A revision of the genus Disparalona (Cladocera, Chydorinae) in South America

The genus Disparalona Fryer, 1968 comprises a well-defi ned species complex, the hamatagroup, which might have sibling species in South America. This hamata-group needs urgent revision. Besides that, a complete morphological evaluation of the endemic species D. leptorhyncha (Daday, 1905) is lacking. Thus, the aim of the present study is to revise populations of species of the hamatagroup in South America and to redescribe D. leptorhyncha. Our fi ndings pointed to an occurrence of species which are part of the Disparalona (Mixopleuroxus) linage. Currently, the Neotropics have the highest diversity to the genus, with three species of the hamata-complex – D. (M.) hamata (Birge, 1879), D. (M.) lucianae sp. nov., D. (M.) tenuispina sp. nov. – in addition to D. (M.) leptorhyncha. These species can be differentiated from each other by the morphology of their rostrum, labrum, and postabdomen.


3
To enumerate the setae of limbs, we used the homology proposed by Kotov (2000aKotov ( , 2000b, which presented stability when tested in different cladoceran groups (Kotov et al. 2010). Drawings were prepared using a camera lucida attached to an Olympus BX41 phase contrast microscope.  European Journal of Taxonomy 460: 1-34 (2018) 6 IP about 0.35. Labrum (Fig. 1M-N). Elongated, without keel; lateral horns present. Maxilla (Fig. 1L). Well developed, with three setulate setae. Antennule -A1 (Fig. 1O). Length about two times the width, not exceeding the tip of rostrum; antennular sensory seta slender, about 1.3 times length of antennular body, inserted near to apex of the antennular body; nine aesthetascs which do not exceed the length of antennular body. Antenna -A2 (Fig. 1P). Coxal setae very long, about 1.5 times as long as fi rst exopodite segment; basal segment thick with short and thin spine; fi rst exopodite segment longer than fi rst segment of endopodite; second exopodite segment with a long seta reaching or exceeding midlength of longest apical seta; fi rst endopodite segment armed with a short spine about 0.4 of length of the apical spine; exopodite apical spine about 1.8 times longer than the segment itself, endopodite apical spine of similar length to apical segment; apical spines armed with short denticles. Antennal formula (exo/endo): spines 001/101, setae 113/003. THORACIC LIMBS ( Fig. 2A-G). Five pairs of thoracic limbs. First limb ( Fig. 2A). Epipodite oval shaped without projection; ODL armed with a short seta and a thin serrated seta longer than IDL fi rst seta; IDL (en 4) with two groups of short setulae; three setae present, third seta relatively slender, hook-like, heavily chithinized and with a group of short spines inserted at mid-length of seta; seta 1 slender and naked, about 1.6 times shorter than second seta; seta 2 about 1.3 times shorter than third seta, armed with long setulae; endite 3 with four setae, posterior setae (a-b) of similar length and shorter than anterior seta 1, seta (c) of similar length to setae (a-b); endite 2 with three posterior setae present (d-f), seta (f) about 1.2 times shorter than seta (e), both setae bear thin setulae on the lateral face, seta (d) setulated and about two times shorter than seta (e); endite 1 with three posterior setae of similar length (g-i), bisegmented and densely setulated on distal part; stiff setae long, of similar length to seta (f), seta (j) not studied; ejector hooks of similar length to corm of limb and armed with spines; ventral face of the limb with nine groups of setulae organized in clusters, increasing in length towards the distal portion. Gnathobase as a setulated setae. Second limb ( Fig. 2B-C). Exopodite short, armed with a seta shorter than exopodite itself; inner limb portion armed with eight slender scrapers; setulation on the scrapers is inconspicuous; scraper 1 with an accessory seta at its base, scraper 2 about 0.6 of length of scraper 1; proximal portion of gnathobase fl attened; distal portion armed with three elongated elements, fi rst element with distal portion sharp, geniculated, and setulated, second element armed with strong denticles, third element longer than mid length of others elements; fi lter comb with seven setulated setae. Third limb (Fig. 2D). Epipodite oval with a long projection; exopodite rectangular, about three times as heigh as wide, with four distal and three lateral setae; seventh seta setulated and longer than sixth and fi fth setae; fourth seta setulated, about twice as long as third seta; third seta similar in length to exopodite corm; second seta setulated, longer than third and fi rst setae, about 0.68 of length of the fourth seta; fi rst seta thin and naked; distal endite with three slender setae, seta 2 about 0.7 of length of seta 1 (third seta not studied); six long and plumose posterior setae decreasing in length towards gnathobase (a-f); basal endite with four setae of similar length (4-7); gnathobase armed with a long and cylindrical sensillum; fi lter comb with eight setulated setae. Fourth limb (Fig. 2E-F). Pre-epipodite oval and densely setulated; epipodite oval with a long projection; exopodite with seven marginal setae; setae 2-7 plumose; seventh seta shorter than sixth and fi fth setae and similar in length to third seta; fourth setae relatively long, about twice as long as second seta; fi rst seta very slender and short, about 0.4 of length of the second seta; distal endite with four setae (1)(2)(3)(4), one scraperlike (1), three setae fl aming-torch-like slightly setulated (3-4) and similar in length to seta 1; basal endite armed with three setulated setae of similar length; gnathobase thick, with three elements, armed with one curved setulated seta which is about twice as long as width of endite; fi lter plate with six setae. Fifth limb (Fig. 2G). Pre-epipodite rounded and densely setulated; epipodite oval with a long projection; exopodite wide, rounded, armed with four plumose setae and two setulated hillocks implanted near fi rst seta; fi rst seta about 0.75 of length of the second seta; third and fourth setae similar in length and longer than other setae of limb; internal lobe elongated, relatively rectangular and with many setulae, two setae of similar length on inner face of the lobe, seta 1 armed laterally with thick spinulae and setulae; seta 2 setulated; fi lter comb with four setulated setae, about 0.75 of length of internal lobe setae. ABDOMINAL AND POSTABDOMINAL STRUCTURES. Abdomen (Fig. 1A). About three times shorter than thorax. Postabdomen ( Fig. 3A-D). About 3.2 times as long as wide, ventral margin relatively straight with two rows of short spinulae; anal margin about 0.7 of length of the preanal margin, armed with a group of thin spines; postanal margin uniformly rounded and very long, about 2.5 times as long as anal margin, armed with 12-13 marginal merged denticles, distalmost denticles with width-at-base/height ratio about 0.45; lateral fascicle formed by thin spinulae not separated in groups; postabdominal setae about 0.7 of length of the postabdomen, bisegmented, armed with setulae from distal segment. Postabdominal claw (Fig. 3A, D). With two basal spines, about 0.28 of the length of the postabdomen and 1.3 times longer than anal margin, base naked; inner group of pecten with thin proximal spinulae; outer group separated in two groups, proximal with ticker spinulae. Basal spines (Fig. 3A, D). Naked, distal about 0.28 of length of the postabdominal claw, proximal about 2.5 times shorter than distal one.

Remarks
According to Smirnov (1996aSmirnov ( , 1996b, Disparalona (M.) leptorhyncha was initially described as Leptorhynchus rostratus. As the name rostratus was preoccupied in the genus of Disparalona, Smirnov (1996a) designated the epithet leptorhyncha. Disparalona (M.) leptorhyncha differs from all species of the genus by the absence of longitudinal lines on the carapace, antenna with very long coxal setae, long and curved rostrum, closely-set tiny pores located halfway between main head pores, labrum elongated and without keel ( Fig. 1M-N); third seta on the IDL relatively long and slender, ejector hook in the fi rst limb very long, stiff setae on the endites 2-3 long, of similar length to seta (f); seta on the exopodite of second limb short, scrapers long and slender, proximal portion of gnathobase fl attened and naked ( Fig. 2B-C); third limb exopodite three times as heigh as wide; postabdomen with short anal margin, postanal margin rounded with 12-13 merged denticles. This set of characters makes Disparalona (M.) leptorhyncha the most peculiar species of the genus when compared with hamata-complex and Holarctic species (Alonso 1996;Flössner 2000;Kotov & Sinev 2011;Klimovsky et al. 2015).

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 fi rst exopodite segment, fi rst 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 fi rst 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.  ODL armed with a short seta and a thin and serrated seta longer than IDL fi rst 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 fi ve long setulae; distal portion armed with four elements, fi rst 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; fi lter comb with eight setulated setae, fi rst 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 fi fth 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 fi rst setae, about 0.9 of length of the fourth seta; fi rst 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; fi lter 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, fi fth and fourth setae, slightly longer than third seta; fi fth seta slightly longer than fourth seta; fourth seta about two times as long as second seta; fi rst 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 fl aming-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; fi lter 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 fi rst seta; fi rst 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; fi lter 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 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.

Remarks
The

Distribution and biology
Disparalona (M.) hamata has a wide distribution in the American continent. Reports from Rey & Vasquez (1986)

Diagnosis
Body elongated, oval, length 0.36-0.48 mm, dorsal margin uniformly arched, ventral margin with wide aperture which is located after middle of body, toward the anterior part. Carapace covered by longitudinal lines and striae between them, posterodorsal corner without evident notch; posteroventral corner rounded, posterior part bears a group of long and thin spinulae that exceed the border. Rostrum short, about 1.7 times longer than antennular body, slightly curved with tip sharp. Antennule length about two times the width, not exceeding the tip of rostrum. Antenna with coxal setae slightly shorter than fi rst exopodite segment, fi rst endopodite segment armed with a short spine about 0.3 of length of the apical spine. First limb IDL seta 1 slender and armed with thin spines at distal part, about 2.2 times shorter than third seta; seta 2 about 1.2 times shorter than third seta, armed with long hard setulae, third seta relatively thick, hook-like, heavily chithinized and with a group of short spines at distal part. Second limb exopodite with a long seta about 2.8 times as long as exopodite itself, inner limb portion armed with one element and eight scrapers, of which 1 and 3 of similar length. Third limb exopodite about 2.5 times height than width, fourth seta setulated, about 1.4 times as long as third seta. Fourth limb fi rst seta slender and short, about 0.4 of length of the second seta. Fifth limb exopodite wide, rounded, armed with four plumose setae. Postabdomen with anal margin about 1.2 times shorter than preanal margin, 10 merged marginal denticles; distalmost denticle with width-at-base/height ratio about 0.23. Basal spines naked, distal about 0.22 of length of the postabdominal claw, proximal about 1.8 times shorter than distal one.

Etymology
The epithet 'lucianae' is given in honor to limnologist Luciana de Mendonça Galvão, an active researcher in the fi eld of aquatic ecosystems in the Brazilian Cerrado Biome.

Description of adult parthenogenetic female
GENERAL HABITUS (Figs. 7A-H). Body elongated, oval, length 0.36-0.48 mm, maximum height located after middle of body, height/length ratio about 0.55; 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 after middle of body, toward the anterior part. CARAPACE (Fig. 7D-E). Covered by longitudinal lines and striae between them; anteroventral margin with a widened fl ange; ventral margin with slight depression before mid length of body; 80 plumose setae per valve (the real number of setae is not represented in the fi gures), of which the posterior and anterior groups are longer than middle group, anterior and posterior setae internally articulated, setae around median part located exactly at the valve margin and clearly not articulated; posterodorsal corner without evident notch; posteroventral corner rounded, posterior part bearing a group of long and thin spinulae that exceed the border. CEPHALIC STRUCTURES (Fig. 7I-N). Ocellus smaller than eye. Rostrum short, about 1.7 times longer than antennular body, slightly curved with sharp tip; straight in frontal view. Head shield (Fig. 7J-K). Anterior and posterior region triangular shaped, covered by striae; two main pores with two closely-set tiny pores between them, which are located closer to the anterior than posterior main pore, PP/IP about 0.5. Labrum (Fig. 7L). Keel not prominent, distal portion short and rounded, lateral horns present. Maxilla. Not studied. Antennule -A1 (Fig. 7M). Length about two times the width, not exceeding the tip of rostrum; antennular sensory seta slender, slightly shorter than antennular body, inserted in the last third of antennular body; nine aesthetascs which do not exceed the length of antennular body. Antenna -A2 (Fig. 7N). Coxal setae slightly shorter than fi rst exopodite segment; basal segment thin with a short and thin spine; fi rst exopodite and endopodite segments of different lengths; fi rst endopodite segment armed with a short spine about 0.3 of length of the apical spine; exopodite apical spine about 1.3 times as long as apical segment and about 1.6 times as long as endopodite apical spine. Antennal formula (exo/endo): spines 001/101, setae 113/003. THORACIC LIMBS (Fig. 8A-J). Five pairs of thoracic limbs. First limb (Fig. 8A-B). Epipodite not studied; ODL armed with a thin seta and a serrated seta similar in length to IDL third seta; IDL (en 4) with two 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; seta 2 about 1.2 times shorter than third seta, armed with long hard setulae; endite 3 with four setae, posterior setae (a-b) of different length and longer than anterior seta 1, seta (c) of similar length to seta (a); endite 2 with three posterior setae (d-f), seta (f) about 1.5 times shorter than seta (e) being that both setae bear thin setulae on the lateral face, seta (d) setulated and about 2.5 times shorter than seta (e), stiff setae similar in length to seta (d); endite 1 with three posterior setae of similar length (g-i), bisegmented and densely setulated from distal part; stiff setae about 1.5 times shorter than seta (f), seta (j) not studied; ejector hooks about 1.3 times shorter than corm of limb and armed with spines; ventral face of the limb with seven groups of long setulae organized in clusters. Gnathobase as a setulated seta. Second limb (Fig. 8C). Exopodite with a long seta armed laterally with short spinulae, about 2.8 times longer than exopodite itself; inner limb portion armed with one element and eight scrapers; scrapers 1-5 armed with thin setulae, 6-8 armed with thick spinulae; scrapers 1 and 3 of similar length; scraper 2 slightly shorter than scrapers 1 and 3; scraper 4 about 1.5 times shorter than scraper 3; scraper 5 longer than scraper 4; scraper 6 about 1.5 times as long as scrapers 7 and 8; proximal portion of gnathobase not elongated and armed with six long setulae; distal portion armed with four elements, fi rst element is a sensillum, second element elongate with distal portion sharp, setulated and geniculated, third element armed with strong denticles, fourth element longer than mid-length of other elements; fi lter comb with eight setulated setae, fi rst seta densely setulated and shorter than others. Third limb (Fig. 8D-E). Epipodite oval with a short projection; exopodite rectangular, about 2.5 times as heigh as wide, with four distal and three lateral setae; seventh seta long, about 2.4 times as long as sixth seta and about twice as long as fi fth setae; fourth seta setulated, about 1.4 times as long as third seta; third seta setulated, longer than length of exopodite corm, about 1.6 times shorter than second seta; second seta setulated, about 1.3 times as long as fi rst setae, longer than fourth seta; fi rst seta thin, armed laterally with short spinulae; setae (1-2) of distal endite slender, seta 2 about 0.75 of length of the seta 1, seta 3 not studied; six long and setulated posterior setae of similar lengths (a-f); basal endite with four setae (3-6), distalmost seta longer than others; gnathobase armed with a long and cylindrical sensillum; fi lter comb with eight setulated setae. Fourth limb (Fig. 8F-I). Pre-epipodite rounded and densely setulated; epipodite oval without projection; exopodite with seven marginal setae; setae 3-7 plumose; seventh seta shorter than sixth, fi fth, fourth and third setae; fi fth seta slightly shorter than fourth seta; fourth seta about twice as long as second seta; fi rst 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 fl aming-torch-like slightly setulated (3-4) longer than seta 1; basal endite armed with three setulated setae of similar length; gnathobase thick, with two elements, armed with one curved setulated seta about longer than width of endite; fi lter plate with six setae. Fifth limb (Fig. 8J). Pre-epipodite rounded and densely setulated; epipodite oval without projection; exopodite wide, rounded, armed with four plumose setae and two setulated hillocks implanted near fi rst seta; fi rst seta about 0.75 of length of the second seta; second seta about 0.7 of the length of the third setae; fourth seta about 0.9 of the length of the third seta; internal lobe elongated, relatively oval 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 0.6 of length of the seta 1; gnathobase without elements; fi lter comb with four setulated setae longer than seta 2 of internal lobe.
ABDOMINAL AND POSTABDOMINAL STRUCTURES. Abdomen (Fig. 7A). About three times shorter than thorax. Postabdomen (Fig. 9A-E). About 3.4 times as long as wide, ventral margin slightly arched with two rows of short spinulae; anal margin about 1.2 times shorter than preanal margin; anal margin armed with one group of thin spines; postanal margin almost straight and long, about 1.6 times as long as anal margin, armed with 10 merged marginal denticles; distalmost denticle with width-at-base/height ratio about 0.23; lateral fascicle separated in 11 groups with short and thin spinulae, distalmost group bears 2-5 spinulae; postabdominal setae about 0.46 of length of the postabdomen, bisegmented, armed with setulae from distal part. Postabdominal claw ( Fig. 9A-B, D-E). 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. 9E). Naked, distal about 0.22 of length of the postabdominal claw, proximal about 3.2 times shorter than distal one.

Male
Unknown.

Distribution and biology
Disparalona (M.) lucianae sp. nov. is the most common species of the hamata-complex occurring in Brazil. This species can be found in lentic water bodies such as, reservoirs, fl oodplain environments, ponds, and shallow lakes. Disparalona (M.) lucianae sp. nov. is somewhat common in lotic systems.

Diagnosis
Body elongated, oval, length 0.33-0.52 mm, dorsal margin uniformly arched, ventral margin with wide aperture which is located after middle of body. Carapace covered by longitudinal lines and striae between them, posterodorsal corner with a notch; posteroventral corner rounded, posterior part bearing two groups of short and thick inner spinulae that do not exceed the border. Rostrum short, about 1.3 times longer than antennular body, curved with sharp tip. Antennule length about 2.6 times the width, not exceeding the tip of rostrum. Antenna coxal setae shorter than fi rst exopodite segment, fi rst endopodite segment armed with a short spine about 0.4 of length of the apical spine. First limb IDL seta 1 about 1.2 times shorter than second seta, armed with setulae, third seta relatively thick, hook-like, heavily chithinized, and with a group of short spines at distal part. Second limb exopodite with a long seta about 2.8 times longer than exopodite itself, inner limb portion armed with one element and eight scrapers.
Third limb exopodite about two times as high as wide, fourth seta about two times as long as third seta. Fourth limb fi rst seta very short, about 0.2 of length of the second seta. Fifth limb exopodite with characteristic shape, armed with four plumose setae. Postabdomen with anal margin about 1.4 times shorter than preanal margin, postanal angle evident; one unmerged marginal denticle, three merged marginal denticles, and six groups of long and thin spines; distalmost denticle with width-at-base/height ratio about 0.5. Basal spines naked, distal about 0.22 of length of the postabdominal claw, proximal about four times shorter than distal one.

Etymology
The epithet comes from the joining of the Latin words tenuis (fi ne) and spina (spine). The species' name refers to the groups of marginal setulae on the postabdomen.

Description of adult parthenogenetic female
GENERAL HABITUS (Fig. 10A-E). Body elongated, oval, length 0.33-0.52 mm, maximum height at located after middle of body, height/length ratio about 0.6; 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 after middle of body. CARAPACE (Fig. 10A-E). Covered by longitudinal lines and striae between them; anteroventral margin with a widened fl ange; ventral margin with slight depression before mid-length of body; 80-83 plumose setae per valve, of which the posterior group is longer than median and anterior groups, anterior and posterior setae internally articulated (the real number of setae is not represented in the fi gures); posterodorsal corner with a notch; posteroventral corner rounded, posterior part bears two groups of short and thick inner spinulae that do not exceed the border. CEPHALIC STRUCTURES (Fig. 10F-L). Ocellus smaller than eye. Rostrum short, about 1.3 times longer than antennular body, curved with sharp tip. Head shield (Fig. 10G). Covered by striae, posterior border with a projection behind main pores; two main pores with two closely-set tiny pores between them, which are located closer to the anterior than posterior main pore, PP/IP about 0.52. Labrum (Fig. 10H-I).
Keel not prominent, distal portion short and rounded, lateral horns present. Maxilla (Fig. 10J). Well developed, with three setulated setae. Antennule -A1 (Fig. 10K). Length about 2.6 times the width, not exceeding tip of rostrum; antennular sensory seta slender, about 1.2 times shorter than antennular body, inserted in the last third of antennular body; nine aesthetascs not exceeding the length of antennular body. Antenna -A2 (Fig. 10L). Coxal setae shorter than fi rst exopodite segment; basal segment thin with a short and thin spine; fi rst exopodite and endopodite segments of different lengths; fi rst endopodite segment armed with a short spine about 0.4 of length of the apical spine; exopodite apical spine slightly longer than apical segment and about 1.5 times longer than endopodite apical spine. Antennal formula: spines 001/101 (exo/endo), setae 113/003 (exo/endo).
THORACIC LIMBS (Fig. 11A-F). Five pairs of thoracic limbs. First limb (Fig. 11A-B). Epipodite not studied; ODL armed with a short seta and a thin seta armed with short setulae, similar in length to IDL third seta; IDL (en 4) with two groups 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 2 slender and armed with thin setulae at distal part, about 1.8 times shorter than third seta; seta 1 about 1.2 times shorter than second seta, armed with setulae; endite 3 with four setae, posterior setae (a-b) of different length and longer than anterior seta 1, seta (c) of similar length to seta (a) and shorter than seta (b); endite 2 with three posterior setae (d-f), seta (f) about 1.5 times shorter than seta (e) being that both setae bear thick spinulae on the lateral face, seta (d) setulated and about two times shorter than seta (e), stiff setae similar in length to seta (d); endite 1 with three posterior setae of similar length (g-i), bisegmented and densely setulated from distal part; stiff setae about 1.3 times shorter than seta (f), seta (j) not studied; ejector hooks about two times shorter than corm of limb and armed with spines; ventral face of the limb with seven groups of setulae organized in clusters, the distalmost groups with setae shorter than others. Gnathobase as a setulated seta. Second limb (Fig. 11C). Exopodite with a long seta armed laterally with short spinulae, about 2.8 times longer than exopodite itself; inner limb portion armed with one element and eight scrapers; scrapers 1-5 armed with thin setulae, 6-8 armed with thin spines; scrapers 1-3 of similar length; scraper 4 about 1.4 times shorter than scraper 3; scraper 5 longer than scraper 4; scraper 6 about 1.6 times as long as scraper 7 and 1.2 times as long as scraper 8; proximal portion of gnathobase not elongated and armed with fi ve long setulae; distal portion armed with four elements, fi rst element is a sensillum, second element elongated with distal portion sharp setulated and geniculated, third element armed with strong denticles, fourth element longer than mid-length of others elements; fi lter comb with eight setulated setae, fi rst seta densely setulated and shorter than others. Third limb (Fig. 11D). Epipodite oval with a short projection; exopodite rectangular about two times as high as wide, with four distal and three lateral setae; seventh seta setulated, about 1.7 times longer than sixth and fi fth setae; fourth seta setulated, about two times longer than third seta; third seta setulated, longer than exopodite corm, about 1.4 times shorter than second seta; second seta setulated, about 1.2 times longer than fi rst setae, shorter than fourth seta; fi rst seta thin, naked; setae (1-2) of distal endite slender, seta 2 about 0.6 of length of seta 1, seta 3 not studied; six setulated posterior setae (a-f) decreasing in length towards gnathobase; basal endite with four setae (3-6), distalmost seta longer than others; gnathobase armed with a long and cylindrical sensillum; fi lter comb with eight setulated setae. Fourth limb (Fig. 11E). Pre-epipodite rectangular and densely setulated; epipodite oval without projection; exopodite with seven marginal setae; setae 3-7 plumose; seventh seta slightly shorter than sixth and fi fth setae, longer fourth and third setae; fi fth seta shorter than fourth seta; fourth seta about 1.8 times as long as second seta; fi rst seta very short, about 0.2 of length of the second seta; distal endite with four setae (1)(2)(3)(4), one scraper-like (1), three setae fl aming-torch-like, slightly setulated (3-4), fi rst fl aming-torch longer than seta 1; basal endite armed with three setulated setae of similar length; gnathobase thick, with two elements, armed with one curved setulated seta about 1.3 times longer than width of endite; fi lter plate with six setae. Fifth limb (Fig. 11F). Pre-epipodite rounded and densely setulated; epipodite oval with long projection; exopodite with characteristic shape, armed with four plumose setae and two setulated hillocks implanted near fi rst seta; fi rst seta about 0.8 of length of the second seta; second seta slightly shorter than third seta; fourth seta about 0.9 of the length of the third seta; internal lobe elongated, relatively oval 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 0.7 of length of the seta 1; gnathobase without elements; fi lter comb with four setulated setae similar in length to seta 2 of internal lobe.
ABDOMINAL AND POSTABDOMINAL STRUCTURES. Abdomen (Fig. 10A). About three times shorter than thorax. Postabdomen (Fig. 12A-B). About three times as long as wide, ventral margin straight with two rows of short spinulae; anal margin about 1.4 times shorter than preanal margin, postanal angle evident; anal margin armed with one group of thin and long spines; postanal margin wavy , about 2.2 times as long as anal margin, armed with one unmerged marginal denticle, three merged marginal denticles and six groups of long and thin spines; distalmost denticle with width-at-base/height ratio about 0.5; lateral fascicle on the postanal face separated by a long gap, fascicules composed of short and thin spinulae; postabdominal setae about 0.42 of length of the postabdomen, bisegmented, armed with setulae from distal part. Postabdominal claw (Fig. 12A). With two basal spines, about 0.25 of the length of the postabdomen and slightly longer than anal margin, base naked; pecten composed of thin spinulae. Basal spines (Fig. 12A). Naked, distal about 0.22 of length of the postabdominal claw, proximal about four times shorter than distal one.

Male
Unknown.

Remarks
Disparalona (M.) tenuispina sp. nov. differs from all species of the genus by its markedly short and curved rostrum about 1.3 times as long as antennular body, the postabdomen armed with one unmerged marginal denticle, three merged marginal denticles and six groups of long and thin spines, and the lateral fascicle separated for a long gap. Furthermore, it can be differentiated from D.

Distribution and biology
Disparalona (M.) tenuispina sp. nov. is an hyporheic species. It is rare, having been observed in the type locality only so far. Disparalona (M.) tenuispina sp. nov. was collected together with Pseudosida ramosa (Daday, 1904), Macrothrix sp., Chydorus eurtynotus Sars, 1901, and Ilyocryptus spinifer Herrick, 1992. Fryer (1968) raised evidence to separate Alonella rostrata to a new genus (Disparalona Fryer, 1968), namely, the structure of a seta on the endite of the third limb and the way it is used for feeding. Afterwards, Fryer (1971) observed the same feeding behavior and morphology of the third limb in specimens of Alonella acutirostris Birge, 1879 [= Disparalona acutirostris (Birge, 1879)]. Although lacking in solid taxonomic basis to support Disparalona, Fryer's initial conclusions were correct.
Disparalona (Disparalona) seems to be a linage with a mainly Holarctic distribution and with some species presenting intrusion in the Oriental zone. Species of Disparalona (Disparalona) also share a long spine of the fi rst endopodite segment of the antenna (see Alonso 1996;Flössner 2000;Hudec 2010;Kotov & Sinev 2011;Klimovsky et al. 2015;Neretina et al. 2018). For species of this linage which have the fi rst limb described, relatively short ejector hooks are observed when compared to length of corm limb. Another species that might belong to Disparalona (Disparalona) is D. acutirostris (Birge, 1879), but, limb description is lacking (for external morphology, see Frey 1961). It seems that the long seta on the fi lter plate of the third limb mentioned by Fryer (1968) is exclusive to (i.e. only belongs to D. (D.), no other subgenus) or only observed in fresh animals.

Concluding remarks
The Disparalona