Alona rigidicaudis ( Smirnov, 1971 )

Sinev, Artem Y., 2022, Redescription of two Australian cladocerans of the subfamily Aloninae Dybowski & Grochowski (Cladocera: Anomopoda: Chydoridae), Zootaxa 5124 (2), pp. 188-204 : 189-195

Alona rigidicaudis ( Smirnov, 1971)

( Figs. 1–5 )

Smirnov, 1971: 476, 479, Fig. 598, 599 ( Biapertura ); Smirnov & Timms, 1983: 61, Fig. 71 ( Biapertura ).

Type locality. “ Doughboy Creek, Ebor, New South Wales, Australia ” ( Smirnov, 1971). This is locality 107 in Smirnov and Timms (1983).

Type material. Holotype. A parthenogenetic female from type locality, slide NNS 1288 in personal collection of N.N. Smirnov deposited at Zoological Museum of M. V. Lomonosov Moscow State University, Moscow, Russia . Paratype. A parthenogenetic female from type locality, deposited at Zoological Museum of Russian Academy of Sciences, Saint-Petersburg, slide 53847 (1232) .

Material examined. Over 30 parthenogentic females from type locality not officially designated as paratypes; over 40 parthenogenetic females, 8 ephippial females, 6 adult males from L. Dunn, Bore overflow, 70 km NE Aramac, region of Longreach , Queensland, Australia, 17.07.1974, coll. B. V. Timms, NNS 1997-180. Both samples from personal collection of Prof. N.N. Smirnov, kept at A.N. Severtsov Institute of Ecology and Evolution of Russian Academy of Sciences, Moscow .

Description. Parthenogenetic female. General. Body ( Figs. 1A , 2 A–E ) moderately compressed laterally, oval in lateral view, moderately high, maximum height at body middle. Height-length ratio about 0.7 in adults. Dorsal margin strongly convex, postero-dorsal and postero-ventral angles broadly rounded. Posterior margin almost straight, ventral margin almost straight, antero-ventral angle rounded. Ventral margin ( Fig. 1B ) with about 35 setae; about 10 anterior setae moderately long, about 20 middle setae very short, posterior 10 setae ( Fig. 2F ) longer than setae of anterior group. Postero-ventral angle ( Figs. 1C–D , 2F ) with 2–3 small spinules, followed by 15–20 setules of variable length and thickness. Valves with weakly developed sculpture as a system of longitudinal lines ( Figs. 2B–C, E ) or without it ( Fig. 2A ).

Head relatively small, triangular in lateral view. In lateral view rostrum short, protruding downwards. Ocellus smaller than eye. Distance from tip of rostrum to ocellus about 1.5 times of that between ocellus and eye.

Head shield ( Fig. 2G ) with shape typical for the alonines, with maximum width behind maxillar articulation, oblique. Rostrum elongated, clearly separated from anterior portion of head shield, only weakly narrowing distally, with broad rounded tip. Posterior margin of head shield rounded. Two main head pores ( Figs. 1E–F , 2H ) with a broad connection between them, PP less than 1 IP. Lateral head pores minute, located at more than 1 IP distance from midline, at level of anterior main head pore. Behind the posterior margin of head shield, at midline of carapace, two posterior dorsal pores (very small apertures visible only at SEM), appear ( Fig. 2I ).

Labrum ( Figs. 1G–H ) of moderate size, without lateral projections. Labral keel of moderate width, height about 1.5 times width. Anterior margin of keel evenly convex, apex broadly rounded. Posterior margin weakly convex, without clusters of setules.

Thorax two times longer than abdomen. Dorsal surface of abdominal segments not saddle-shaped.

Postabdomen ( Figs. 1I–K ) short and wide. Length about 2.1–2.3 heights. Ventral margin weakly convex. Basis of claws separated from distal margin by clear incision. Distal margin convex to straight, distal angle very broadly rounded. Dorsal margin convex in postanal portion and strongly concave in anal one; distal part as long or slightly shorter than preanal one; postanal portion as long or slightly longer than anal one. Preanal angle well-defined, prominent, postanal angle not defined. Preanal margin concave. Postanal margin with 5–6 composite denticles of similar length, with several spinules along anterior margin; length of denticles slightly exceed the width of postabdominal claw base. Anal margin with 3–4 groups of marginal spinules and setules, preanal margin with 2–3 groups of thick setules near preanal angle. Eight–nine lateral fascicles of setules in main row. In postanal portion fascicles narrow, composed of 3–7 setules each, distance between fascicles about half width of fascicle; posteriormost setula very long and thick, in distalmost groups with length about 2.5 widths of postabdominal claw base. In anal portion fascicles wider, with shorter and much thinner distalmost setule; additional fascicles are located above the main row. Postabdominal claw slender, weakly curved, of moderate length, much shorter than preanal portion of postabdomen. Basal pecten of claw composed of thin, moderately long densely spaced setules. Basal spine slender, curved, about 0.2–0.25 of the claw length. A cluster of spinules located near the base of basal spine.

Antennule ( Fig. 1L ) narrow elongated, length about three widths, with 2 clusters of long setules at anterior face. Antennular seta of about 1/3 length of antennule, arising at 1/2 distance from the base. Nine terminal aesthetascs, longest of them about half length of antennule.

Antenna ( Figs. 3A , 5A ) with antennal formula: setae 0-0-3/1-1-3 and spines 1-0-1/0-0-1. Basipodite robust, with short seta between branches; branches relatively short; basal segments in both branches 1.5 times longer than apical segments. Basal and middle segment of endopod with clusters of short setules. Seta arising from basal segment of endopod as long as endopod. Seta arising from middle segment of endopod of similar size to shortest apical setae. One of apical setae on both branches significantly shorter and thinner than two others. Spine on basal segment of exopod slightly longer than middle segment. Spine on apical segment of exopod slightly longer than apical segment. Spine on apical segment of endopod much longer than apical segment.

Mandibles of typical for Aloninae morphology.

Maxilla I ( Fig. 3B ) with three short setae.

Limb I ( Figs. 3C–D ). Epipodite oval, with a finger-like projection as long as epipodite body itself. Accessory seta two times shorter than ODL seta. ODL seta with short setules in distal portion. IDL without seta (1); setae (2–3) much thicker than ODL seta, armed with long setules in distal portion; seta (3) as long as ODL seta; seta (2) significantly shorter. Endite 3 with four short setae, inner setae (1) much shorter than setae (a–c). Endite 2 with two long setulated setae (e–f) and a shorter seta near their base (d); seta (e) almost as long as limb itself with long setules in distal portion, seta (f) about 2/3 length of seta (e). Endite 1 with two long 2-segmented setae of similar size (g–h), setulated in distal part, and a long flat setulated seta (i) pointing to the limb base. Inner setae of endites 1–2 not found. Anterior face of limb with 5–6 clusters of short setules. Two ejector hooks of similar size.

Limb II subtriangular ( Figs. 3E–F ). Exopodite elongated, without seta. Eight short scraping spines, spines 1–5 long, decreasing in length basally, spine (3) thicker than others and armed with thicker spinules. Spines (6–8) short, with wide bases and short distal part. Distal armature of gnathobase with four elements. Filter plate II with seven setae, distalmost setae much shorter than others.

Limb III ( Figs. 3G–H ). Epipodite rounded. Exopodite subrectangular, with six setae. Seta (3) very long, about 4 times as long as exopodite; seta (5) about 1/2 length of seta (3); setae (2) and (6) very short. Setae (1–4) plumose, seta (5) armed bilaterally with short hard setules in distal portion, seta (6) with short setules in distal portion. Distal endite with 3 setae, two distalmost members slender, with short denticles in distal part, seta (3) flattened, armed with long setules. Gnathobase not clearly separated from basal endite. Four inner setae (4–7) slightly increasing in size basally; a sensillum near the base of distalmost seta. Distal armature of gnathobase with four elements; the first one an elongated, slightly curved sensillum; the second a geniculated seta; two others are short spines with fused bases. Filter plate with seven setae.

Limb IV ( Figs. 3I–J ). Pre-epipodite setulated; epipodite with short process. Exopodite rounded, with six setae. Seta (3) being longest, setae (1–2) slightly shorter than seta (3), seta (4) about 1/2 length of seta (3), setae (5–6) about 2/3 length of seta (3). Setae (1–4) flat, plumose, setae 5–6 thin, with short setules in distal portion. Inner portion of limb IV with four setae and sensillum. Scraping seta (1) slender, first flaming-torch seta (2) much wider than two other (3–4), sensillum curved. Small sensillum located between bases of setae (2) and (3). Three soft setae of similar size. Gnathobase with two-segmented seta and a small hillock distally. Filter plate IV with five setae.

Limb V ( Fig. 3K ). Preepipodite setulated, epipodite oval, with short projection. Exopodite oval, with four plumose setae. Setae (1–2) long, similar in length; setae (3) and (4) of 2/3 and 1/2 length of seta (2), respectively. Inner lobe narrow oval. At inner face, two setae, distal seta slightly longer than basal. Filter plate V absent.

Ephippial female ( Figs. 4A , 5B–D ) with body slightly higher than in parthenogenetic female; dorsal margin without depression between valves and head shield. Ephippium ( Fig. 5E ) yellow-brown in preserved specimens, with egg locules not clearly visible, covered by irregular robust polygonal sculpture, more pronounced than on the rest of the valves.

Male. General. Body ( Figs. 4B , 5F–G ) low oval, lower than in female, strongly compressed laterally, maximum height before the middle of the body, height/length ratio about 0.6. Ocellus larger than in female. Head pores ( Fig. 5H ) similar to those of female, but PP is less than 0.7 IP. Posterior dorsal pores ( Fig. 5I ) present, located much closer to each other than in female.

Postabdomen ( Figs. 4C–D ) short, narrowing distally in both anal and postanal portion, without defined distal margin, dorso-distal and ventro-distal angles. Postabdominal claws located on a weakly defined bulge at distal end of postabdomen. Sperm duct openings located ventrally at some distance from the end of postabdomen. Preanal angle well-defined, obtuse, postanal angle not defined. Distal part of postabdomen 1.5 times longer than preanal, anal portion almost two times longer than postanal one. Clusters of short thick setules in place of marginal denticles. Lateral fascicles of setules in postanal portion similar to those of females, but less developed; distalmost setulae in each fascicle shorter and thinner. Postanal portion laterally with irregularly spaced clusters of short setules. Postabdominal claw 1.5 times shorter than that of female, with blunt tip, bearing several short thick spinules laterally near the base. Basal spine long, about 0.3–0.4 length of claw.

Antennule ( Fig. 4E ) shorter and thicker than in female, with 11 terminal and 1 lateral aesthetascs. Lateral aesthetasc about half length of antennule, located at half-distance between end of antenna and the base of male seta; length of terminal aesthetascs about 1/2–2/3 lengths of antennule. Male seta arising at 1/3 length from tip, of about 1/3 of antennule length.

Limb I ( Figs. 4F–G ) with short U-shaped copulatory hook two times shorter than limb itself. Copulatory brush present, copulatory brush seta short. Ventral face of limb below them with short setules, Inner distal lobe without seta (1); setae (2) and (3) of similar length, much shorter and thinner than in female, with distal portions armed with thin setules; male seta curved, as long as setae (2–3). Endite 3 with seta (1) much thinner than in female.

Size. Adult female length 0.33–0.38 mm, height 0.24–0.27 mm; adult male length 0.28–0.3 mm, height 0.17– 0.19 mm.

Differential diagnosis. Alona rigidicaudis clearly differs from other species of Alona s. lato in its unusual morphology of postabdomen, with short distal and long preanal portions; being preanal portion equal in size or slightly shorter than distal (anal + postanal) one.

Distribution: Australia, pancontinental.

Figures

FIGURE 1. Alona rigidicaudis (Smirnov, 1971), parthenogenetic female from Doughboy Creek, Ebor, New South Wales, Australia (type locality). A, lateral view. B, ventral margin of valves. C–D, posteroventral angle of valves. E–F, head pores. G–H, labrum. I–K, postabdomen. L, antennule.

FIGURE 2. Alona rigidicaudis (Smirnov, 1971), parthenogenetic female from L. Dunn, Bore overflow, 70 km NE Aramac, region of Longreach, Queensland, Australia.A–C, lateral view. D, frontolateral view. E, ventral viev. F, posterior setae of ventral margin of valves. G, head shield. H, head pores. I, posterior dorsal head pores.

FIGURE 3. Alona rigidicaudis (Smirnov, 1971), parthenogenetic female from Doughboy Creek, Ebor, New South Wales, Australia (type locality). A, antenna. B, maxilla I. C, limb I. D, ODL and IDL of limb I. E–F, limb II. G, exopodite of limb III. H, inner portion of endite of limb III. I, exopodite of limb IV. J, inner portion of limb IV. K, limb V.

FIGURE 4. Alona rigidicaudis (Smirnov, 1971), ephippial female (A) and adult male (B–G) from L. Dunn, Bore overflow, 70 km NE Aramac, region of Longreach, Queensland, Australia. A, ephippial female, lateral view. B, male, lateral view. C–D, postabdomen. E, antennule. F–G, thoracic limb I.

FIGURE 5. Alona rigidicaudis (Smirnov, 1971), parthenogenetic female (A), ephippial female (B–E) and adult male (F–I) from L. Dunn, Bore overflow, 70 km NE Aramac, region of Longreach, Queensland, Australia. A, parthenogenetic female, antenna. B, ephippial female, lateral view. C, dorsolateral view. D, dorsal view. E, ephippium. F, male, lateral view. G, dorsal view. H, head pores. I, posterior dorsal pores.