Chydorus sphaericus
Creators: name: Sousa, Francisco Diogo Rocha
name: Silva, Barbara Aparecida Souza Da
name: Elmoor-Loureiro, Lourdes M. A.
name: Moreira-Silva, Camila
name: Espolau, Greyce
name: Sarmento, Hugo
name: Isumbisho, Mwapu
name: Borges, Alberto V.
name: Perbiche-Neves, Gilmar
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Branchiopoda
Diplostraca
Chydoridae
Chydorus
Chydorus sphaericus

description: Chydorus sphaericus (O.F. Müller, 1776)
(Figs. 1–2)
Lynceus sphaericus O.F. Müller, 1785: p. 71 –72, figs. 7–9; Chydorus arcticus Røen, 1987: p. 125 –130, figs. 1–5; Chydorus caelatus Schoedler, 1862: p. 15, fig 44; Chydorus lynceus Langhans, 1911 in Smirnov (1971): p. 345; Chydorus mutilus Kreis, 1921: p. 272, figs. 42–45; Monoculus infusorius Schrank, 1781: p. 536.
Material Examined. Fifteen parthenogenetic females from Bjørndalsvatn Lake Sveindal-Hornnes, Norway (58°30’32.9”N, 7°33’27.1”E), material collected by Lourdes M. A. Elmoor-Loureiro on 11.viii.2018 (FDRS0698).
Description. Parthenogenetic female. General. (Fig. 1A–C). In lateral view body sub-globular or rounded; dorsal margin arched, without a keel or projections; without lateral compression, lateral projections absence.
Head. Ocellus smaller than the eye (Fig. 1A–B). Rostrum short, with a sharp tip, about 1.2 times longer than the antennular body. Head shield not completely studied. Two major head pores of similar diameter, and two very small, closely-set pores between them, these tiny pores located approximately at half the distance between the major head pores (Fig. 1F).
Labrum (Fig. 1G–H). Keel not prominent, without notch, naked, distal portion elongated and triangular, with short lateral projections in frontal view.
Carapace (Fig. 1A–E). Covered by slight tubercles or hexagons; anteroventral margin with an evident flange; anterior part of ventral margin with 6–8 slender setae which are not plumose; posterior part of ventral margin naked; each valve bear 34–35 plumose setae inserted at a distance from valve ventral margin, posterior and anterior setae shorter than middle setae. Posteroventral margin without spines or denticles. Posterior margin with an acute or obtuse projection formed by the junction of dorsal and ventral margins.
Abdomen (Fig. 1A). About 3.6 times shorter than the thorax, three transverse rows of setulae at its dorsal surface.
Postabdomen (Fig. 1M). About 3.5 times longer than wide, ventral margin relatively straight; preanal margin about 1.4 times longer than the anal margin, angle clearly prominent; anal margin somewhat concave, with similar length to postanal margin, armed with several spinulae; postanal margin with distalmost portion slightly narrow, armed with 7–8 denticles which have the base about 2.5–4.5 times shorter than the length itself; lateral fascicles formed by thin and short spinulae not organized in groups. Postabdominal setae about 1.8 of postabdomen length, bisegmented, provided with setulae in the distal segment. Postabdominal claw. Bearing two basal spines and a subterminal flagellum, about 0.28 of postabdomen length; pecten organized in two groups, proximal group with short spines, distal group with long spines. Basal spines. Naked, proximal spine about 1.8 times shorter than the distal; distal spine about 0.15 of postabdominal claw length.
Antenna I (Fig. 1I). Approximately two times longer than wide, never reaching tip of the rostrum; antennular sensory seta about 3–3.2 times shorter than antennular body, inserted near middle of antennular body; nine aesthetascs do not extend middle of antennular body.
Antenna II (Fig. 1J–L). Basal segment with a short and thin spine. First exopodite segment longer than first endopodite segment; second exopodite segment with a long seta, about 2.8 times longer than exopodite length itself; third exopodite segment with three apical setae, plumose, two setae about 1.9 times longer than the branch length itself, one setae of similar length to length of branch itself; apical spine of the exopodite about 2 times shorter than endopodite apical spine. First endopodite segment armed with a spine similar in length to apical spine of exopodite; third endopodite segment with tree setae, plumose, about 1.9 times longer than the branch length itself. Antennal formula (exo/endo): spines 001/101, setae 013/003.
Limb I (Fig. 2A–D). Epipodite not studied. ODL armed with a short seta and a thin serrated seta, longer than IDL third seta. IDL (en 4) with a single group of setulae on the corm, three setae present; seta 1 hook-like, about 1.5 times shorter than the second seta; seta 2 bisegmented, about 1.3 times shorter than the third seta; seta 3 long, hook-like; all setae armed with spines. Endite 3 with four setae, anterior seta 1 (Fig. 2B) similar in length to posterior seta (b), posterior seta (a) about 1.1 times shorter than seta (b), posterior seta (c) about 0.9 times longer than the seta (b). Endite 2 with three posterior setae present (d–f) and a single stiff setae (Fig. 2C); seta (d) plumose about 1.7 times shorter than seta (e); seta (f) armed laterally with short and thick setulae, about 1.3 times shorter than seta (e); seta (e) bears thick setulae on lateral face; stiff seta armed with short denticles, about 1.6 shorter than posterior seta (d). Endite 1 with three posterior setae of similar length (g–i), which are bisegmented and densely setulated on the distal part, seta (j) plumose and approximately 2.3 times shorter than seta (i); ejector hooks of similar length among themselves and armed with spines; ventral face of the limb with 7 cluster of setulae. Gnathobase with a setulated setae.
Limb II (Fig. 2E). Exopodite armed with a seta about 1.3 times longer than e exopodite itself; inner limb portion armed with eight scrapers and a single element (sensillum); scraper 1 markedly longer than the others, length of scraper 2 about 0.89 of scraper 1 length; scraper 3 about 0.9 of scraper 2 length; scraper 4 about 0.78 of scraper 3 length; scraper 5 armed with thick denticles, about 0.8 of scraper 4 length; scrapers 6–7 of similar length; scraper 8 about 1.2 times longer than scrapers 6–7. Proximal portion of the gnathobase flattened, armed with long setulae and one element; distal portion armed with three elongated elements; filter comb with seven setulated setae.
Limb III (Fig. 2F–G). Pre-epipodite densely setulated. Epipodite oval, with a short projection; exopodite with four distal (1–4) and three lateral setae (5–7); seventh seta setulated, about 1.3 times longer than the sixth and fifth setae; fourth seta setulated, about two times longer than the third seta; third seta setulated and about 1.5 times shorter than the second seta; second seta setulated, longer than the first seta, about 0.7 of fourth seta length. Distal endite with tree setae (1–3), setae 1–2 slender, armed with spines and with similar length; seta 3 about 2.6 times shorter than the setae 1–2 (not represented in Fig. 2F); six setulated posterior setae decreasing in length towards the gnathobase (a–f). Basal endite with four setae (4–7) increasing in length towards the gnathobase. Gnathobase armed with a long and cylindrical sensillum (s) and three elements; filter comb with eight setulated setae.
Limb IV (Fig. 2H–I). Pre-epipodite oval and densely setulated. Epipodite with a projection relatively long. Exopodite with seven marginal setae; setae 2–7 plumose; setae 5 and 7 similar in length; sixth seta about 1.2 times shorter than the setae 5 and 7; fourth seta about 1.6 times longer than the third seta; third seta about 1.4 times shorter than the second seta; first seta about 0.63 of second seta length. Distal endite with four setae (1–4), seta 1 scraper-like, armed with thin spines at its middle-length; flaming-torch-like setae (2–4) longer than seta 1, armed with long setulae. Basal endite armed with four setae (a–d) increasing in length towards the gnathobase. Gnathobase with a single sensillum (s) and two elements, armed with one curved setulated seta, which is shorter than the width of the endite itself; filter plate with six setae.
Limb V (Fig. 1J–K). Pre-epipodite densely setulated. Epipodite oval with, a relatively long projection; exopodite wide, rounded, and armed with four plumose setae and two setulated hillocks implanted near the first seta; fourth seta about 1.3 times longer than first seta; setae 2–3 of similar length. Internal lobe elongated, with rounded apex armed with many setulae; setae 1–2 setulated and similar in length; filter comb with four setae.
Ephippial female. Ephippium occupying posterior part of carapace, covered by lines and regular polygons, slightly pigmented on the central part. The head shield is strongly tapered posteriorly and the postpore distance is longer than in parthenogenetic female (Frey, 1980; Alonso, 1996).
Adult male. According to Frey (1980), Smirnov (1996) and Alonso (1996), the males of C. sphaericus are smaller than the mature females. The postabdomen has a postanal margin strongly contracted, finger-like. The postabdominal claws do not bear basal spines.
Size. Parthenogenetic female. Length 0.31–0.49 mm, height/length ratio 0.86–0.
Differential diagnosis. Chydorus sphaericus should be considered a complex with several sibling species worldwide still waiting for description. Members of the sphaericus -complex have a globular body, labral keel with distal portion elongated and triangular, postabdomen with preanal angle clearly prominent, postanal margin with distalmost portion slightly narrow armed with 7–8 denticles which have the base about 2.5–4.5 times shorter than the length itself. Members of the sphaericus -complex bear ODL with two setae and IDL of the first limb, with setae 1 and 3 hook-like. Chydorus sphaericus s.str. might be differentiated from other species within the group because the basal spines on the postabdominal claws are absent in males (Frey, 1980; Alonso, 1996; Klimovsky & Kotov, 2015).
Distribution and biology. The species was described from Denmark; however, it has been reported worldwide (Rey & Saint-Jean 1969; Frey 1980; Smirnov 1996; Elmoor-Loureiro 1997; Kotov et al 2010; Makino et al., 2023). In Eurasia, there are some populations with clear geographic and genetic separation (Kotov et al. 2016; Karabanov et al. 2022). At the same time, zoochory and anthropogenic dispersion explain several events of colonization of Chydorus sphaericus s.str. in Australia (Karabanov et al. 2022).
description: Published as part of Sousa, Francisco Diogo Rocha, Silva, Barbara Aparecida Souza Da, Elmoor-Loureiro, Lourdes M. A., Moreira-Silva, Camila, Espolau, Greyce, Sarmento, Hugo, Isumbisho, Mwapu, Borges, Alberto V. & Perbiche-Neves, Gilmar, 2024, One more step in solving the Chydorus puzzle-a morphological comparison between Chydorus tilhoi Rey & Saint-Jeans, 1969 and Chydorus sphaericus (O. F. Müller, 1776) (Crustacea: Cladocera), pp. 308-322 in Zootaxa 5424 (3) on pages 310-314, DOI: 10.11646/zootaxa.5424.3.2, http://zenodo.org/record/10821168