Mysidetes illigi Zimmer, 1914, bona species

Wittmann, Karl J. & Chevaldonne, Pierre, 2021, First report of the order Mysida (Crustacea) in Antarctic marine ice caves, with description of a new species of Pseudomma and investigations on the taxonomy, morphology and life habits of Mysidetes species, ZooKeys 1079, pp. 145-227 : 145

Mysidetes illigi Zimmer, 1914, bona species

Figures 7 , 8 , 9 , 10 , 11 , 12 , 13 , 25A-C

Mysidetes illigi Zimmer, 1914: 404-405, Figs 47-49 in Fig.-Tab. XXVI (first description).

Mysidetes Illigi , Hansen 1921: 5 (proposed validity check).

Mysidetes illigi referred to as synonym of Mysidetes posthon : W.M. Tattersall 1923: 275, 288; Illig 1930: 470, 581; Müller 1993: 164; Mees and Meland 2021: AphiaID = 451694 (unaccepted).

Type series.

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.

Non-types from ice caves.

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.

Diagnosis.

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.

Description of holotype.

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.

Description of adult females from ice caves.

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.

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.

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.

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.

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.

Notes on non-adult males.

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.

Gut contents.

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).

Colour and microdistribution.

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).

Distribution and type locality.

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.

Figures

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.

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).

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.

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.

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.

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.

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.

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.

Figure 25. Surface details of appendix masculina and its female homologue in series of increasing body size for Mysidetes illigi (A-C) and M. hanseni (D) from Antarctic ice caves. A immature male BL 12.1 mm B subadult male 14.7 mm C adult female 18.4 mm D adult male 24.7 mm.