Acroperus africanus Neretina & Kotov, 2015, sp. nov.

Acroperus africanus sp. nov.

(Figs. 1–5)

Etymology. This new species is named after Africa, the continent where it was discovered.

Type locality. Bay of Bahir-Dar, Lake Tana, Ethiopia (11.6111° N, 37.3780° E). From this locality, 70 parthenogenetic females were collected in November 27th of 2014 by A.N. Neretina.

Type material. Holotype: an adult parthenogenetic female deposited at the collection of Zoological Museum of Moscow State University, MGU Ml 142. The label of holotype is: “ Acroperus africanus sp. nov., 1 parth. ♀ from Lake Tana (Ethiopia), HOLOTYPE ”. Paratypes from Lake Tana preserved in 4% formaldehyde: 24 undissected parthenogenetic females (MGU Ml 143: 11.6111 N °, 37.3780 E °, collected in November 27th of 2014 by A.N. Neretina); 17 undissected parthenogenetic females (ANN 2014-008: 11.6023° N, 37.3947° E, collected in November 27th of 2014 by A.N. Neretina) and 3 undissected parthenogenetic females (ANN 2014-017: 11.6117° N, 37.3727° E, collected in November 28th of 2014 by A.N. Neretina).

Other material studied. Bahir-Dar Bay of Lake Tana: 3 adult parthenogenetic females (an ethanol-fixed sample AAK M-2633: 11.6172° N, 37.3831° E, collected by S. Cherenkov in April 2nd of 2013) and 5 juvenile parthenogenetic females (an ethanol-fixed sample AAK M-1454: 11.6116° N, 37.3779° E, collected by A.A. Kotov in February 22nd of 2009).

Eastern Cape, Republic of South Africa: 10 parthenogenetic females from the pool (31.1972° S, 28.1705° E) located near McClear Road, collected in March 26th of 1993 by K. Martens, de Moor & Barber), NNS 2002-119; 10 parthenogenetic females from the Wildebeest River Dam (31.0508° S, 28.3147° E), McClear, collected in March 27th of 1993 by K. Martens, de Moor & Barber, NNS 2002-120.

Description. Parthenogenetic female. General. In lateral view, body ovoid (Fig. 1 A–B, 3A, 5A), with expressed dorsal keel, maximum height anterior to the middle or at the middle of body (body height/length ratio about 0.6 for adults and juveniles). Body strongly compressed laterally (Fig. 3 D). Dorsal margin regularly arched in anterior portion, slightly convex in adults (Fig. 1 A, 5A) and nearly straight in juveniles (Fig. 1 B) in its posterior half, without a depression between valves and head. Posterodorsal angle expressed (Fig. 1 A, 5A), posteroventral margin weakly to moderately convex (Fig. 1 A–B, E, 5A). At inner face of posteroventral margin a row of fine setules, thinning towards posterodorsal angle (Fig. 1 E–F, 3H). Posteroventral angle broadly rounded (Fig. 1 A–B, E, 3A, 5A), provided with 1-2 small denticles (Fig. 1 E, G, 3I –K, 5B). Ventral margin from almost straight to weakly convex, with two groups of setae differing in length: setae are long in anterior portion, then becoming shorter in the first third of margin and then becoming longer again, decreasing in size towards posterior end (Fig. 1 A–B, E, 5A). A row of setules between each neighboring setae (Fig. 1 I, 3L–M). Anteroventral angle rounded (Fig. 1 A, E, 5A). Valves with prominent sculpture consisting of diagonal, never anostomosing lines (Fig. 1 E, 3A, N, S).

Head small, keeled (Fig. 1 C, 3B–C). Rostrum relatively short, compound eye larger than ocellus, distance from tip of rostrum to ocellus almost equal to distance between ocellus and eye (Fig. 1 C). Head keel with three connected major head pores of equal size (Fig. 3 E), lateral pores small, rounded, located at the level before anterior main pore (Fig. 3 E: arrow).

Labrum with a narrow subtriangular keel which bears several rows of fine setules (Fig. 1 C–D, 3F–G); apex of keel rounded.

Thorax relatively long (Fig. 1 A, 5A). Abdomen short, distal surface of abdominal segments with transverse rows of fine setules (Fig. 1 A, 2A, 5A).

Postabdomen (Fig. 1 A, 2A, 5A) long and narrow, with almost parallel dorsal and ventral margins, postabdomen length/height ratio about 2.5-2.7. Ventral margin almost straight, with no ridges or setules (Fig. 2 A– B, 3 O). Preanal margin short, length of anal margin almost equal to that of preanal margin, postanal margin two times longer than preanal margin (Fig. 2 A). Preanal and postanal angles well-expressed (Fig. 2 A). Basis of claws with a group of long setules (Fig. 2 A–C, 3 O, Q), bordered from distal margin by a clear depression. Postanal margin armed with series of small denticles (Fig. 2 A–B, 3 O, P); fascicles of thin setules on lateral sides of postabdomen; distalmost setule in each fascicle on postanal portion of postabdomen remarkably longer than following setules (Fig. 2 B–C, 3P–Q), while all setules in fascicles at anal margin subequal in size (Fig. 2 B). Postabdominal claw long (as long as anal portion of postabdomen) (Fig. 2 A–C), almost straight, external side armed in its proximal portion with a row of teeth fluently increasing in size distally; distalmost tooth specially strong; very fine setules on distal half of claw (Fig. 2 B–C, 3Q); internal side of claw with two successive pectens of setulae. Basal spine long (about 0.3 of claw length), straight and thin, with a row of setulae along its dorsal margin; also there are several short setules at claw base (Fig. 2 B–C, 3Q).

Antenna I relatively long and narrow (length/diameter at base about 2.7-3), with four transverse rows of short setulae at anterior face (Fig. 2 D, 3R). Eight terminal aesthetascs, one of them almost two times longer and thicker than others, as long as body of antenna I; a single long lateral aesthetasc located near a slender sensory seta of antenna I (Fig. 2 D).

Antenna II long for a chydorid (Fig. 1 A, 2E, 3A, S, 5A). Antennal formula: setae 0-0-3/1-1-3; spines 1-0-1/0- 0-1. Branches of antenna II subequal in size. In adults and juveniles seta on proximal segment of endopod specially short (remarkably shorter that half of middle segment). This feature is difficult to observe in the light microscope due to fine distal part of seta (Fig. 2 E, F, H–I, 3U, 5C). Seta on middle segment only somewhat longer than apical segment with its spine (Fig. 2 G) and makes up about half length of endopod itself (in adults and juveniles) (Fig. 2 E, F, H–I, 3T, 5C). All apical setae subequal in size (Fig. 1 A, 2E, G, 3A, S, 5A), covered by fine setulae (Fig. 3 S, V). Spine on proximal exopod segment very short (shorter than 1/4 length of second segment) (Fig. 2 E, F, H–I, 3S, T, 5C). Spines on both apical segments of exopod and endopod short (less than 0.3 of segment length), subequal in size (Fig. 2 F–I, 3S, T, 5C).

Thorax limbs: six pairs.

Limb I (Fig. 4 A–B, 5D–E) relatively large. Accessory seta short. Outer distal lobe (Fig. 4 B, 5E: ODL) conical and large, bearing a single bisegmented seta with minute setulae on distal segment. Inner distal lobe (Fig. 4 B, 5E: IDL) with three bisegmented setae of different length: smallest seta short with tiny setulae on the distal segment; middle and largest setae armed by relatively short and thick setulae distally. Endite 4 with three posterior setae (Fig. 4 A, 5D: a–c) and single anterior seta (Fig. 4 A, 5D: 1). Endite 3 with three posterior setae (Fig. 4 A, 5D: d–f), among which setae e and f very long and a rudimentary anterior seta (Fig. 4 A, 5D: 2). On endite 2 three bisegmented posterior setae (Fig. 4 A, 5D: g–i) (among which seta “i” very short) and an anterior seta (Fig. 4 A, 5D: 1). Endite 1 (gnathobase) reduced, without setae. Two serrate ejector hooks of almost equal size (Fig. 4 A, 5D). Remarkably, that ratio of seta 3/seta 2 length on the limb I is about 3–3.2 (larger, than ratios for these setae in sibling Palearctic species, A. harpae and A. angustatus (see below)).

Limb II (Fig. 4 C) subtriangular. Exopodite small, ovoid, with a single rudimentary seta (Fig. 4 C: ext); inner portion of limb with eight scrapers (Fig. 4 C: 1–8) forming three groups according to size: setae 1-2 longest, with fine setulation; setae 3–5 somewhat shorter, with larger setulae; setae 6-8 shortest, setae 6-7 with thick setulae and seta 8 with fine setulae. Distal armature of gnathobase with four setae; seta 1 represented by a small sensillum. Filter plate with seven seta: distalmost seta thick and short, with specially long setulae, next seta also shorter than other setae increasing in size proximally (Fig. 4 C).

Limb III (Fig. 4 D–E). Exopodite trapezium-shaped, with five distal setae (among which two middle setae specially short), and two lateral setae (Fig. 4 D). Distal endite (in terms of Kotov 2013) with three anterior setae (Fig. 4 D: 1–3): setae 1 and 2 supplied with robust denticles distally, a small sensillum between their bases (Fig. 4 D: s); seta 3 with fine setulae. Proximal endite with four small anterior setae (4–7); a small sensillum near seta 4 (Fig. 4 D–E: s). Four soft setae on posterior face of limb (Fig. 4 D, a–d). Distal armature of gnathobase with three setae and a thick sensillum (Fig. 4 E: s). Filter plate with seven setae of subequal size (Fig. 4 D).

Limb IV (Fig. 4 F) with ovoid preepipodite and globular epipodite supplied with a long finger-like projection; exopodite subrectangular, with three distal and three lateral setae (Fig. 4 F). Inner distal portion with four anterior setae: distalmost setae 1 stout, naked; setae 2-4 shorter with inflated basal portion and relatively thick setules distally (Fig. 4 F). Three thin setae on posterior limb face (Fig. 4 F). Distal armature of gnathobase with three elements, distalmost one represented by a thick sensillum (Fig. 4 F: s). Filter plate with five setae subequal in size (Fig. 4 F).

Limb V (Fig. 4 G) with ovoid preepipodite and globular epipodite supplied with a long finger-like projection. Exopodite bearing a single distal seta and three lateral setae. Inner limb portion as elongated flap, with setulated margin, with two setulated setae of subequal length (Fig. 4 G). Filter plate with three small setae (Fig. 4 G).

Limb VI (Fig. 4 H) small as compared with other five limbs, represented by a small rounded lobe with setulated margin.

Ephippial female, male. Unknown.

Size. Juvenile females 0.35–0.48 mm in length (height 0.21–0.27 mm; thickness 0.13–0.15 mm). Adult parthenogenetic females 0.55–0.86 mm in length (height 0.37–0.52 mm; thickness 0.22–0.24 mm). Holotype 0.65 mm in length (height 0.39 mm; thickness 0.24 mm).

Variability. The number of denticles on posteroventral margin of valve varies from one to two in different individuals. Remarkably, the most frequent morphotype in the population from Lake Tana has a single denticle on the posteroventral margin (Fig. 1 A, B, E, 3I, J), whereas individuals with 2 denticles (Fig. 3 K) are rarely found. Some of the investigated specimens from Lake Tana have asymmetric valves: one with a small denticle (Fig. 1 G) and the other without denticles (but with tiny hillock on its position) (Fig. 1 H). On the contrary, specimens from the two localities in Republic of South Africa usually have two denticles (Fig. 5 B), being the morphotype with a single denticle less common. It is not surprising because the variability of the number of denticles on the posteroventral angle was also shown for other taxa of Acroperus (Sinev 2009; Sinev & Elmoor-Loureiro 2010; Sinev et al. 2012).

Differential diagnosis. Acroperus africanus sp. nov. could be confused with Palearctic A. harpae and A. angustatus. But the former could be easily distinguished from the latter two taxa in: (1) smaller seta on the proximal segment of antenna II endopod; (2) larger ratio of seta 3/seta 2 length on the first thoracic limb. Other differences from well-described species of Acroperus are summarized in Table 1.

Distribution and ecology. The species is known to date from several localities in Africa (Ethiopia and Republic of South Africa). Despite the intensive sampling in Ethiopia (there are more than 600 samples from different water bodies in the personal collection of AAK), to date A. africanus sp. nov. was found only in Lake Tana. Type locality, the Bahir-Dar Bay of Lake Tana, is a shallow water body (the maximum depth in the central part is 8.5 m) with a well-developed littoral zone covered by macrophytes (Ceratophyllum sp., Nymphaea sp., Potamogeton sp. among others). At time of sample collection in November of 2014, temperature was 19.3–23 °C; pH 7.8 and conductivity 125 ΜS cm -1. The findings of A. africanus sp. nov. in the water bodies located in the south part of Africa allows us to presume, that this species is rather common in the continent, but it was misidentified by previous authors as Palearctic Acroperus species.