Rheoalona Sinev & Tiang-Nga & Sanoamuang 2017, gen. nov.

Rheoalona gen. nov.

Type species. Rheoalona mekongensis sp. nov.

Etymology. The name is composed from Greek “rheos” (river) and generic name Alona to indicate the genus adaptations to the riverine habitat.

Diagnosis. A member of Aloninae of small size; maximum length of adult female 0.4 mm. Body low oval, compressed laterally. Maximum height at mid-body.

Valves and head shield without keel; valves from previous molts not retained. Dorsal, posterior and ventral margins of valves evenly curved; postero-dorsal, postero-ventral and anterior-ventral angles of valves broadly rounded. Posteroventral angle of valves without denticles.

Head small, triangular in lateral view, rostrum elongated. Ocellus and eye present, of similar size. Head shield broad, with elongated rostrum. Posterior margin of head shield broadly rounded. Two major head pores with a wide connection between them. Lateral head pores minute. Labrum with small oval keel, keel apex rounded, posterior margin of keel without clusters of setulae.

Thorax 1.5 times longer than abdomen. Abdomen without abdominal joint, middle abdominal segment not saddle-shaped.

Postabdomen elongated, with parallel margins; length about 2.3–2.4 height. Ventral margin weakly convex. Distal margin convex, distal angle rounded. Dorsal margin straight in postanal part and convex in anal part, with postanal portion two times longer than anal one. Preanal and postanal angles well-defined. Postabdomen provided with 6–8 clusters of 1–3 very small elementary marginal denticles on postanal margin. Ten-twelve lateral fascicles of setulae, posteriormost setae of postanal fascicles very thick and long; in distal fascicles their length about 3 width of claw base. Postabdominal claw weakly curved, thin, longer than preanal margin of postabdomen, basal spine very short.

Antennule short, two times shorter than rostrum. Antennular seta thin. Nine terminal aesthetascs of variable length.

Antenna with antennal formula setae 0–0–3/1–1–3; spines 1–0–1/0–0–1. Basipodite robust; branches with basal segment of each branch 1.5 times longer than two others. Seta of basal segment of endopodite not reaching end of endopodite. Seta of middle segment of endopodite and apical setae of same morphology, similar length and thickness. Antennal spines very long.

Five pairs of thoracic limbs.

Limb I. With long accessory seta. IDL with 2 setae (seta 1 absent), setae 2 and 3 thin, long, armed with thin setulae. Endite 3 with four setae. Endite 2 with three outer setae and rudimentary inner seta. Endite 1 with two 2- segmented setae, long flat seta and rudimentary inner setae.

Limb II. Exopodite narrow, elongated, without seta. Inner portion of limb with eight scraping spines, spines 1– 5 long, decreasing in size basally, scrapers 6–8 short, of similar size. Filter plate II with seven setae.

Limb III. Exopodite rounded, with six setae, seta 3 being longest, setae 4–5 of about 1/2 and 1/3 length of seta 3, other setae short. Distal endite with three setae, two distalmost ones slender, sharp, with denticles; basalmost seta shorter, geniculated, with thin setulae. Basal endite with four stiff setae. Four soft setae increasing in size basally. Filter plate III with seven setae.

Limb IV. Exopodite rounded, with six setae, setae 1–5 plumose, seta 6 naked. Inner portion of limb IV with scraping seta and only two flaming-torch setae. Three short soft setae increasing in size basally. Filter plate IV with five setae.

Limb V. Exopodite V oval, 4 plumose setae. Inner limb portion as rounded lobe. At inner face, two setae. Filter plate V absent.

Male unknown.

Differential diagnosis. Main diagnostic features of Rheoalona gen. nov. include: two major head pores with a wide connection between them; postabdomen with extremely strong lateral groups of setulae; IDL with only two setae; exopodite III with six setae; only two flaming-torch setae on inner portion of limb IV; absence of filter plate V. In many features, Rheoalona gen. nov. is similar to Karualona, but differs from it in elongated rostrum, only two IDL setae instead of three, and strongly inflated gnatobase of limb II. Rheoalona gen. nov. differs from Coronatella in having two major head pores and in having two flaming-torch setae on inner portion of limb IV. Rheoalona gen. nov. differs from Anthalona in lack of cosmaria of lateral pores. The other differences between these genera are summarized in Table 1.