Biapertura ossiani Sinev 2020, comb. nov.

Biapertura ossiani (Sinev, 1998) comb. nov. Sars 1901: 48–49, pl. IX, figs 1a–d (Alona affinis); Daday 1905: 171–172, Pl. 10, Fig. 25–24 (Alona affinis); Sinev 1998: 103110, Fig. 1–3 (Alona); Sinev & Silva–Briano 2012: 20, Fig. 10 K–M (Alona); Sinev 2013: 330–335, Fig. 1–3 (Alona). Type locality. Neighborhood of Sao Paolo, Brazil. Type material. Holotype (adult male) and paratypes (2 disected parthenogenetic females, 63 parthenogenetic females, 7 ephippial females, 10 males, 1 juvenile male of instar II) at Zoological Museum of Oslo University, access numbers F12324a for holotype, F12324b-d for paratypes. Material studied earlier. See Sinev (1998, 2013) for the list of material studied earlier and for detailed description. Diagnosis. Parthenogenetic female. Body oval or slightly ovoid, maximum height at the midline or behind it, height/length ratio about 0.6 in adults. Postero-dorsal angle with 6–8 groups of setulae, with 5–8 setulae in each. Main head pores with PP 1.5–2 in adults. Lateral head pores located about 1 IP distance from midline, at level of anterior major head pore or before it. Postabdomen (Fig. 16 E) subrectangular, with parallel margins, weakly narrowing at the end; length about 2.5 height; 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 2.5–2.8 times longer than anal one; postanal margin (Fig. 16 F) with 12–14 massive denticles, each with 1–7 (usually 3–5) spinulae along anterior margin; postabdominal claw (Fig. 16 G) with basal spine about 0.25–0.35 length of the claw. Antenna (Fig. 16 H) with basal and distal segments of both branches of similar length, middle segment being shortest, about 1.5 shorter than two others; spine on basal segment of exopodite longer than middle segment; spines on apical segments shorter than apical segments. Limb I (Fig. 16 I) with IDL claw-like seta 1; seta e of endite 2 two times longer than seta f. Male. Length of adult 0.60–0.63 mm. Body low oval (Fig. 16K, N), with maximum height near the middle, height/length ratio about 0.54. Postabdomen (Fig. 16L, O) of moderate size, with almost parallel margins; length about 2.5 height; ventral margin straight; postabdominal claw situated on small protrusion in ventral portion of convex distal margin; the sperm ducts open above the protrusion; posteroventral and posterodorsal angles rounded; dorsal margin weakly convex in postanal portion and weakly concave in anal one; preanal angle weakly defined; postanal angle not defined; preanal margin almost straight; distal portion of postabdomen two times longer than preanal one, postanal portion two times longer than anal one. Postabdominal claw weakly curved, shorter than in female, as long as preanal margin; basal spine about 0.25–0.3 length of claw. Thoracic limb I (Fig. 16 I,M) with IDL seta 1 well-developed, about 2/3 length of IDL seta 2, located terminally; anterior face of the limb under the copulatory brush with row of 10–12 stiff setulae of moderate length; outer face of endite 3 with two rows of about 10 shorter setulae, separated by a wide gap. Differential diagnosis. B. ossiani clearly differs from B. affinis in morphology of male postabdomen and in well-developed seta 1 on IDL of male limb I; from B. martensi and B. elliptica in female postabdomen not narrowing distally and in smaller IP/PP ratio; from B. sibirica in absence of denticles on posteroventral margin of valves, from B. kendallensis in much smaller IP–PP ratio; from B. lepida in claw-like IDL seta 1. Distribution. Nearctic and Neotropic regions from South Canada to South Argentina and Chile.

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