Chydorus obscurirostris tasekberae Frey, 1987

Sinev, Artem Y., Dadykin, Ivan A., Umi, Wahidah A. D. & Yusoff, Fatimah M., 2025, New data on Cladocera (Crustacea: Branchiopoda) of Peninsular Malaysia, Zootaxa 5604 (3), pp. 255-284 : 271-278

Chydorus obscurirostris tasekberae Frey, 1987

Fig. 11–14

Idris & Fernando 1981a: 243–246, Figs. 44–46 (faviformes); Idris,1983: (faviformes); Frey 1987a: 369–371, Fig. 78–85, 88– 115, 130–143; Kotov 2006: Fig. 4a –ж.

Material examined: over 30 parthenogenetic females from several localities in Chini Lake , Pahang state .

Description. Parthenogenetic female.

Body shape is typical of the genus Chydorus , oval in juveniles ( Fig. 11A) and rounded in adults ( Fig. 11B–C, 12A–D) in lateral view, height/length ratio about 0.9, maximum height at midline. In frontal and posterior view ( Fig. 12E–F, 13A), body subtriangular, with weakly convex dorsal surface and moderately developed egg locules, which narrow significantly in the ventral half. Valves and head shield completely covered by numerous honeycomb-like meshes, not organized into ridges or collars ( Fig. 13B–C). Height of mesh walls of variable height, about 20 µm in highest meshes. Mesh walls edge with irregularly spaced, very short dense setulae. Surface of valves and head shield inside the meshes with a characteristic, irregular spider web-like patterns of tiny ridges of variable height. Valves. Anterior angle broadly rounded, with a submarginal flange at inner side. Ventral margin of valves with about 50 setae; 10–12 anterior setae very thin, short, located on the inner side of valves close to margin, followed by about ten very short setae located maginally. Posterior group consists of about 28 long setae, armed bilaterally with long setules ( Fig. 11D), located on inner side of valve; maximum length of setae in the middle of the group. Two–three posteriormost setae straight, thick, spine-like. Postero-ventral angle without denticles. Valves covered by a honeycomb-like polygonal sculpture, about 120 cells per valve. Largest cells are located at egg locules, at the border with head shield, size of cell gradually decreases toward outer margins of valves.

Head with a short rostrum, protruding downward and posteriorly ( Fig. 11E, 13B). Length of rostrum about 1.5 length of antennule. Ocellus 1.5 times smaller than eye. Head shield large, with maximum width at the middle of posterior portion; its posteriormost portion widely oval ( Fig. 12F, 13A). Rostrum ( Fig. 11E, 13B) wide triangular in frontal view, apex divided into two small asymmetric lobes, lateral meshes obscure lateral margins of rostrum. Head shield covered by a honeycomb-like polygonal sculpture of less than 100 asymmetrically spaced, irregularly varying in size meshes ( Fig. 13A). Largest meshes are located at posterior margin of head shield. Two major head pores ( Fig. 13D), each at the break of mesh wall between two meshes on surface of head shield. PP is about 1 IP. Lateral head pores minute ( Fig. 13E), located asymmetrically, close to midline of head shield, at the middle between anterior and posterior major head pores. Labrum with a small, oval labral keel ( Fig. 11F) having a rounded apex. Height of keel about 1.5 widths. Anterior margin of labral keel convex, posterior margin weakly convex.

Postabdomen ( Fig. 11G, 13F) short, rather narrow, weakly narrowing distally. Length about 3.5 heights. Ventral margin clearly concave. Base of claws delimited from the distal margin by a clear incision. Distal margin convex, distal angle rounded. Dorsal margin straight in postanal portion and weakly concave in the anal one. Distal part of postabdomen about 1.5 times longer than the preanal portion, postanal portion two times shorter than anal portion. Preanal angle well-expressed, triangular, well-prominent; postanal angle not defined. Preanal margin unequally concave. Dorsal margin with about eight narrow sharp denticles and 2–3 broad groups of short setules in anal portion. Length of longest denticles slightly greater than width of base of postabdominal claw. A row of 7–8 lateral groups of short setulae, the number of setulae in the group increases basally.

Postabdominal claw ( Fig. 13F) slender, weakly curved, slightly shorter than preanal portion of postabdomen, with a distinctive pecten of setules on dorsal margin. The basal portion of pecten consists of about 10 short spinules, distal portion of about 15 longer setules. Several long setulae located ventrally near the claw end. Two basal spines; distal spine length 0.25 length of the claw, proximal spine two times shorter.

Antennula ( Fig. 11H) of moderate size; length about two widths. Antennular seta thin, about half length of antennule, arising at 2/3 distance from the base. Nine terminal aesthetascs, six of them about 2/3 length of antennule, three about 1/3 length of antennule.

Antenna is relatively short ( Fig. 11I–J). Antennal formula: setae 0-0-3/0-1-3; spines 1-0-1/0-0-1. Branches relatively short; proximal segment of both branches 1.5 times longer and thicker than two others. Seta arising from middle segment of endopodite as long as most apical setae. Apical segment of endopodite with a single subapical seta and two apical setae, one of apical setae two times shorter and thinner than two others. Apical segment of exopodite with a single subapical seta and two apical setae of similar size. All antennal spines very short.

Thoracic limbs: five pairs.

Limb I of moderate size ( Fig. 14A–C). Epipodite oval, with process 1.5 times longer than exopodite itself. ODL with two setae, one of them very small. IDL with three setae and several clusters of setules. IDL ( Fig.14 B-C) with setae 1–2 thin, about 1/3 and 1/2 length of longest ODL seta, respectively; seta 3 thick, strong, claw-like, little shorter than longest ODL seta, armed with about 20 hard setulae in the distal part. Base of IDL seta 3 two times wider than base of setae 1–2. A small sensillum located near bases of IDL setae 2–3. Endite 3 with four setae, inner seta (1) ( Fig.14A) shorter than outer setae (a–c) ( Fig. 14B). Endite 2 with seta d ( Fig.14B) slightly longer than setae of endite 1, setae e–f ( Fig.14A) almost as long as limb body, seta e slightly longer than seta f, and an inner seta (2) on anterior face of limb. Endite 1 with three two-segmented setae (g-i) ( Fig.14B), flattened shorter seta i close to limb base ( Fig.14A), and an inner seta armed with spinules (3) on anterior face of limb. Six-seven rows of thin long setules on ventral face of limb. Two ejector hooks, first one slightly shorter than the other one.

Limb II subtriangular ( Fig. 14D). Exopodite oval, with seta two times longer than exopodite. Eight scraping spines; spines 1–3 long, slightly decreasing in size basally, armed with fine spinules; spines 4–8 short, with length of setae 4–5 and 7–8 evenly decreasing basally, seta 6 much thinner and shorter than neighbours; seta 5 thicker than others, armed with thick spinules, seta 4 and 6–8 armed with spinules of moderate size. An elongated sensillum located between spines 3 and 4. Distal armature of gnathobase with four elements. Filter plate II with eight setae, the two posteriormost members much shorter than others, about 1/2 and 2/3 lengths of other setae, respectively.

Limb III ( Fig. 14E–F). Epipodite oval, without projection. Exopodite subquadrangular, with three lateral (1–3) and four terminal (4–7) setae. Seta 4 being longest; setа 6 slightly shorter than seta 4, setae 1–2 and 7 subequal in length, about 2/3 length of seta 4, setae 3 and 5 about 1/2 length of seta 4. Setae 1–5 flattened, plumose; seta 6 slender, with row of very long, thick setules at the middle and small spinules in distal portion; seta 7 slender, naked. Distal endite with 3 scraping setae (1–3) and two small sensillae located between their bases, setae 1–2 ( Fig.14E) slender, of similar length, with small denticles in distal part, seta 3 ( Fig.14F) small and thin, three times shorter than setae 1–2. Basal endite with six plumose setae (a–f) ( Fig.14E) slightly increasing in size basally. Four pointed inner setae (4–7) ( Fig.14F) increasing in size basally; an elongated sensillum near the base of seta 4. Distal armature of gnathobase ( Fig.14F) with four elements: one very large sensillum with curved distal portion, strongly geniculated seta, short spine and sensillum of moderate size. Filter plate III with eight setae.

Limb IV ( Fig. 14G–H). Pre-epipodite setulated; epipodite elongated, with process longer than epipodite itself. Exopodite rounded with seven setae; seta 1 being longest, length of setae decreases evenly from seta 1 to seta 4; seta 4 about 2/3 length of seta 1; seta 5 and 7 about 1/3 length of seta 1; seta 6 about 1/2 length of seta 1. Setae 1–5 flat, plumose; seta 6 setulated unilaterally in basal part, seta 7 naked. Inner portion of limb IV with four setae. Scraping seta (1) slender; three flaming-torch setae (2–4) slightly increasing in size basally, each armed with 7–8 thick long setulae. Small sensilla located near base of seta 3. Four inner setae (a–d) increasing in length basally. Gnathobase with one 2–segmented setae, a small hillock distally, and two small sensillae. Filter plate with six setae.

Limb V ( Fig. 14I–J). Pre-epipodite setulated; epipodite oval, with two processes protruding at the different sides of exopodite, one as long as epipodite, another two times longer. Exopodite ovoid, with four setae, length of setae 1–3 evenly decreasing basally, seta 3 about 2/3 length of seta 1, seta 4 as long as seta 3. Setae 1–3 plumose, seta 4 bilaterally armed with thick setulae in distal poirtion. Two small hillocks with thick setulae located on basal side of exopodite near seta 4. Inner lobe long, narrow, with setulated inner margin. At inner face, two setae of similar length, in distal portion armed unilaterally with thick setulae. Filter plate with four setae.

Ephippial female and male unknown.

Size. Length of females (excluding honeycomb meshes) in studied material was up to 0.38 mm, height up to 0.3 mm; length of smallest juvenile female 0.24 mm, height 0.2 mm. According to Frey (1987a), length of adult females from Bera Lake was 0.28–0.39 mm.

Taxonomic notes. The morphology of thoracic limbs for most species of the faviformis -group was described only by Frey (1987a); a full set of drawings was provided only for C. obscurirostris obscurirostris Frey, 1987 . Frey (1987a) did not find any significant differences between the species of the faviformis -group in limb morphology, except in the morphology of IDL seta. Comparison of limb morphology of C. o. obscurirostris ( Frey, 1987a: fig. 86– 94) with that in our populations reveals that C. o. tasekberae has a more robust seta 1 of IDL of limb I. Despite Frey (1987a) did not give information on epipodite morphogy in C. obscurirostris , our study shows unusual structure of epipodite V in C. o. tasekberae, which has been never reported for any other Chydorus taxa. Within Cladocera , doubled epipodite process occurs very rarely: to date, it was observed in limb I of rare African Aloninae Matralona simoneae ( Dumont, 1981) ( Van Damme & Dumont 2009) and in thoracic limb I of Bosminopsis Richard, 1895 (see Garibian et al. 2021; Kotov & Garibian 2021).

The only detailed description of thoracic limbs for the faviformis -group was produced for C. izvekovae Sinev, Novichkova & Chertoprud, 2022 ( Sinev et al. 2022) recently described from North-East Russia. In Chydorus , differences in the thoracic limb morphology between species of the same group are generally either absent( Klimovsky & Kotov 2015), or sometimes closely related species, like these of the eurynotus -group, differ by proportions of IDL setae ( Sinev 2014). However, there are numerous differences between limbs of C. obscurirostris tasekberae and C. izvekovae , which are summarized in Table 1. These two species differ in morphology of IDL setae of limb I, in proportions of limb I endites setae, in morphology of scrapers of limb II, and in proportions of setae of exopodite III. Chydorus izvekovae has epipodite V with single process, as common for the majority of Chydoridae .

izvekovae (see Sinev et al. 2022).

Recent studies of the genus Chydorus ( Sinev 2014; Sousa et al. 2024) revealed that two species, South-East Asian C. breviceps ( Stingelin, 1905) and African C. tilhoi Rey & Saint-Jeans, 1969 have numerous differences from the majority of the genus in outer morphology, morphology of head pores, postabdomen and appendages, and probably should be placed into a separate genus. These authors did not discuss taxonomic status of the faviformis - group. Morphology of thoracic limbs of C. obscurirostris tasekberae (our data) and C. izvekovae ( Sinev et al. 2022) is generally similar to that of C. sphaericus group (see Klimovsky & Kotov 2015; Sousa et al. 2024). While outer morphology of faviformis -group species is outstanding, the group shares main characters of Chydorus s. str. in morphology of head pores, postabdomen, labrum and head appendages, and its position within the genus is wellsupported.

Distribution and ecology. Chydorus obscurirostris tasekberae Frey, 1987 was described from Bera Lake in Peninsular Malaysia ( Frey 1987a). It was recorded in North-East Thailand ( Maiphae et al. 2008; Kotov 2006) and Vientiane Province, Laos ( Kotov et al. 2013), but the latter record is doubtful, as provided illustrations show specimens with central meshes of the valves only slightly larger than meshes near valve edges. In Malaysia, C. obscurirostris tasekberae is found only in Bera Lake and Chini Lake among macrophytes, it is quite rare there.