Mysidetes hanseni Zimmer, 1914

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 hanseni Zimmer, 1914

Figures 14 , 15 , 16 , 17 , 18 , 19 , 20 , 25D

Mysidetes hanseni Zimmer, 1914: 403, 404, Figs 43-46 in Fig.-Tab. XXV (first description); Brandt et al. 1998: Tab. 1 (endemism); Price 2001: 111 (in list, distribution); San Vicente 2011a: 53, Tab. 4, Figs 38N, O (diagnosis, in key); Petryashov 2014: map 11 (biogeography); Mees and Meland 2021: AphiaID = 226498 (accepted).

Type series.

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.

Non-type material from ice caves.

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.

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.

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.

Description of types.

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.

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.

Colour.

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.

Description of ice cave specimens.

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.

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.

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.

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.

Distribution and type locality.

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.

Colour and microdistribution.

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 ).

Gut contents.

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.

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.

Figures

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 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.

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.

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.

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 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.

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.

Figure 20. Nauplioid larva of Mysidetes hanseni from ice cave of Damiers Islands, Antarctica A larva at late substage N 2, lateral B antennae and mouth field, ventral, lower-case labels indicate antennula (a 1), antenna (a 2), 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.

Figure 21. Phylogenetic placement of Pseudomma kryotroglodytum sp. nov. from Antarctic ice caves at Bernard Island, amongst the Pseudomma taxa available in DNA databases (GenBank accession numbers shown), based on 18 S rDNA. The root is the Erythropini Holmesiella affinis. Neighbour-Joining (NJ, shown here) and Maximum Likelihood (ML) reconstruction methods gave a similar topology. Bootstrap (1000 replicates) values, higher than 700, are shown at nodes (NJ / ML) in this order.

Figure 22. Distance tree (Neighbour-Joining) of mitochondrial COI sequences of Mysidetes hanseni and Mysidetes illigi specimens collected in Antarctic ice caves at Curie and Damiers Islands, Dumont d'Urville Station, rooted with Antarctomysis maxima from the same area. Most relevant Bootstrap (1000 replicates) values are shown at nodes.

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.