting mesially.

Maxilla (Fig. 11E) normal, densely setose, with various types of setae, but no spines or teeth. Mesial margin of sympod with 1 (2) basally thick seta, barbed in central portions, microserrated by minute stiff bristles along distal third; slightly or not extending beyond dense brush of barbed setae. Terminal segment of endopod plus sympod and all three large endites of sympod, with densely setose distal margins. Basal segment of endopod with three basally thick, barbed setae (below drawing plane in Fig. 11E). Terminal segment 1.4–1.7 times longer than wide. The setae along its lateral margin slender, sparsely barbed near basis, not resembling spines. Leaf-like exopod extends shortly beyond the distal margin of basal segment of endopod. Exopod with 32–37 barbed setae all along lateral margin, the subapical setae on inner margin longest, the neighbouring apical seta second longest, both longer and thicker than the remaining ones (all located on outer margin).

Foregut (Fig. 12). Lateralia mostly covered by smooth acute spines, apically pronged spines (Fig. 12E) and fewer apically forked spines (Fig. 12D), the two latter spine-types with minute spinules along their shaft. Posterior part of lateralia on each side of foregut with lobe bearing dense set of 5–7 unilaterally serrated spines (Fig. 12C). Dorsolateral infoldings on each side with 5–6 spines decreasing in length dorsally-medially, unilaterally serrated in median to subapical portions (Fig. 12B). Foregut not covered by pigment bodies.

Thoracic sternites. Sternite 1 anteriorly produced into an anterior lobe contributing to the caudal closure of the mouth field as usual in Mysidae. Pair of comparatively large barbed setae on intersegmental joint between thoracic sternite 2 and sympod 2. No such setae on sternites 1 and 3–8.

Thoracopods general (Figs 10E–K, 13A). Length increasing from exopod 1 to 3, remaining subequal from 3 to 7 and decreasing from 7 to 8. Length of flagella 1.8–2.3 times length of basal plates (Fig. 13A). Exopods with basal plates laterally expanded; length of plates 1.9–2.3 times maximum width. Disto-lateral edge of plates angular, tip rounded to varying degrees. Endopods 5–8 long and slender. Ischium becomes increasingly slender from endopods 1 to 5. Length of ischium increasing from endopods 1 to 5, remaining subequal amongst endopods 5–8. Ischium shorter than merus in endopods 1–4, but longer than merus in endopods 5–8. Dactyli of endopods 1–2 larger than those of endopods 3–4, these latter larger than those of 5–8. Claw 1 strong, weakly bent; claw 2 not developed; claws 3–4 needle-like and shorter than claw 1; claws 5–8 distinctly or indistinctly curved, shorter than claw 3. The first thoracopods with large, leaf-like, smooth epipod.

Figure 11. 

Mysidetes illigi from ice cave of Bernard Island, Antarctica. Adult females BL 18.4 mm (A, C, E), 18.1 mm (B, D) A labrum, ventral aspect B mandibles with right palp, rostral C labium D maxillula, caudal E maxilla, caudal.

Maxillipeds. Coxa of maxilliped 1 (thoracic endopod 1) with small endite bearing one barbed seta at its tip. This seta extends across mid-line, thus setae from left and right endite slightly overlapping. Basis with large, prominent endite densely setose on mesial margin. Ischium and merus each with one smaller, but distinct, medially setose endite. Basis of maxilliped 2 (endopod 2) with setose, medially projecting endite. Combined praeischium plus ischium 0.6–0.7 times merus length. Combined carpopropodus plus dactylus measure 1.2–1.3 times merus. Dactylus very large, with dense brush formed by large numbers of normal setae and 14–19 modified setae, the latter apically bent, bearing two symmetrical series of denticles (stiff barbs) on either side in sub-basal to median portions.

Figure 12. 

Cardiac portion of foregut in Mysidetes illigi from ice cave of Bernard Island, Antarctica. Adult female BL 18.1 mm A foregut in dorsal view, food removed from right half, lower-case labels indicate dorsolateral infoldings (di), lateralia (la), mid-gut (mg) and storage space (sp) B spine group from dorsolateral infoldings C spinose lobe of posterior part of lateralia D, E spines from median portions of lateralia.

Marsupium. Thoracopods 7 and 8 with large oostegites 1, 2, respectively. Each oostegite without setae on upper (dorsal) margin. Ventral margin and part of posterior margin, from sub-basal region up to rounded tip, with dense series of plumose setae, together with bilaterally opposite oostegite forming gate contributing to ventral and caudal closure of marsupium. Basal portions of marsupium inside with comparatively long setae, microserrated on their distal half. Oostegite 1 near basis with about 20 microserrated setae, oostegite 2 with about 8–10. No setae on outer face of marsupium. Thoracopod 6 with rudimentary oostegite (Fig. 13A) represented by small, rounded, smooth lobe bearing 10–13 smooth setae (n = 2) on terminal margin. This rudiment not contributing to wall of brood chamber.

Pleon (Fig. 13C–E). Pleonites 1–5 are 0.6, 0.5–0.7, 0.5–0.6, 0.6–0.7 and 0.6–0.7 times the length of pleonite 6, respectively; thus combined pleonites 4, 5 longer than pleonite 6. No pores found on tergites. Length and slenderness of exopodal portion increasing from first to fifth pleopods. By contrast, thickness of exopodal portion and length of endopodal portion decreasing in this direction (Fig. 13C–E). Scutellum paracaudale subtriangular, terminally well rounded.

Tail fan (Fig. 13H, I). Exopod of uropods 1.3–1.5 times length of endopod and 1.4–2.0 times telson, endopod 1.0–1.3 times telson. Exopod extends 0.2–0.4 times its length beyond endopod and 0.3–0.8 times beyond telson, endopod 0.1–0.3 times its length beyond telson (partly due to telson inserting more rostrally). Exopod of uropods with slightly sigmoid, almost straight lateral margin and clearly convex mesial margin. Endopod with proximal four spines discontinuously increasing in length distally; distally followed by 6–9 longer and more slender spines, subequal amongst each other. Endopod basally with large statocyst containing one egg-shaped, irregularly-discoid statolith with partly moruloid surface, diameter 208–213 µm, height 90–98 µm (n = 6 statoliths from four specimens). Statoliths discoidal, composed of the mineral fluorite. Statolith formula (3–4) + (1–2) + (4–7) + (6–9) + (4–9) = 19–25. Telson (Fig. 13I) 1.2–1.4 times length of ultimate pleonite. Its lateral margins slightly sigmoid, almost straight.

Immature males are recognised by knob-like appendix masculina with setae bases present, but not yet bearing setae (Fig. 25A). Subadult males by appendix up to half the length of terminal segment of antennular trunk, in part with short setae (Fig. 25B). Penes (Fig. 13B) slender, large, already reaching to thoracic sternites 4–5 in immature males (body size 12–13 mm, n = 2), to sternite 4 up to the maxillula in subadults (11–15 mm, n = 8). No spermatozoa seen inside penes. Pleopods of subadult males (Fig. 13F, G) with same structure and almost same size as in adult females (Fig. 13C–E). No adult males available.

Five adult females (♀0–) and five subadult males inspected in this respect with 30–70% foregut volume filled with largely masticated organic material (debris) plus varying amounts of mineral particles; additional three females (♀0–) with empty foregut. Abundant detritus and mineral particles are visible in Fig. 12A (content artificially removed from the right half of this foregut).

Live colour was documented in the laboratory (Fig. 7A, B) and in the field (Fig. 7C). Eyestalks, carapace, posterior half of pleomeres and telson densely covered by red pigment spots. Ovarian tubes and brood pouch content red; cornea orange to brown. The animals appear fully red upon ‘expanded’ chromatophores. Many specimens as in Fig. 7C show red cephalothorax and tail fan, but transparent pleomeres 1–5, suggesting a differential ‘expansion’ of chromatophores as also found in many other Mysidae species. The mysids swam several centimetres to several metres away from the substrate, in part within and close to swarms of early stages (Fig. 7C, D) of the nothotheniid fish Pa. borchgrevinki (identification R. Causse, MNHN Paris).

Figure 13. 

Mysidetes illigi from ice cave of Bernard Island, Antarctica. Adult females BL 18.1 mm (A, C–E, H), 18.4 mm (I); subadult males 13.7 mm (B), 13.1 mm (F, G). A thoracopod 6 including rudimentary oostegite B penis of subadult male C–E series of female pleopods 1, 3, 5 F, G pleopods 4, 5 in subadult male H uropods dorsal, setae omitted I telson. C–G, many setae broken.

First described from samples below ice at the type locality by monotypy, this is Gauss Station, 66°02'S, 89°38'E, coast of East Antarctica. Data of Zimmer (1914) and Lüdecke (2013) combined and refined by present authors: Gauss Station is the ‘Winterstation’ of the ‘Deutsche Südpolar-Expedition 1901–1903’ about 85 km north of the continental coast, where the research vessel ‘Gauß’ was locked in ice and drifting with ice for almost one year. Locality with perennial ice cover, except for transient breaks, fissures and holes. The respective sample was taken in 1902 during the austral summer through an artificial hole in the ice, sampling depth from 200–0 m, bottom depth 385 m.

Our findings are the second published with the original name, obtained upon two diving excursions to an ice cave of Bernard Island, in 6–10 m depth at 66°39.64'S, 140°01.55'E; this is also at the coast of East Antarctica. It is unclear whether and from where this species previously might have been reported as M. posthon. The latter taxon was considered the senior synonym of the present species for almost a century, 1923–2021; the taxon M. illigi is now reinstalled.

Jar (ZMB 18283) labelled “Mysidetes hanseni Zimmer. Typus. Gauß-Station, leg. D.S.P. Exp.” contains two vials each with one specimen preserved in ethanol. All types sampled (S#9–10) at Gauss Station, 66°02'S, 89°38'E, coast of East Antarctica. For prerequisites of lectotype designation, see Discussion.

Lectotype by present designation (Fig. 15). Adult male BL 18.6 mm (ZMB 18283a), vial inside labelled as “D. Südpol.-Exp. 21.12.02 vertikal 200 m. Mysidetes hanseni Typ”; S#9.

Paralectotype. Immature male BL 8.7 mm (ZMB 18283b), vial inside labelled “D. Südpol.-Exp. 22.12.02 vert. 250 m. Mysidetes hanseni”; S#10.

An additional [transl.] “younger male specimen” reported by Zimmer (1914) is not in the ZMB collection.

Total of four samples (S#5–8) taken by P. Chevaldonné and S. Hourdez upon diving in austral summer 2017–2018 in ice caves at the coasts of Curie and Damiers Islands, near Dumont d’Urville Station, Adélie Land, Antarctica:

Six incubating females (♀♀B–) BL 19.3–22.8 mm, 1 ♀0– 19.3 mm, 2 ♂♂A 20.5–22.2 mm, 3 ♂♂S 13.3–17.8 mm, 12 ♂♂I 7.5–12.0 mm (in vials, NHMW 27302, SMF-57649, ZMB 34484, ZMH-K-60868) and 1 ♀B– 23.4 mm (on slides, NHMW 27303), Curie Islands, S#5; 5 ♀♀B– 22.0–22.5 mm, 1 ♀S– 20.5 mm, 1 ♂S 18.9 mm (in vials, SMF-57650, ZMH-K-60869) and 1 ♂A 24.7 mm (on slides, NHMW 27304), S#7; 13 ♀♀B– 13.5–24.2 mm, 2 ♀♀0– 16.6–17.8 mm, 5 ♂♂A 17.3–18.1 mm, 2 ♀♀S⁺ 18.8–21.3 mm, 1 ♀S– 19.3 mm, 4 ♂♂S 10.9–13.1 mm, 3 ♂♂I 11.5–12.1 mm, 2 juv. 7.7–7.9 mm (in vials, NHMW 27305, SMF-57651, ZMB 34485, ZMH-K-60870), S#6; 17 ♀♀B– 10.5–22.1 mm, 6 ♀♀0– 17.8–25.7 mm, 3 ♀♀S⁺ 17.2–21.1 mm, 6 ♂♂S 12.3–14.2 mm, 10 ♂♂I 9.2–13.4 mm, 1 ♀I 7.9 mm (in vials, NHMW 27306, SMF-57652, ZMB 34495, ZMH-K-60871), Damiers Islands, S#8.

Diagnosis covers adults of both sexes. Eyes (Figs 15C, 16A, B) well-developed, clearly longer than wide. Cornea roughly calotte-shaped with or without indentation of proximal margin, its length 0.5–0.7 times length of conical eyestalk, diameter 1.0–1.4 times length of terminal segment of antennular trunk. Cornea occupies distal third to half of eye surface. Eyestalk without papilla; length 0.9–1.2 times its maximum width at conjunction with cornea. Rostrum (Figs 15C, 16B, 17D) subtriangular, terminally well-rounded; lateral margins concave (Fig. 17D) to almost straight (Fig. 15C), margins slightly tilted up; 0.3–0.8 times length of terminal segment of antennular trunk (measured along dorsal median line). Antero-lateral edges of carapace well-rounded (Figs 15C, 17D).

Antennae s.l. (Figs 15D, 16C, 17A–C). Appendix masculina (Figs 16C, 17A) strongly setose, measured without setae 0.5–0.8 times as long as terminal segment of antennular trunk, shortly extending beyond anterior margin of this segment. Antennal sympod (Figs 16A, 17C) with one large, acute tooth on disto-lateral edge and, more caudally, an additional shorter tooth. Dorsal face of sympod with lappet-like to triangular lobe, in every case apically rounded. Antennal scale (Fig. 17C) setose all around, apically rounded, two-segmented with apical segment only 2–4% total scale length; scale 4–5 times as long as its maximum width; scale projecting 0.3–0.5 times its length beyond antennular trunk (0.2–0.4 in subadults) and 0.4–0.6 times beyond antennal peduncle.

Thorax (Figs 15, 16C, 17G–K, 19A, B). Right mandible with digitus mobilis and pars centralis modified as in Fig. 11B; remaining mouthparts normal; labrum not produced rostrally; maxilla without spines. Thorax without mid-sternal processes in both sexes. Flagella of thoracic exopods 1 and 8 with eight segments (Fig. 19A), flagella 2–7 with nine segments. Carpopropodites of thoracic endopods 1–8 with 2, 2, 7–8, 7–8, 9–11, 9–10, 9 and 8–9 segments, respectively. Claw of endopod 1 strong, subapically bilaterally serrated; claws 3–8 (Fig. 17H–K) weak, slender, smooth. Female thoracopods 7 and 8 with large oostegites, thoracopod 6 with rudimentary oostegite. Penes (Fig. 19B) tube-like, stiff, slender, smooth all along, without setae. Size variable in adult males: length 1.5–2.5 times length of ischium 8 and 2–3 times merus 8; penes anteriorly extending to thoracic sternites 2–5.

Figure 14. 

Mysidetes hanseni in its natural habitat inside ice cave of Damiers Islands, Antarctica A adult male, dorsal B incubating female, dorsal C physical aspect of habitat.

Pleopods (Fig. 19C–E) reduced to undivided, bifid, setose plates with comparatively long endopodal portion (pseudobranchial lobe) in both sexes. All pleopods without spines, no modified setae. Uropods (Figs 16D, 19F) entire, slender, setose all around, no spines; exopod extends by 18–29% its length beyond endopod.

Telson (Figs 15B, 19G) trapezoid, length twice maximum width near basis and 5–6 times width shortly above bifid terminus; 0.7–0.9 times exopod of uropod. Each lateral margin armed almost all along with 45–57 small spines. Sub-basal spine-free portion, if any, up to 1/10 telson length in adults (occasionally longer in non-adults). Spines arranged in consecutive sets on distal half; each set represents series of 2–6 spines increasing in length distally. Triangular apical cleft (Fig. 15B) penetrates 1/10 telson length, margins of cleft lined all along with 14–17 laminae. Telson cleft 1.0–1.8 times as deep as its width at apex. Disto-lateral lobes of telson rounded, each lobe terminally armed with 4–5 strong, subequal spines with 4–5% telson length.

The initial objective for inspection of the types was the unclear state of development of male characteristics. Zimmer (1914) indicated the largest specimen examined by him as [transl.] “adult or subadult”. We found a damaged appendix masculina (left arrow in Fig. 15D) with a few setae, apex broken, on the right antennula of the lectotype, suggesting that this appendix was longer in vivo, ergo the lectotype considered adult.

Both available type specimens not dissected. Body proportions (Fig. 15A) slender in both specimens as normal in males of Mysidetes species. Terminal segment of antennular trunk with 3–4 large plumose setae plus a number of smaller barbed setae on disto-mesial corner; additional large plumose seta inserted subterminally on mesial margin in both specimens. Rostrum of both specimens short, terminally broad, with slightly sigmoid, almost straight lateral margins (Fig. 15C).

Lectotype (Fig. 15). Cornea roughly calotte-shaped, dorsally with proximal indentation, length 0.5–0.6 times eyestalk, diameter equals length of terminal segment of antennular trunk. Eyestalk without papilla. Median segment of antennular trunk with its mesial face inflated (right arrow in Fig. 15D), indicative of male adulthood. Antennal scale as in diagnosis, apical segment 4% total scale length; scale five times as long as its maximum width; scale projecting 0.3–0.4 times its length beyond antennular trunk and 0.6 times beyond antennal peduncle (Fig. 15D). Flagella of thoracic exopods 1–6 as in diagnosis, flagella 7–8 broken. Carpopropodites of thoracic endopods 4–8 with 8, 10, 9, 9, and 9 segments, respectively. Claw of endopods 4–8 weak, slender, weakly bent and smooth. Penes reach to sternite 4. Pleopods as normal in the genus; length increases from pleopod 2 to 5; pleopod 1 slightly longer than pleopod 2. Uropods as in diagnosis; exopod extends by 1/5 its length beyond endopod (Fig. 15A). Telson as in diagnosis, length five times width shortly above bifid terminus. Length 1.2 times sixth pleonite, 0.9 times endopod of uropod and 0.7–0.8 times exopod of uropod. Left (undamaged) lateral margin all along with total of about 54 spines. Most proximal portion of each lateral margin with seven crowded spines; sub-basal portion with six subequal spines positioned with lower density in continuous series; median to distal portions with about 41 spines densely arranged in consecutive sets of 2–6 spines increasing in length distally. Apical cleft penetrates by 9% telson length. Margins of cleft (Fig. 15B) all along with total of 15 laminae increasing in size distally; largest lamina with 2/5 cleft length. Disto-lateral lobes as in diagnosis; terminal spines longer than subterminal spines.

Figure 15. 

Lectotype of Mysidetes hanseni Zimmer, 1914, adult male BL 18.6 mm A body, lateral B terminal fifth of telson C anterior body region, dorsal D anterior body region, obliquely ventral, left arrow points to remnant of broken appendix masculina, right arrow to mesial swelling of median segment of antennular trunk. A, B, objects artificially separated from background.

Paralectotype. Median segment of antennular trunk not inflated as normal in immatures. Penes reaching to sternite 6. Telson conforming well to that of lectotype, taking differences due to body size into account: right lateral margin with total of 35 spines, ten of which in approximately linear arrangement along basal and sub-basal portions, remaining spines more densely set along median to apical portions, arranged in groups as in lectotype. Apical cleft 10% telson length; numbers and relative size of laminae as in lectotype.

Lectotype with well-pigmented dark cornea (Fig. 15) and large dark-brown patches on the body, the latter often observed as artefacts in century-old preserved material. By contrast, the paralectotype is completely bleached, cornea included. This suggests that the two specimens experienced different treatments before being placed in ethanol.

For evaluation of differences between description by Zimmer (1914) and type specimens, see Discussion.

Includes re-description of males and first description of females. All features of the above diagnosis. General appearance of females moderately slender (not considering the marsupium), males even more slender. Body length of adult females 10.5–25.7 mm (n = 52), males 17.3–24.7 mm (n = 8). Rostrum comprising 1–3% body length, cephalothorax 32–39%, pleon 47–53%, telson 14–15% and carapace 26–31%. Frons with at least four horizontal bulges (Fig. 16A; potential additional bulges not well verified).

Carapace (Fig. 17D) with normal gross structure, without apparent sexual dimorphism. Rostrum covering basal portions of normally orientated eyestalks, reaching at most to middle of artificially straight forward-orientated eyestalks (without cornea). Antero-lateral edges of carapace well rounded, not visibly projecting in situ, whereas weakly projecting in artificially expanded carapace. Posterior margin of carapace evenly rounded, mid-caudally well emarginated, leaving ultimate 1–1.5 thoracomeres dorsally exposed. Cervical sulcus strong, cardial sulcus indistinct. Median field of 44–59 crowded pores (Fig. 17E) directly in front of cervical sulcus. Transverse series of 46–81 pores (Fig. 17F) crossing carapace between, if present, cardial sulcus and posterior margin. Except for the here-stated structures, outer surface of carapace smooth in both sexes.

Eyes (Fig. 16A–C). Eyestalks and cornea dorsoventrally (very) weakly compressed (Fig. 16C). In dorsal view, cornea appearing calotte-shaped, in lateral view, oviform with upper margin (= face) slightly flattened.

Antennulae (Fig. 17A, B). Trunk measures 7–9% body length in both sexes, extending 0.4–0.5 times its length beyond eyes, being 1.6–2.1 times longer than maximum width in adult males, 2.2–2.8 in adult females. Measured along dorsal midline, basal segment 42–47% trunk length, median 18–20% and terminal 33–38%. Lateral lobe from basal segment extending beyond median segment. Median segment with its mesial face inflated in adult males only. Terminal segment 0.6–0.9 times as long as wide. Part of terminal segment with cuticle sculptured by minute depressions in males only; due to their small size and density, these depressions drawn as reduced numbers of dots with exaggerated size in Fig. 17A. Details of depressions available in Fig. 16E for oostegite 2. Antennulae of females (Fig. 17B) dorsally with smooth cuticle, not sculptured by minute depressions. Terminal segment of antennular trunk in both sexes with mid-dorsal apophysis bearing four barbed setae on its lateral half and small cilia along its disto-mesial margin; no spiniform anterior projection. Lateral antennular flagellum about as wide as mesial one when measured near basis.

Figure 16. 

Mysidetes hanseni from ice cave of Curie Islands, Antarctica. Adult females BL 23.4 mm (A, B), 21.4 mm (E); adult male 24.7 mm (C, D). A head of female, obliquely lateral B anterior body region of female, dorsal, dashed line enhances the anterior contour of carapace C anterior body region of male, lateral D tail fan, lateral E cuticle structures on outer surface of the large second oostegite.

Antennae (Fig. 17C). Sympod dorsally with terminally rounded, tongue-like process; caudally with bulbous lobe containing end sac of antennal gland. Three-segmented antennal peduncle in both sexes with basal segment 22–25% peduncle length, second 36–43% and third 32–36%. Third segment 1.1–1.4 times as long as wide. Antennal scale with convex mesial margin; proximal half of lateral margin slightly sigmoid, distal half convex. Small apical segment with five plumose setae.

Figure 17. 

Mysidetes hanseni from ice cave of Curie Islands, Antarctica. Adult male BL 24.7 mm (A, C–F, I, J), adult female 23.4 mm (B, G, H, K–M) A right male antennula, dorsal B distal margin of left female antennular trunk, ventral, flagella omitted C antenna with antennal gland, dorsal, setae omitted from antennal scale D carapace expanded on slide, details show cervical (E) and cardial (F) pore groups G–K tarsus in series of thoracic endopods 2–5, 8, setae omitted L tergite expanded on slide together with pleurites of pleomere 1, setae of pleopods omitted, detail (M) shows transverse pore groups. E, F, M, pore diameters not to scale.

Mandibles. Segments 1–3 contributing 11–14%, 53–60% and 29–33% length to three-segmented palp. Proximal segment without setae. Median segment 2.7–3.3 times as long as its maximum width, both margins setose all along. Terminal segment strongly setose along mesial margin; distal 2/3 in addition with series of short setae on caudal face near lateral margin. Pars incisiva with 4–5 teeth. Left mandible normal, its digitus mobilis strong, with 3–4 teeth and its pars centralis with 3–4 separate, spiny teeth. Right mandible modified as in M. illigi (Fig. 11B), its digitus mobilis small with one large and 3–4 very small teeth, pars centralis distally with one thick spiny tooth and proximally with 3–5 acute teeth projecting from a common basis. Pars molaris with well-developed grinding surface in both mandibles.

Labrum and labium as described above for M. illigi.

Maxillula. Distal segment of maxillula terminally with 12–14 strong spines, in part serrated by small denticles in median portions. No such denticles on the largest spine in innermost (mesial) position. Distal segment subterminally with 8–9 barbed setae, of which 7–8 setae densely set in transverse, linear series; 0–2 pore near outermost seta; the remaining 1–2 setae positioned a short distance proximally, out of series. All these setae with barb patterns as in M. illigi. Endite of maxillula terminally with three distally spiny setae accompanied by four proximally thick barbed setae; mesial and lateral margins of endite with numerous less thick setae; innermost seta not longest and not projecting mesially as in M. illigi.

Maxilla normal, densely setose, with various types of setae, but no spines or teeth. Terminal segment of endopod and sympod including its three large endites, with densely setose distal margins. The leaf-like exopod extends to distal margin of basal segment of endopod or shortly beyond. Exopod with 22–25 barbed setae all along lateral margin, the two apical setae longer and thicker than the remaining ones. Basal segment of endopod with three basally thick, barbed setae. Terminal segment 1.3–1.5 times longer than wide; setae along its lateral margin resembling acute spines, but characterised as modified setae rather than spines based on the densely barbed basal half. Mesial margin of sympod with long seta micro-serrated by minute stiff bristles along its distal third; this seta extending beyond dense brush of plumose setae.

Foregut (Fig. 18). Spines on most of the lateralia as in M. illigi, except for modified spines in Figs 18B and C. Posterior part of lateralia on each side of foregut with lobe bearing dense set of 10–14 bilaterally serrated spines (Fig. 18C). Dorsolateral infoldings on each side with two strong spines, unilaterally serrated in median to subapical portions (Fig. 18B). Dorsal and rostral portions of foregut furnished outside with large numbers of pigment bodies.

Thoracic sternites as described above for M. illigi.

Thoracopods general (Figs 17G–K, 19A). Exopods with variable length of flagella and basal plates, no clear size trend along series of exopods 2–7; exopod 1 shorter in both sexes; exopod 8 shorter in females, variable in males. Length of flagella 1.3–2.1 times length of basal plates. Basal plates laterally expanded, length 1.2–2.2 times width. Disto-lateral edge of plates rounded. Endopods becoming longer and more slender from endopod 1 to 5 and decreasing slightly from 5 to 8. Endopods 5–7 long and slender. Ischium becoming increasingly slender from endopods 1 to 5. Length of ischium increasing from endopods 1 to 5 and remaining subequal amongst endopods 5–8. Ischium shorter than merus in endopods 1–4, but longer than merus in endopods 5–8. Dactyli of endopods 1–3 larger than those of endopods 4–8. Claw 1 strong, weakly bent; claw 2 not developed; claws 3–4 slightly bent, equally long; claws 5–8 well or indistinctly curved, shorter than claws 3–4. First thoracopods with large, leaf-like, smooth epipod.

Figure 18. 

Cardiac portion of foregut in Mysidetes hanseni from ice cave of Curie Islands, Antarctica; pyloric parts removed. Adult female BL 23.4 mm. A foregut in slightly oblique lateral view, lower-case labels indicate dorsolateral infoldings (di), lateralia (la), mid-gut (mg), esophagus (oe), pigment bodies (pb), and storage space (sp) B spine from dorsolateral infoldings C spinose lobe of posterior part of lateralia D, E spines from anterior parts of lateralia.

Maxillipeds. Combined praeischium plus ischium of maxilliped 2 are 0.8–0.9 times merus length. Dactylus with large numbers of normal setae and 14–17 setae modified as in M. illigi. Remaining features as described above for M. illigi.

Marsupium (Fig. 16E). Essentially as described above for M. illigi, except for setae numbers and cuticle structure. Oostegite 1 near basis with about 30 micro-serrated setae, oostegite 2 with 9–12. Large oostegite 2 with cuticle sculptured by minute depressions over most of its outer surface. These structures resembling scales in episcopic view, but clearly identifiable as depressions in tangential view (both views in Fig. 16E), visible in situ already with 15× episcopic inspection. Oostegite 1 with a narrow ribbon of such structures along and close to upper margin, but most of its surface with smooth cuticle, not considering setae. Thoracopod 6 with rudimentary oostegite bearing 10–13 (n = 3) apically microserrated setae.

Penes (Fig. 19B) anteriorly bent at basis. Shaft terminally slightly widened, blunt, ending in 2–3 indistinct lobes. Penes extend anteriorly to thoracic sternites 6–7 in immatures (n = 25), to sternites 3–7 in subadults (n = 14) and to sternites 2–5 (mainly sternite 4) in adults (n = 7).

Pleon (Figs 17L, M, 19C–E). Pleonites 1–5 measure 0.6–0.7, 0.6–0.8, 0.6–0.7, 0.6–0.7 and 0.6–0.7 times the length of pleonite 6, respectively, i.e. combined pleonites 4 and 5 exceed pleonite 6. Tergites 1–7 with transverse linear series of various numbers of pores as in Figs 17L, M. Pleopod structure as described above for M. illigi. Pleopods of about same size in both sexes. Length decreasing from pleopod 1 to pleopod 2, remaining subequal amongst 2 and 3 and increasing from 3 to 5. Exopodal portion of pleopod 1 wider than in pleopods 2–5. Its length ranges between that of pleopods 3 and 4. Scutellum paracaudale forming a large acute triangle with slightly concave margins.

Uropods (Figs 16D, 19F). Length of exopod 1.1–1.4 times endopod and 1.1–1.4 times telson, endopod 0.9–1.0 times telson. Exopod extending 0.2–0.3 times its length beyond endopod and 0.2–0.3 times its length beyond telson, endopod 0.1–0.2 times its length beyond telson (partly due to telson inserting more rostrally). Exopod with slightly sigmoid, almost straight lateral margin and clearly convex mesial margin. Endopod basally with large statocyst containing one statolith with diameter of 178–227 µm (n = 8 statoliths from four specimens). Statoliths discoidal, composed of the mineral fluorite. Statolith formula 3 + 1 + (4–7) + (6–7) + (5–9) = 19–23.

Telson (Fig. 19G). Length 1.2–1.3 times length of ultimate pleonite. Basal portion of lateral margins with linear series (rather than aggregated) of 2–5 spines in immatures with 9 mm body length (n = 3) and in three subadults with 11 mm length (n = 3); spine-free sub-basal portion 5–13% of telson length in immatures, 5–17% in subadults and 0–10% in adults (n = 10). Most proximal portion of each lateral margin with 3–7 crowded spines in adults; sub-basal spine-free portion, if any, distally followed by 4–8 subequal spines positioned in a nearly continuous series; median to terminal portions with 31–46 spines densely arranged in consecutive sets of 2–6 spines increasing in length distally.

Larvae (Fig. 20). Nauplioids at substages N2 and N3 more slender than in Heteromysis S.I. Smith, 1873 species, for example (Wittmann and Abed-Navandi 2021). Twenty-one mounted nauplioid larvae with smooth cuticle, except for antennula, antenna and distal portions of abdomen. Antennae 1 and 2 sparsely covered with minute hairs over distal 2/3 of their length. Density of hairs increases up to tip (Fig. 20C). The old cuticle has started to separate from the tip of the antennula in Fig. 20C, therefore appearing flabby there. Antennula and antenna not yet bifid (Fig. 20B) in all N2-larvae and in most N3-larvae examined. The most striking features of the nauplioids are a pair of long cerci (Fig. 20D), together forming a comparatively large caudal furca armed by numerous spine-like setae. Such spiny setae, together with tiny hairs (as on antennae), are also present on (sub)-apical portions of abdomen. Remaining features in Fig. 20 are typical for the state of development.

Figure 19. 

Mysidetes hanseni from ice cave of Curie Islands, Antarctica. Adult male BL 24.7 mm. A thoracopod 8, rostral B penis C–E series of male pleopods 1, 3, 5, caudal (C, D) and rostral (E) face, many setae broken F uropods, dorsal, setae omitted G telson.

Type locality is at the East-Antarctic coast, 66°02'S, 89°38'E (details as given above for M. illigi). The types only there were taken in December 1902 with non-closing vertical hauls from 200–0 m (lectotype) and 250–0 m (paralectotype) below ice, bottom depth 385 m (Zimmer 1914). The present records from ice caves in 2–5 m depth at Curie Islands, 66°38.64'S, 140°02.43'E and in 2 m depth at Damiers Islands, 66°39.21'S, 139°57.61'E, are from the second and third localities ever published; see also Discussion.

Live colour of this species was documented only in the field (Fig. 14A, B). Most specimens showed a whitish tinge of body, eyes, hepatic caeca and brood pouch content. Oil globules (fat bodies) also contributed to the whitish tinge. Globules were found everywhere in the body trunk, with greatest densities above the foregut and in the telson. Comparatively small numbers of red-orange pigment spots were present on eyestalks, carapace, pleomeres, and telson. These were slightly ‘expanded’ mostly on telson, partly also eyestalks. A few specimens (not documented in Fig. 14) showed an overall, weakly red to orange tinge. Corneas appeared white in the field, but were brown in ethanol-fixed materials; therefore, it is not excluded that reflection had contributed to the white tinge in field photos. The mysids were encountered close to and mostly in physical contact with the substrate ice or rock surfaces, with or without epigrowth (Fig. 14A, B).

Upon external inspection of 49 foreguts, all appeared empty in twenty incubating females examined, all in nine spent females available and in eight out of twenty foreguts of immature males. Eight ‘empty’ foreguts dissected and mounted on slides showed that 0–5% volume contained food. Fig. 18 gives an example of a foregut considered ‘empty’ upon external inspection (40´), yet with a few diatoms identified at 200× magnification. The 12 ‘positive’ immature males had 10–40% foregut volume filled. Contents were unidentifiable, masticated organic material (debris), cyanobacteria, diatoms, a few copepod remains and a few mineral particles.

Molecular study of ice cave mysids

Figures 21, 22

Sequencing of Pseudomma kryotroglodytum sp. nov. The 18S DNA sequences obtained from the two here-described specimens of P. kryotroglodytum sp. nov. were identical and 805 bp long. COI sequences of unequal quality were obtained from both specimens and were 614 and 658 bp for individuals 611-1 (paratype) and 612-1 (holotype), respectively. Over the alignable part of these sequences, they differed by only one synonymous position. Only 18S sequences could be compared with GenBank sequences of other Pseudomma species. We aligned our sequences with 18 available GenBank sequences and obtained NJ and ML phylogenetic trees (Fig. 21) of similar topologies (only NJ shown), rooted with Holmesiella affinis Ii, 1937 and bootstrapped (1000 replicates for each method). Not even half of the Pseudomma species described to date are shown in this tree and only one Southern Ocean species (P. sarsii) is available for comparison, but P. kryotroglodytum sp. nov. is molecularly different from every other one in the tree. Bootstrap support is poor for most relevant nodes, but two species appear more closely related to P. kryotroglodytum sp. nov. in this dataset: P. longisquamosum Murano, 1974 and P. maasakii Meland & Brattegard, 2007.

Figure 20. 

Nauplioid larva of Mysidetes hanseni from ice cave of Damiers Islands, Antarctica A larva at late substage N2, lateral B antennae and mouth field, ventral, lower-case labels indicate antennula (a1), antenna (a2), labrum (l) and mandibles (m) with palp (p) C tip of antennula D tip of abdomen with caudal furca, lateral. A, larva artificially separated from background; A–D are from four different specimens.

Sequencing of Mysidetes illigi and M. hanseni. A total of six individuals of M. illigi from Bernard Island and 10 M. hanseni from Damiers and Curie Islands were sequenced at both the COI and 18S loci. No comparison with GenBank was possible because this is the first time Mysidetes sequences are made available. The 18S sequences obtained were 815 and 813 bp long for M. illigi and M. hanseni, respectively. No differences were observed at this 18S fragment within species, whereas there was a 6 bp difference (but no indel) between the two species. COI sequences of variable quality were obtained (658 to 629 bp), of which 629 bp could be aligned. A simple distance tree (NJ) was produced to visualise the differences and similarities amongst sequences (rooted with A. maxima). As evident on Fig. 22, the M. hanseni sequences are quite diverse (each specimen displays a distinct haplotype), but the Damiers and Curie specimens are mixed, indicating that no apparent genetic structuring exists at this locus and this geographical scale (3.8 km). In contrast, the Bernard Island M. illigi sequences cluster in two divergent groups, which cannot be related either to morphological differences or to collection date and station. The divergence between the two groups is ca. 10% – a high value for intraspecific comparisons, but quite low if they were different species. As noted above, 18S is identical between the two M. illigi clusters, as is their translated COI amino acid sequence.