Chydorus tilhoi Rey & Saint-Jeans, 1969

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), Zootaxa 5424 (3), pp. 308-322 : 314-320

Chydorus tilhoi Rey & Saint-Jeans, 1969

( Figs. 3–6 )

Rey & Saint-Jean (1969): p.39–40, figs 20A–D.

Chydorus brevidentatus in Smirnov (1971): p. 385 –386, figs. 353–354.

Material Examined. Three adult parthenogenetic females from the Congo mainstem, Congo River Basin (-2.1329N / -4.1928N / 1.8104N, 16.2017N / 15.5258E / 23.0660E), collected on 16.12.2013, 19.12.2013 and 12.6.2014 ( FDRS701 ) . Two adult parthenogenetic females from the Kasai River , Congo River Basin (- 3.6250N / -3.1764N, 18.6586E / 17.3230E), collected between 25.4.2015 ( FDRS709 ) and 17.4.2015 . Two adult parthenogenetic females from the Sankuru River , Congo River Basin (- 4.2753N / 20.4415E), material collected on 28.4.2015 ( FDRS710 ). Eleven parthenogenetic females from the Congo Basin ( FDRS711 ), material collected between the Congo River and its tributaries in 2010 .

Redescription. Parthenogenetic female. General ( Figs. 3A–B , 6A–B ). Parthenogenetic female. General ( Figs. 3A–B , 6A–B ). In lateral view body sub-oval, dorsal margin arched, without keel or projections; no lateral compression, absence of lateral projections.

Head. ( Figs. 3E–I , 6C–G ). Ocellus smaller than the eye. Rostrum short with sharp tip, similar in length to the antennular body ( Fig. 3A ). Head shield ( Figs. 3E–G , 6F–G ) with posterior part elongated, about 1.3 times longer than the wide, margin posterior rounded. Single major head pore relatively large with a wide rim.

Labrum ( Figs. 3H–I , 6E ). Keel not prominent, with a large denticle, distal portion elongated with rounded or obtuse apex; lateral horns present in frontal view.

Carapace ( Figs. 3A–D , 6A–B, H–J ). Covered by hexagons; the anteroventral margin has an evident flange; margins with thick border in ventral view; anterior part of ventral margin with 5-7 slender setae not plumose; posterior part of ventral margin naked; each valve bear 49-52 plumose setae laterally inserted, posterior and anterior setae shorter than the middle setae. Posteroventral margin with 1 or 2 denticles with sharp apex. Posterior margin clearly low, armed with fine spinules that decrease in length towards the dorsal margin.

Abdomen ( Figs. 3A , 6A ). About 2 times shorter than the thorax, setulae on the dorsal surface not studied.

Postabdomen ( Figs. 5C–D , 6K ). Tapering distally, about 4.1 times longer than wide, ventral margin straight; preanal margin concave, relatively longer than the anal margin, angle clearly prominent; anal margin relatively concave, about 1.8–2.3 times shorter than the postanal margin, armed with 1–4 groups of short spines; postanal margin elongated, straight, marginal denticles organized in two groups, distal group formed by isolated thin denticles, proximal group formed by 3–5 groups of short denticles; lateral fascicles with inconspicuous spinulae. Postabdominal setae about 1.7 of postabdomen length, provided with setulae in the distal segment. Postabdominal claw. Bearing two basal spines and a subterminal flagellum, about 0.17–0.19 of postabdomen length; pecten organized in two groups, proximal group with short and thick spines, distal group with thin and relatively long spines. Basal spines. Naked, proximal spine about 2.8–3.4 times shorter than the distal; distal spine about 0.3 of postabdominal claw length.

Antenna I ( Figs. 3J , 6D ). About 2.5 times longer than wide, never reaching the tip of the rostrum; antennular sensory seta about 1.5 times shorter than the length of the antennular body, inserted near to middle length of antennular body; nine aesthetascs which extend beyond the tip of the rostrum, with half the length of the antennular body.

Antenna II ( Fig. 3K ). Basal segment with short, thin spine. First exopodite segment longer than first endopodite segment; second exopodite segment with a long plumose seta, about 3.1 times longer than the length of the branch itself; third exopodite segment with three apical setae similar in length, plumose, about 3 times longer than the length of the branch itself; apical spine of the exopodite approximately 2 times shorter than the apical spine of the endopodite. First endopodite segment armed with a spine similar in length to apical spine of exopodite; third endopodite segment with three setae of similar length, plumose, about 3 times longer than the length of the branch itself. Antennal formula (exo/endo): spines 001/101, setae 013/003.

Limb I ( Fig. 4A–C ). Epipodite not studied. ODL armed with a thin serrated seta, similar in length to IDL third seta. IDL (en 4) with one group of short spinulae and one group of denticles on the corm, three setae present; seta 1 slender, armed with fine spinulae, about 1.4 times shorter than the second seta; seta 2 bisegmented, armed with short spines, about 1.2 times shorter than the third seta; seta 3 bisegmented, long, chitinized, armed with spines. Endite 3 with four setae, anterior seta 1 similar in length to posterior seta (b), posterior seta (a) about 1.1 times shorter than the seta (b), posterior seta (c) about 0.9 times longer than the seta (b). Endite 2 with three posterior setae present (d–f) and one stiff seta Fig. 4B ); seta (d) plumose, about 2.3 times shorter than seta (e); seta (f) armed laterally with short and thick setulae, about 1.6 times shorter than seta (e); seta (e) bears thick setulae on lateral face; stiff seta armed laterally with tick spines, similar in length to 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 times shorter than seta (i); ejector hooks of similar length among themselves and armed with spines; ventral face of the limb with 7–8 cluster of thick setulae. Gnathobase as a setulated setae.

Second limb ( Fig. 4D ). Exopodite armed with a seta about 1.8 times longer than the exopodite itself; inner limb portion armed with eight scrapers; scraper 1 about 0.9 of scraper 2 length; scraper 3 similar in length to scraper 2; scraper 4 about 0.8 of scraper 3 length; scraper 5 about 0.9 of scraper 4 length; scrapers 6–8 of similar length; scraper 7 armed with thick spines. Proximal portion of the gnathobase flattened, armed with 4–5 long setulae; distal portion armed with three elongated elements; filter comb with eight setulated setae.

Limb III ( Fig. 4E–F ). Epipodite not studied. Exopodite with four distal (1–4) and three lateral setae (5–7); seventh seta setulated, about 2.7 times longer than the sixth and fifth setae; fourth seta setulated, about 2.3 times longer than the third seta; third seta setulated and about 1.6 times shorter than the second seta; second seta setulated, longer than the first seta, about 0.68 of the length of the fourth seta. Distal endite with two setae (setae 1–2), slender and naked, seta 1 about 1.5 times longer than the seta 2; six setulated posterior setae similar in length (a–f). Basal endite with four setae (3–6) increasing in length towards the gnathobase. Gnathobase armed with a long and cylindrical sensillum (s) and two elements; filter comb with seven setulated setae.

Limb IV ( Fig. 4G–H ). Pre-epipodite oval and densely setulated. Epipodite with a short projection. Exopodite with seven marginal setae; setae 2–7 plumose; setae 7 and 6 similar in length; fifth seta about 1.2 times shorter than the setae 6 and 7; fourth seta about 1.8 times longer than the third seta; third seta about 1.4 times shorter than the second seta; first unfeathered seta about 0.6 of second seta length. Distal endite with four setae similar in length (1–4), seta 1 scraper-like armed with thin spinulae at its middle-length, setae 2–4 flaming-torch-like. Basal endite armed with four setae increasing in length towards the gnathobase. Gnathobase with one sensillum (s) and one element, armed with one curved setulated seta which is longer than the width of the endite itself; filter plate with five setae.

Limb V ( Fig. 5A–B ). Pre-epipodite densely setulated. Epipodite oval with a projection relatively long; exopodite wide, trilobed, armed with four plumose setae and two setulated hillocks implanted near the first seta; fourth seta about 2.7 times longer than first seta; seta 3 about 0.7 times shorter than seta 4; seta 2 about 0.7 times shorter than seta 3. Internal lobe elongated, with rounded apex armed with many setulae, two setae present; seta 1 densely setulated, about 1.2 times longer than seta 2; seta 1 with flaming-torch apex; filter comb with four setae.

Male and Ephippial female. Unknown.

Size. Length between 0.32–0.47 mm, height/length ratio 0.8

Differential diagnosis. Chydorus tilhoi has a unique set of morphological traits: a single, relatively large major head pore with a wide rim, elongated labral keel with a large spine and postabdomen with elongated postanal part, tapering distally, and with denticles near its anal margin organized in groups. It is distinguished from C. dentifer by presence of sharp denticles on the posteroventral margin on the carapace, one major head pore, and an elongated labral keel with a large spine. Chydorus tilhoi is differentiated from C. nitidulus by the presence of a single major head pore in adult females and an elongated labral keel with a large spine. Chydorus tilhoi can be distinguished from C. breviceps (Stingelin, 1905) because it has a large spine on the labral keel. Regarding limbs, the species are differentiated by the presence of seta (j) on the first limb of C. tilhoi , proportion of IDL setae 2–3, proportion of exopodite seta of the second limb and presence of an element in the corm of limb in C. breviceps , and the proportion of seta 1 in the exopodite of the fourth limb.

In contrast to C. sphaericus , C. tilhoi has groups of stiff setae on the corm of the first limb, while C. sphaericus has smooth setulae; C. tilhoi has a group of denticles on the IDL corm while C. sphaericus has setulae; the IDL 3 seta in C. tilhoi is bisegmented and relatively thin, while C. sphaericus has a distinctly IDL 3 seta clearly hook-shaped and thick. In C. tilhoi , the third limb exopodite is approximately twice as tall as its wide, while in C. sphaericus this ratio is approximately 1.2; the setae on the basal endite are longer than the setae observed in C. sphaericus , similar in morphology to Disparalona species ( Sousa et al. 2018; Neretina et al. 2018). The exopodite of the fifth limb of C. tilhoi is large and lobed, while in C. sphaericus the exopodite of the fifth limb is relatively small and rounded.

Distribution and biology. Chydorus tilhoi has exclusive geographic distribution in the Afrotropic zone. The presence of the species extends from the Chari River Basin to the Congo River Basin ( Rey & Saint-Jean 1969; Korinek 1984). For a general limnological context for studied localities, see Borges et al. (2019).

Figures

FIGURE 3. Chydorus tilhoi Rey & Saint-Jeans, 1969 from the Congo River Basin, Africa, parthenogenetic female. A, habitus; B, dorsal view; C, ventral margin of carapace, anterior portion; D, ventral margin of carapace, posterior portion with denticles; E, head shield; F, idem, rostrum; G, head pores; H, labral keel; I, idem, frontal view; J, antenna I; K, antenna II. The authors thank to Dr. Alexey A. Kotov and an anonymous reviewer for the valuable suggestions and criticism. Scale bars = 0.05 mm.

FIGURE 4. Chydorus tilhoi Rey & Saint-Jeans, 1969 from the Congo River Basin, Africa, parthenogenetic female. A, limb I; B, idem, arrows showing stiff setae; C, idem, ODL and IDL; D, Limb II; E, limb III; F, idem, basal endite; G, limb IV; H, idem, basal endite and gnathobasic filter plate. Scale bars = 0.05 mm.

FIGURE 5. Chydorus tilhoi Rey & Saint-Jeans, 1969 from the Congo River Basin, Africa, parthenogenetic female. A, limb V; B, idem, outline showing variation on the exopodite shape; C-D, postabdomen. Scale bars = 0.05 mm.

FIGURE 6. Chydorus tilhoi Rey & Saint-Jeans, 1969 from the Congo River Basin, Africa, parthenogenetic female. A-B, habitus; C, rostrum; D, rostrum and antennule; E, denticle on the labral keel; F, arrow showing the position of major head pore; G, major head pore with a thick rim; H, ventral margin of carapace, anterior portion; I, ventral margin of carapace, median and posterior portions; J, denticle on the posteroventral margin of carapace.