Biapertura Smirnov, 1971

Sinev, Artem Y., 2020, Re-evaluation of the genus Biapertura Smirnov, 1971 (Cladocera: Anomopoda Chydoridae), Zootaxa 4885 (3), pp. 301-335 : 303-304

Genus Biapertura Smirnov, 1971 emend. nov.

Type species. Lynceus affinis Leydig, 1860 , typified in the first genus description ( Smirnov 1971).

Emended diagnosis. Female. Large Aloninae , length of adult female up to 1.2 mm. Body of Alona -like habitus, of moderate height, moderately compressed laterally. Head and carapace without dorsal keel, ventral margins of valves in ventral view straight, can be closed without any gaps between them. Dorsal margin of carapace convex, postero-dorsal and postero-ventral angles broadly rounded. Posterior margin weakly convex. Antero-ventral angle rounded. Ventral margin weakly convex to straight, with 70–100 setae. About 25 anteriormost setae very long, next 10–15 seta short, posterior setae of moderate length, decreasing in length posteriorly. Postero-dorsal angle with 3–9 groups of large setulae, length and thickness of setulae in groups increasing posteriorly, or with 1–3 closely spaced denticles with few setulae between them. A row of about 80–120 setulae of variable length along posterior margin on inner side of carapace, these setulae not organized into groups. Carapace ornamentation is weakly to well-developed longitudinal lines and dense longitudinal striae between them.

Head relatively small, triangular-rounded in lateral view. In lateral view rostrum relatively narrow, protruding downwards. Ocellus and eye present. Head shield with maximum width behind mandibular articulation and triangular posterior portion, its rostrum short and rounded. Posterior part of head shield triangular, with prominent distal angle. Two major head pores with a narrow connection between them. PP about 1–3.5 IP in adults. Lateral head pores minute. Behind the posterior angle of head shield located “posterior dorsal pore”, a longitudinal depression at the dorsal midline of carapace (see Fig. 1E , 3 A–E).

Labral keel moderately wide, with a blunt or rounded apex, shape of labral keel is highly variable. Anterior margin of keel convex or polygonal, posterior margin with two clusters of short setulae.

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

Postabdomen large, moderately high, length about 2.5 height. Ventral margin straight. Basis of claws bordered from distal margin by clear incision. Distal margin from almost straight to convex; distal angle rounded. Dorsal margin weakly convex in postanal portion and weakly concave in anal one, with distal part about 2.5 times longer than preanal one, with postanal portion 2.5 or more times longer than anal one. Preanal angle well expressed, postanal angle weakly to not defined. Preanal margin almost straight. Postanal margin with 11–15 well–developed, sharp composite denticles, each with 1–8 spinulae along anterior margin; size of denticles increasing distally. Length of longest denticles about 1.5 the width of base of postabdominal claw, more than 3 times exceeding the width of the denticle base. Postanal portion with 11–12 broad lateral fascicles, posteriormost setae of each fascicle longest, thicker than others, slightly shorter than neighboring marginal denticles. Anal portion with several smaller fascicles, spaced irregularly. Postabdominal claw of moderate length, slightly longer than preanal portion of postabdomen. Basal spine long and slender, about 0.25–0.35 length of the claw.

Antennule of moderate size, length about three times width, with a cluster of 2–7 long setulae about one third the length of antennule at anterior face. Antennular seta thin, about 1/2–3/ length of antennule, arising terminally. Nine terminal aesthetascs, two of them long and thin, little shorter than antennule itself, all others much shorter.

Antenna relatively short. Antennal formula: setae 0-0-3/1-1-3, spines 1-0-1/0-0-1. Basipodite robust, with very short seta between branches, branches relatively short. Seta arising from basal segment of endopodite thin, much shorter than other setae. Several apical setae with long spinule at the point of articulation. Antennal spines welldeveloped, of variable length.

Thoracic limbs: six pairs.

Limb I of moderate size. Accessory seta long, only slightly shorter than ODL seta. IDL with three setae, seta 1 large, at least half as long as seta 2, in most species claw-like, strongly curved. IDL setae 2 and 3 with thin setulae in distal part. Endite 3 with four setae subequal in length. Endite 2 with two long distally setulated setae (e–f), a shorter seta near their base (d) and a naked inner seta (2) and small sensillum on anterior face of limb. Endite 1 with two 2-segmented setae, both setulated in distal part, a flat plumose seta pointed the limb base, and a naked inner seta (3) and small sensillum on anterior face of limb.

Limb II. Exopodite elongated, of irregular shape, with slender seta as long as exopodite itself and cluster of very long setulae in distal part. Eight scraping spines, three basalmost spines (6–8) subequal in length, others increasing in length distally. Distal armature of gnathobase with four elements. Filter plate II with seven setae, the posteriormost member shorter than others.

Limb III. Exopodite with seven setae. Seta 3 being longest, it continues distal margin of exopodite. Setae 6 and 7 about 1/3–1/4 length of seta 3; other setae short; setae 4–5 of same size. Setae 1–5 plumose; seta 6 with three rows of hard setulae in distal part; seta 7 with thin setulae in distal part. Morphology of inner portion typical for subfamily; scraping setae of distal endite moderately long. Filter plate III with seven setae.

Limb IV. Exopodite with six setae. Setae 1–4 flat, plumose; setae 5–6 with short setuleae. Inner portion of limb IV with four setae and bottle-shaped sensillum. Scraping seta slender; three flaming-torch seta well-developed, with long setulae. Three soft setae slightly increasing in size basally. Gnathobase with one long 2-segmented setae, a small hillock distally and a sensillum. Filter plate IV with five setae.

Limb V. Exopodite divided into two lobes, with four plumose setae; setae 1–3 long, subequal in length; seta 4 three-four times shorter. Inner lobe broad, rounded with setulated inner margin. At inner face, two setae densely setulated in distal part, one of them very long, similar in length to exopodite setae 1–3. Filter plate V with three setae.

Limb VI as oval lobe with setulated margin, much smaller than exopodite V.

Male. Body low oval, with maximum height at the middle or behind it.

Postabdomen of moderate size, with parallel margins. Length about three heights. Shape of distalmost portion of postabdomen variable, sperm ducts open on distal margin. Dorsal margin almost straight in postanal portion and weakly concave in anal one. Preanal angle weakly defined; postanal angle not defined. Clusters of thin setulae in place of female marginal denticles on postanal margin, in some species spine-like denticles near the postanal angle. Lateral fascicles of setulae as in female. Postabdominal claw shorter than in female, with long basal spine.

Antennule broader than in female, with 12 terminal aesthetascs. Male seta subterminal.

Thoracic limb I with U-shaped copulatory hook, half as long as limb itself. The IDL with seta 1 present, much smaller than in female; IDL setae 2 and 3 much thinner and shorter than in female; the male seta curved, long.

Differential diagnosis. As a member of the Hexalona-branch, Biapertura clearly differs from the genera of Coronatella -branch ( Coronatella , Anthalona , Karualona and others) in much greater size, presence of limb VI and seven setae on exopodite III. It differs from Alona s. str. ( quadrangularis -group) and Ovalona in two main head pores, presence of filter plate V and limb VI. It clearly differs from other genera of Hexalona namely Armatalona , Flavalona, Prendalona , A. intermedia and A. guttata -groups in greater size, triangular posterior part of head shield with prominent distal angle, broad massive postabdomen armed with over 10 large composite denticles, and in presence of well-developed seta 1 on IDL, at least half as long as setae 2-3.

Figures

FIGURE 1. Biapertura affinis (Leydig, 1860) from Glubokoe Lake, Moscow Area, Russia.A–K, adult parthenogenetic female. A, lateral view. B–C, posteroventral angle of valves. D, head shield. E, head pores. F, labrum. G, postabdomen. H, postanal margin of postabdomen. I, postabdominal claw. J, antennule. K, antenna. L, ephippial female. M–O, adult male. M, lateral view. N, postabdomen. O, postabdominal claw. (A–L, orig.; M–O, after Sinev et al. 2020)

FIGURE 3. Biapertura affinis (Leydig, 1860) from Glubokoe Lake, Moscow Area, Russia, parthenogenetic female.A–B, head pores. C–E, posterior dorsal pores.