Disparalona (Mixopleuroxus) hamata ( Birge, 1879 )

Sousa, Francisco Diogo R., Elmoor-Loureiro, Lourdes M. A., Mugnai, Riccardo, Panarelli, Eliana Aparecida & Paggi, Juan César, 2018, A revision of the genus Disparalona (Cladocera, Chydorinae) in South America, European Journal of Taxonomy 460, pp. 1-34 : 9-15

Disparalona (Mixopleuroxus) hamata ( Birge, 1879)

Figs 4–6 , 13

For synonymy, see Neretina et al. (2018).

Diagnosis

Body elongated, length 0.48–0.50 mm, dorsal margin uniformly arched, ventral margin with wide aperture which is located at the middle of the body. Carapace covered by longitudinal lines and striae between them, posterodorsal corner with a notch, posteroventral corner rounded, posterior part bears a group of long and thin spinulae exceeding the border. Rostrum short, about 2.8 times as long as antennular body, not curved, pointed downwards. Antennule length about 1.7 times the width, not exceeding the tip of rostrum. Antenna with coxal setae about 0.55 the length of the first exopodite segment, first endopodite segment armed with a short spine about 0.35 of the length of the apical spine. First limb with IDL seta 1 slender and armed with thin spines at distal part, about 2.2 times shorter than third seta; setae 2 about 1.2 times shorter than third seta, armed with long setulae, third seta relatively thick, hook-like, heavily chithinized and with a group of short spines at distal part. Second limb with exopodite armed with a long seta, about three times longer than exopodite itself, inner limb portion armed with eight slender scrapers decreasing in length towards gnathobase. Third limb exopodite rectangular, about 2.5 times as heigh as wide, fourth seta about 1.2 times as long as third seta. Fourth limb first seta slender and short, about 0.4 of length of the second seta. Fifth limb exopodite wide, rounded, armed with four plumose setae. Postabdomen anal and preanal margins similar in length, 11 marginal denticles, distalmost denticles not bearing spinulae or spines, proximalmost denticles are merged; distalmost denticles with width-at-base/ height ratio about 0.25. Basal spines naked, distalmost about 0.27 of length of the postabdominal claw, proximal about 1.7 times shorter than distal one.

Material examined

BRAZIL: 5 parthenogenetic ♀♀, Rio Grande do Sul, Santa Maria, fish pond at Universidade Federal de Santa Maria , 29°43ʹ21.87ʺ S, 53°43ʹ42.86ʺ W, Jul. 2014, F.D.R. Sousa and M.M. Dallosto leg. (FDR 0517).

ARGENTINA – Provincia de Santa Fé: 4 parthenogenetic ♀♀, Sirgadero Island, Paraná River , 31°40´51ʺ S, 60°40´17ʺ W, Oct. 1972, J.C. Paggi leg. ( FDRS 0520 ) ; 6 parthenogenetic ♀♀, Clucellas Island, Laguna Los Matadores , Paraná River, 31°42´33ʺ S, 60°44´30ʺ W, Oct. 1975, J.C. Paggi leg. ( FDRS 0520 ) .

Description of adult parthenogenetic female

GENERAL HABITUS ( Fig. 4A–E ). Body elongated, length 0.48–0.50 mm, maximum height at middle of body, height/length ratio about 0.5; dorsal margin uniformly arched, without dorsal keel or lateral projection; in dorsal and ventral view, body not laterally compressed; ventral margin with wide aperture which is located at the middle of the body.

CARAPACE ( Fig. 4A–E ). Covered by longitudinal lines and striae between them; anteroventral margin with a widened flange; ventral margin with slight depression at mid length of body; 73–87 plumose setae per valve (the real number of setae is not represented in the figures) of which the posterior and anterior groups are longer than the middle group, anterior and posterior setae internally articulated, setae around median part located exactly at the valve margin and clearly not articulated; posterodorsal corner with a notch; posteroventral corner rounded, posterior part bearing a group of long and thin spinulae exceeding the border.

CEPHALIC STRUCTURES ( Fig. 4F–M ). Ocellus smaller than eye. Rostrum short, about 2.8 times as long as antennular body, not curved, pointed downwards, with sharp or bifid tip; straight in frontal view. Head shield ( Fig. 4H ). Covered by striae, posterior border triangular; two main pores with two closelyset tiny pores between them, located closer to the anterior than posterior main pore, PP/IP about 0.4. Labrum ( Fig. 4I –J ). Keel not prominent, distal portion elongated and triangular, lateral horns present. Maxilla ( Fig. 4K ). Well developed, with three long setulated setae. Antennule – A1 ( Fig. 4L ). Length about 1.7 times the width, not exceeding the tip of rostrum; antennular sensory seta slender, about same length of antennular body, inserted in the second third of antennular body; nine aesthetascs not exceeding the length of antennular body. Antenna – A2 ( Fig. 4M ). Coxal setae about 0.55 the length of the first exopodite segment; basal segment thick with short and thin spine; first exopodite and endopodite segments of similar length; first endopodite segment armed with a short spine about 0.35 of the length of the apical spine; exopodite apical spine about 1.2 times longer than apical segment and about 1.5 times longer than endopodite apical spine. Antennal formula (exo/endo): spines 001/101, setae 113/003.

THORACIC LIMBS ( Fig. 5A–H ). Five pairs of thoracic limbs. First limb ( Fig. 5A–C ). Epipodite not studied; ODL armed with a short seta and a thin and serrated seta longer than IDL first seta; IDL (en 4) with a group of short setulae, three setae present, third seta relatively thick, hook-like, heavily chithinized and with a group of short spines at distal part; seta 1 slender and armed with thin spines at distal part, about 2.2 times shorter than third seta; setae 2 about 1.2 times shorter than third seta, armed with long setulae; endite 3 with four setae, posterior setae (a–b) of similar length and longer than anterior seta 1, seta (c) of similar length to setae (a–b); endite 2 with three posterior setae (d–f), seta (f) about 1.7 times shorter than seta (e) being that both setae bear thin setulae on the lateral face, seta (d) setulated and about two times shorter than seta (e), stiff setae about 1.5 times shorter than seta (d); endite 1 with three posterior setae of similar length (g–i), bisegmented and densely setulated from to distal part; stiff setae about 1.2 times shorter than seta (f), seta (j) not studied; ejector hooks about mid length of corm of limb and armed with spines; ventral face of the limb with eight groups of long setulae organized in clusters. Gnathobase not studied. Second limb ( Fig. 5D ). Exopodite with a long seta armed laterally with short spinulae, about three times longer than exopodite itself; inner limb portion armed with eight slender scrapers decreasing in length towards gnathobase, scrapers bear short and thin spinulae; proximal portion of gnathobase not elongated and armed with five long setulae; distal portion armed with four elements, first element is a sensillum, second element elongated with distal portion sharp and geniculated, third element armed with strong denticles, fourth element longer than mid-length of others elements; filter comb with eight setulated setae, first seta densely setulated and shorter than others. Third limb ( Fig. 5E ). Epipodite subrectangular without projection; exopodite rectangular about 2.5 times as heigh as wide, with four distal and three lateral setae; seventh seta setulated and longer than sixth and fifth setae; fourth seta setulate, about 1.2 times as long as third seta; third seta setulated, longer than length exopodite corm, shorter than second seta; second seta armed laterally with short spinulae, about 1.6 times as long as first setae, about 0.9 of length of the fourth seta; first seta thin and naked; setae (1–2) of distal endite slender, seta 2 about 0.7 of length of the seta 1, seta 3 not studied; six long and setulated posterior setae increasing in length towards gnathobase (a–f); basal endite with four setae (3–6), distalmost seta longer than others; gnathobase armed with a long and cylindrical sensillum; filter comb with eight setulated setae. Fourth limb ( Fig. 5F–G ). Pre-epipodite rounded and densely setulated; epipodite rectangular without projection; exopodite with seven marginal setae; setae 3–7 plumose; seventh seta shorter than sixth, fifth and fourth setae, slightly longer than third seta; fifth seta slightly longer than fourth seta; fourth seta about two times as long as second seta; first seta slender and short, about 0.4 of length of the second seta; distal endite with four setae (1–4), one scraper-like (1), three setae flaming-torch-like slightly setulated (3–4), longer than seta 1; basal endite armed with three setulated setae of similar length; gnathobase thick, with three elements, armed with one curved setulated seta about 1.3 times as long as width of endite; filter plate with six setae. Fifth limb V ( Fig. 5H ). Pre-epipodite rectangular and densely setulated; epipodite oval with a very short projection; exopodite wide, rounded, armed with four plumose setae and two setulated hillocks implanted near first seta; first seta about 0.7 of length of the second seta; third and fourth setae different in length, longer than other setae of limb; fourth seta about 0.9 of the length of the third seta; internal lobe elongated, relatively rectangular and with many setulae, two setae of different length on inner face of the lobe, seta 1 armed laterally with thick spinulae and setulae; seta 2 setulated, about 0.75 of length of the seta 1; gnathobase with an element; filter comb with four setulated setae longer than seta 2 of internal lobe.

ABDOMINAL AND POSTABDOMINAL STRUCTURES. Abdomen ( Fig. 4A ). About two times shorter than thorax. Postabdomen ( Fig. 6A–C). About 3.7 times as long as wide, ventral margin slightly arched with two rows of short spinulae; anal and preanal margins similar in length; anal margin armed with four groups of thin spines; postanal margin almost straight and long, about 1.4 times as long as anal margin, armed

with 11 marginal denticles, distalmost denticles do not bear spinulae or spines, proximalmost denticles are merged; distalmost denticles with width-at-base/height ratio about 0.25; lateral fascicle separated in ten groups with short and thin spinulae; postabdominal setae about 0.5 of length of the postabdomen, bisegmented, armed with setulae. Postabdominal claw ( Fig. 6A, C). With two basal spines, about 0.25 of the length of the postabdomen and slightly shorter than anal margin, base naked; inner group of pecten separated in two groups, proximal with thick spinulae, distal thin spinulae; outer pecten armed with thin spinulae. Basal spines ( Fig. 6A–C). Naked, distalmost about 0.27 of length of the postabdominal claw, proximal about 1.7 times shorter than distal one.

EPHIPPIAL FEMALE, EPHIPPIUM. Unknown.

Remarks

The morphology of South American populations of D. (M.) hamata is very similar to the description presented in Neretina et al. (2018). The most relevant aspect of variability was observed just in the distalmost marginal denticles of the postabdomen, which were smooth and unmerged; the first seta on the exopodite of the fifth limb is longer when compared to the redescription performed by Neretina et al. (2018). Disparalona (M.) hamata differs from D. (D.) rostrata , D. (D.) leei , D. (D.) ikarus and D. (D.) smirnovi in armature of seta 3 of the IDL which is thick, hook-like, and heavily chithinized. The labrum of D. (M.) hamata has a distal portion elongated and triangular, differently from D. (M.) lucianae sp. nov. and D. (M.) tenuispina sp. nov. which has a labrum with distal portion very short and rounded. Compared to D. (M.) caudata, Disparalona (M.) hamata is a small species it's body reaching up to 0.50 mm (height/length ratio about 0.5), while D. (M.) caudata reaches up to 0.6 mm (height/length ratio about 0.45); seta 1 of IDL in D. (M.) hamata is slender and armed with thin spines at distal part and about 1.8 times shorter than second seta, while in D. (M.) caudata the same seta is naked and about 2.5 times shorter than second seta; the postabdomen of D. (M.) hamata bears up to 11 marginal denticles while D. (M.) caudata has 14 marginal denticles ( Smirnov 1996a, 1996b). Disparalona (M.) hamata differs from D. (M.) acutirostris in the morphology of rostrum, length of basal spines and proportion between them ( Frey 1961). Disparalona (M.) hamata can be differentiated from D. (M.) leptorhyncha in the morphology of habitus, postabdomen, and limbs.

Distribution and biology

Disparalona (M.) hamata has a wide distribution in the American continent. Reports from Rey & Vasquez (1986) and Fuentes-Reines et al. (2018) probably belong to Disparalona (M.) hamata .

Figures

Fig. 4. Disparalona (Mixopleuroxus) hamata (Birge, 1879), parthenogenetic female. A. Habitus. B. Ventral view of carapace. C. Anterior portion of ventral margin of carapace. D. Middle portion of ventral margin of carapace. E. Posterior portion and posteroventral corner of carapace. F. Rostrum. G. Idem, frontal view. H. Main head pores. I. Labrum. J. Idem, frontal view. K. Maxilla. L. Antennule. M. Antenna. Scale bars: C–L = 50 μm; A–B = 100 μm.

Fig. 5. Disparalona (Mixopleuroxus) hamata (Birge, 1879), parthenogenetic female. A. First limb. B. Idem, stiff setae. C. Idem, ODL and IDL. D. Second limb. E. Third limb. F. Fourth limb. G. Idem, distal endite. H. Fifth limb. Scale bar = 50 μm.

Fig. 13. Pictorial key to separation of species of Disparalona Fryer, 1968 occurring in South America.