Alonopsis Sars, 1862

Sinev, Artem Y. & Atroschenko, Margarita M., 2011, Revision of the genus Alonopsis Sars, 1862 and its position within Aloninae (Cladocera: Anomopoda: Chydoridae), Zootaxa 2800, pp. 1-17 : 4-9

Genus Alonopsis Sars, 1862

Figures 1–9

Type species. Alona elongata Sars, 1861 .

Emended diagnosis. Female. Body elongated oval, valves and head shield without keel (see Fig. 1 C–D, 6E–F). Adult females frequently retain 2–3 valves from the previous molt. Dorsal, posterior and ventral margins of valves evenly curved, postero-dorsal, postero-ventral angles and anterior-ventral angles of valves broadly rounded. Postero-ventral angle of valves with 0–3 denticles (see Fig. 2 D, 6H), number of denticles varies on valves from different molts and on the left and right valves of the same specimen. Valves with distinctive diagonal lines with horizontal fine striae between them.

Head small, triangle-round in lateral view, in lateral view rostrum short, pointing downward. Eye larger than ocellus. Head shield broad, covered by similar striae as valves, with short, broadly rounded rostrum, posterior margin of head shield broadly rounded (see Fig. 3 C, 7A). Three narrowly connected major head pores of same size. Minute lateral head pores located near the margin of head shield on level of major head pores. Labrum with moderately broad subtriangular keel, keel apex rounded, posterior margin of keel without clusters of setae.

Thorax 1.5 times longer than abdomen. Abdomen with well-developed abdominal joint (see Fig. 4 D).

Postabdomen long and narrow, with parallel margins or, in larger specimens, slightly narrowing distally, length about four times height (see Fig. 3 E–G, 6I –J). Ventral margin weakly convex. Basis of claws bordered from distal margin by clear incision. Distal margin straight, distal angle broadly rounded. Dorsal margin concave in both anal and preanal portions. Distal part of dorsal margin 3–3.5 times longer than preanal one, with preanal portion about 2 times longer than anal. Preanal angle weakly defined, postanal angle not defined. Postanal margin bears up to 15 well-developed, single marginal denticles. 14–20 lateral fascicles of long setules, in distalmost fascicles length of setules exceed the width of base of postabdominal claw. A cluster of long setules located near the base of postabdominal claws. Postabdominal claw long, almost straight, 1.5 times longer than preanal portion of postabdomen, with pecten of 7–9 large spines at the middle. Basal spine about 0.2 claw length, armed with setules.

Antennule protruding beyond the tip of rostrum, with 3 transverse rows of short setules at anterior face (see Fig. 4 B, 7E). Antennular seta thin, of about 1/3 length of antennule, arising at 2/3 distance from the base. One lateral and eight terminal aesthetascs, one of terminal aesthetascs long and thick, about 3/4 length of antennule, other terminal aesthetascs short and thin, about 1/3 length of antennula. Lateral aesthetasc long and thick, about 2/3 length of antennule, located near the base of antennular seta.

Antenna with antennal formula setae 0-0-3/1-1-3, spines 1-0-1/0-0-1 (see Fig. 4 C, 7F). Basal segment robust, branches elongated, all segments narrow, cylindrical, subequal in length. Seta of basal segment of endopodite short, little longer than middle segment. Seta of middle segment of endopodite and apical setae of same morphology, similar length and thickness. Spine of basal segment of exopodite shorter than middle segment. Spines of apical segments long, little shorter than these segments.

Six pairs of thoracic limbs.

Limb I with accessory seta almost as long as ODL seta. IDL with 3 setae: seta 3 longest, subequal to ODL seta; seta 1 quite large, about half-length of the other two; setae 2 and 3 strongly curved, two-segmented, with setules in distal part (see Fig. 4 E–G, 7G–H). Endite 3 with four setae. Endite 2 with three setae, long naked setae and sensillum on anterior face of limb. Endite 1 with two 2-segmented setae, very long naked setae, two times longer than the same seta on endite 2 and sensillum on anterior face of limb. Six rows of long setules on ventral face of limb. Two ejector hooks, one little larger than other. Maxillar process with single seta.

Limb II. Exopodite narrow, elongated, with slender seta. Inner portion of limb with eight scraping spines; scrapers 7 and 8 much thicker than others, armed with strong denticles (see Fig. 4 H–I, 7I). Small naked seta located between near the base of scraper 1, small sensillum between bases of scrapers 3 and 4. Distal armature of gnathobase with four elements. Filter plate II with seven setae, the posteriormost one considerably shorter than others.

Limb III. Epipodite elongated, without any process. Exopodite trapezium-shaped, with seven setae: seta 3 being longest; length of setae 6 and 7 between 1/3 and 2/3 length of seta 3; other setae short (see Fig. 4 J, 7J). Distal endite with three setae: two distalmost ones long, slender, sharp, with long denticles; basalmost seta shorter, geniculated, with thin setules (see Fig. 4 K, 7K–L). Basal endite with four stiff setae. Four soft setae increasing in size basally. Distal armature of gnathobase with four elements: elongated, cylindrical sensillum; thin, geniculated seta; and two short pointed spines. Filter plate III with seven setae.

Limb IV. Pre-epipodite setulated. Epipodite with finger-like process longer than epipodite itself. Exopodite with six setae, seta 3 being longest, setae 1–2 about half length of seta 3, setae 5–6 about 1/3 length of seta 3, and seta 4 very short (see Fig. 4 L–M, 8A–B). Setae 1–2 plumose, seta 3 plumose in basal portion and covered by short setules in distal portion, setae 5–6 with 5–7 long setules in the middle and peculiar fork-like structure at the end. Inner portion of limb IV with four setae and ovoid sensillum (see Fig. 4 N, 8C). Distalmost seta slender, sharp, first flaming torch seta is more robust than two others, armed with short thick denticles instead of setules. A small sensillum located near the base of middle flaming-torch seta. Three soft setae increasing in size basally. Gnathobase with one long 2-segmented seta and a blunt process near it. Filter plate IV with five setae.

Limb V. Pre-epipodite setulated. Epipodite with finger-like process longer than epipodite itself. Exopodite V separated into two lobes, with 4 plumose setae decreasing in size basally (see Fig. 4 O, 8D). Inner limb portion as elongated lobe with parallel margins. At inner face, two short setae with wide bases, one 1.5 times longer than other. Filter plate V absent.

Limb VI consisting in a rounded lobe with four clusters of long setules on margin (see Fig. 4 P, 8E).

Male: body lower than in female, similar to that of juveniles, with dorsal margin almost straight. Postabdomen straight or curved, narrowing distally, spermoducts located at small protrusion above the base of postabdominal claws. Clusters of setules in place of female marginal denticles, lateral fascicles of setules same as in female. Postabdominal claw much shorter than in female, curved, with pecten of about ten narrow pointed spines at the middle, without basal spine. Antennule shorter and wider than in female, with eight lateral and two terminal aesthetascs, male seta located at about 2/3 distance from the base, as long as antennal seta. Limb I with V-shaped copulatory hook. IDL retains seta 1, male seta of moderate size.

Figures

FIGURE 1. Alonopsis elongata (Sars, 1861), Russia, Murmansk Area, Khibiny Mountains, Lake Malyi Vud'yavr: A – B—juvenile females of instar I and II; C – E—adult parthenogenetic females, E—valve; F—ephippial female; G – H—juvenile males of instar I and II; I—adult male. All are lateral views. Scale bars: 0.2 mm for A – D, F – I, 0.1 mm for D.

FIGURE 2. SEM photographs of Alonopsis elongata (Sars, 1861), Russia, Murmansk Area, Khibiny Mountains, Lake Malyi Vud'yavr: A – B—juvenile female of instar I: A—habitus, lateral view, B—head pores; C – H—adult parthenogenetic female: C—habitus, lateral view, D—postroventral corner of valves, E—postabdomen, F—distal denticles of postabdomen, G—postabdominal claws, H—antenna; I – J—adult male: I—habitus, lateral view, J—postabdomen. Scale bars 0.1 mm for A, C, I, 0.05 mm for E, J, 0, 0 2 mm for D, H, 0.01 mm for B, F, G.

FIGURE 3. Alonopsis elongata (Sars, 1861), Russia, Murmansk Area, Khibiny Mountains, Lake Malyi Vud'yavr: A – B—postabdomens of juvenile females, instar I and II; C – F—adult parthenogenetic female: C—head shield, D—abdominal joint, Epostabdomen, F—denticles of postabdomen, G—postabdominal claw; H – I—postabdomens of juvenile males, instar I and II; J—postabdomen of adult male. Scale bars 0.1 mm for A – C, E and D, 0.05 mm for F – J.

FIGURE 4. Alonopsis elongata (Sars, 1861), Russia, Murmansk Area, Khibiny Mountains, Lake Malyi Vud'yavr, рathenogenetic female: A—labrum, B—antennule, C—antenna, D—maxillule, E—limb I, F—ODL and IDL of limb I, Gendites 1 and 2 of limb I, H—limb II, I—exopodite and distal scrapers of limb II, J—exopodite of limb III, K—inner portion of limb III, L—exopodite of limb IV, M—setae 5 – 6 on exopodite of limb IV, N—inner portion of exopodite IV, O—limb V, Plimb IV. Scale bars: 0.1 for A – C, E – F, H, J, L, O – P, 0.05 mm for D, G, I, K, M – N.

FIGURE 5. Alonopsis elongata (Sars, 1861), Russia, Murmansk Area, Khibiny Mountains, Lake Malyi Vud'yavr: A—limb I of juvenile male of instar I; B – C—juvenile male of instar II: B—antennule, C—limb I; D – F—adult male: D—antennule, E – F—limb I, inner and outer view. Scale bar 0.05 mm.

FIGURE 6. Alonopsis aureolata Doolittle, 1912, USA, Maine, Sebago lake: A – B—juvenile female of instar I: A—habitus, lateral view, B—postabdomen; C – D—juvenile female of instar II: C—habitus, lateral view, D—postabdomen; E – H—parthenogenetic female: E – F—habitus, lateral view, G—valve, H—postero-ventral corners of valves, I – J—postabdomen. Scale bars: 0.2 mm for A, C, E – F, 0.1 mm for B, D, G, I – J, 0.05 mm for H.

FIGURE 7. Alonopsis aureolata Doolittle, 1912, USA, Maine, Sebago lake, parthenogenetic female: A—head shield, Bmain head pores, C – D—labrum, E—antennula, F—antenna, G – H—thorachic limb I in inner and outer view, I—limb II, Jexopodite of limb III, K – L—inner portion of limb III. Scale bars 0.1 mm for A and C – L, 0.05 mm for B.

FIGURE 8. Alonopsis aureolata Doolittle, 1912, USA, Maine, Sebago lake: A – E—thoracic limbs of parthenogenetic female: A – B—exopodite of limb IV and its setae 5 – 6, C—inner portion of limb IV, D—limb V, E—limb VI; F—outline of male postabdomen, after photo in Kubersky (1997), no scale bar. Scale bars: 0.1 mm for A, C – E, 0.05 mm for B.

FIGURE 9. Bootstrap 50 % majority-rule consensus phylogenetic tree of Arthrocauda. Bootstrap-support of each branch is indicated.