Disparalona (Mixopleuroxus) Hudec, 2010

Neretina, Anna N., Garibian, Petr G., Sinev, Artem Y. & Kotov, Alexey A., 2018, Diversity of the subgenus Disparalona (Mixopleuroxus) Hudec, 2010 (Crustacea: Cladocera) in the New and Old World, Journal of Natural History 52 (3 - 4), pp. 155-205 : 160-161

Subgenus Disparalona (Mixopleuroxus) Hudec, 2010

Type species

Mixopleuroxus striatiodes ( Šrámek-Hušek, 1946) . Type species was fixed by Hudec (2010) by monotypy (International Commission on Zoological Nomenclature ( ICZN) 2000, case 68.3) .

Emended diagnosis

Parthenogenetic female. Middle-sized Chydorinae , length of adult parthenogenetic female up to 0.63 (0.80 according to Hudec 2010) mm. In lateral view, body elongatedsubovoid, with maximum height almost at the middle. Height/length ratio about 0.60. In dorsal and ventral view body moderately compressed laterally, lateral processes on valves absent. Head relatively small, with a relatively long rostrum, distance between centre of eye and ocellus subequal in length to distance between centre of ocellus and tip of rostrum or significantly (two or three times) larger than it. Dorsal and posterior margins of valves convex, posterodorsal angle expressed; posteroventral and anteroventral angles broadly rounded. Posteroventral angle of valve without any denticles. Inner side of valve posterior margin covered by a row of numerous fine setules located quite close to the margin, these setules decreasing in size posteriorly. Sculpture of valves and head strongly developed. Sculpture of posterior and anterior portion of valves as long parallel sometimes anastomosing lines. Outer surface between them covered by additional fine striation, represented by more short lines. Head shield reticulation with short and relatively long parallel lines. Structure of head pores typical for the subfamily Chydorinae : two major pores and two minute pores between them at head shield midline, located slightly asymmetrically to midline, closer to anterior major pore than to posterior one. Labral keel relatively long, with a drawn tip, or short, with a rounded tip. Postabdomen subrectangular, postanal margin almost subequal in size to anal margin or almost in two times longer. Postanal margin almost straight. Distal angle of postabdomen almost right. Postanal margin armed with composite teeth, decreasing in size proximally. Anal margin and lateral surface of postanal margin covered by bunches of fine setules. Postabdominal claw long, subequal in length to anal margin, with two basal spines of different size. Antenna I typical for chydorids, antennular sensory seta slender, arising subdistally, its length subequal to antennular body length. Antenna II with a very short spine on proximal exopod segment (shorter than 1/4 of next exopod segment). Five thoracic limbs. Number of setae on thoracic limbs typical for subfamily Chydorinae . Inner distal lobe of thoracic limb I with two thin setae of different length and one thick, hook-like seta. Two lateralmost setae on exopodite of limb IV covered by short setules, unequal in size, lateralmost seta almost two times shorter than nearest seta. Ephippial female similar with parthenogenetic female in lateral view. Ephippium not bordered from the rest of valves, brownish, contains a single large egg.

Male. In lateral view, body oval, elongated, up to 0.48 mm in length. Armature of valves and ornamentation of valves and head shield as in parthenogenetic female. Dorsal side of postabdomen almost straight or strongly concave. Postabdominal claw with two basal spines. Gonopores open laterally or ventrally almost near claw bases. Antenna I with male seta located almost at middle of antennular body, near sensory seta. Limb I with a massive copulatory hook, that only slightly thicker than hook-like seta on IDL.

Differential diagnosis. D. ( Mixopleuroxus ) differs from the nominotypical taxon in (1) a very short spine on proximal exopod segment of antenna II; (2) distal angle of postabdomen is almost right; (3) two thin setae unequal in size and one long thick hook-like seta on inner distal lobe of thoracic limb I; (4) two thin setae covered by short setules are unequal in length on exopodite IV, lateralmost seta is almost two times shorter than nearest seta. In contrast, Disparalona s. str. has: (1) a massive and long spine on proximal exopod segment of antenna II; (2) distal angle of postabdomen is broadly rounded; (3) two long thin setae subequal in size and one short seta on inner distal lobe of thoracic limb I; (4) two thin setae covered by short setules are subequal in size on exopodite IV.

Other features (proportions of labral keel, armature of posteroventral angle and posterior margin of valve, its ornamentation; presence of lateral processes on valves; length of basal spines on postabdominal claw; ratio between setae on IDL of thoracic limb I; characteristics of adult males) are important for discrimination of certain species within each subgenus. A further detailed comparison of considered species confirms our observation ( Table 1). Below we analyse in detail populations of Disparalona (M.) hamata species group based on original data and available references.