Acanthalona, Sinev, Artem Y. & Kobayashi, Tsuyoshi, 2012

Sinev, Artem Y. & Kobayashi, Tsuyoshi, 2012, Redescription of the endemic Australian cladoceran Alona willisi (Smirnov, 1989) and its assignment to Acanthalona gen. nov. (Cladocera: Anomopoda: Chydoridae), Zootaxa 3390, pp. 43-55 : 44-45

Acanthalona gen. nov.

Type species — Biapertura willisi Smirnov, 1989

Etymology. name is derived from Greek Acanthos —spined and Alona , and reflects unusually well-developed marginal denticles on postabdomen.

Diagnosis. Aloninae of moderate size, length of female about 0.5 mm.

Body in lateral view regular oval, moderately high, maximum height at middle of body. Ventral margin of valves almost straight. Body moderately compressed laterally. Valves with linear sculpture in dorsal and posterior portions. Anteroventral corner of valves without setae, ventral setae start at some distance from it. About 45–50 ventral setae starting at some distance from anteroventral angle, about 20 anterior setae very short, other setae of moderate size, reaching to postero-ventral corner of valves. Postero-ventral angle bears about 40 short setules of similar length, not organised in groups.

Head of moderate size, narrow in lateral view, rostrum elongated, pointing downward. Eye and ocellus large. Head shield wide, with quite small distal portion, length of anterior portion of shield 4 times greater than that of posterior portion. Rostrum elongated, broad, with broadly rounded tip; posterior margin of head shield broadly rounded. Two major head pores with narrow connection between them. Lateral head pores absent.

Labrum of moderate size; labral keel moderately wide, with rounded apex; anterior margin of keel unevenly convex, posterior margin without any setules.

Thorax 2 times longer than abdomen, dorsal surface of abdominal segments not saddle-shaped.

Postabdomen short and wide, strongly truncated in postanal portion, with a length-to-height ratio of about 2. Distal margin straight, distal angle not prominent, broadly rounded. Dorsal margin with distal part 1.5–1.6 times longer than preanal one, with postanal portion 1.5-2 times shorter than anal. Postanal portion of distal margin almost straight, anal portion strongly concave. Preanal angle prominent, postanal angle well-defined. 11–12 welldeveloped groups of marginal denticles; 5-6 postanal groups consisting of 2-5 (usually 4) denticles, distalmost denticle in each group very long, sharp and spine-like, about three width of postabdominal claw base; other denticles in group at least two times shorter. Preanal margin with 3–4 clusters of long setules in posterior portion. 8–10 lateral fascicles of short setules in the main row, only two-three of them in postanal portion, consisting of 2–4 setules only. Postabdominal claw short, strongly curved, 1.5 times shorter than preanal portion of postabdomen. Basal spine about one-third of length of claw.

Antennule elongated with length-to-width ratio of about 2.5. Antennular sensory seta slender, three times shorter than antenna, arising at 2/3 distance from the base. Nine terminal aesthetascs of similar length, about 2/3 length of antennule.

Antenna of moderate size. Antennal formula, setae 0–0–3/0–1–2, spines 1–0–1/0–0–1. Seta arising from middle segment of endopodite as long as endopodite. Spine on basal segment of exopodite about half length of the middle segment. All apical setae with very long thin spinules at the point of articulation. Apical spines slightly shorter than apical segments. Basal segment of endopodite distally with a cluster of several very long setules (about 2/3 length of segment), middle segment with a cluster of shorter setules (about 1/3 length of segment).

Six pairs of thoracic limbs.

Limb I of moderate size. Epipodite ovoid. Accessory seta present, short. ODL seta armed with long setules. IDL with three setae, setae 2 and 3 only slightly shorter than ODL seta, armed with long setules, seta 1 well-developed, about 1/3 length of seta 3. Endite 3 with four setae. Endites 1 and 2 both with naked seta and small sensillum on anterior face. Endite 2 with very long setae f and e. Endite 1 with large flat seta (i), armed with long, thick setules.

Limb II. Exopodite elongated, with a single naked seta of moderate length. Eight scraping setae, setae 1–5 long, decreasing in length basally; setae 6–8 short, of similar size, armed with large, thick spinules. Distal armature of gnathobase with four elements. Filter plate with seven setae, the posteriormost much shorter than others.

Limb III. Exopodite elongated, narrow, somewhat folded, with six setae. Setae 3 and 5 being longest, of similar length, setae 6 only slightly shorter, other setae short. Setae 1–4 plumose, setae 5 armed with short setules, seta 6 naked. Distal endite with three setae, two distalmost members very long, slender and sharp, with distal parts unilaterally armed with sharp denticles. Inner portion of limb of typical for subfamily morphology. Filter plate III with seven setae.

Limb IV. Exopodite almost quadrangular, with six setae. Seta 3 longest, setae 1, 2 and 5 of about 3/4 length of seta 3, setae 4 and 6 of 1/3 and 1/2 length of seta 3, respectively. Setae 1–2 and 4 plumose, seta 3 armed with long, unarranged setules in distal part, seta 5–6 armed with very short, thick setules. Scraping seta of inner portion of limb large, with small denticles in distal portion. The first flaming-torch seta broad and massive, armed with about long setules with thickened basal portion, two other flaming torch setae thin and elongated, armed with very thin, short setules. Three soft setae of similar size. Filter plate with five setae.

Limb V. Exopodite subrectangular, not divided into two lobes, with three plumose setae. Inner limb portion as narrow oval lobe. At inner face, two setae, one about 2/3 length of exopodite seta 2, other 1.5 times shorter, a large sensillum located between its base and filter plate. Filter plate with three setae.

Limb VI as setulated rounded lobe with a length-to-width ratio of about 1.5.

Ephippial female and male unknown.

Differential diagnosis. The most distinctive feature of Acanthalona gen. nov. is the armament of the postabdomen which is unique for the subfamily with very long, thick marginal denticles. This feature, together with the characteristic postabdomen, separates it from all other genera of the subfamily. Acanthalona gen. nov. differs from most Aloninae in antennal formula (setae 0-0-3/0-1-2, spines 1-0-1/0-0-1), such antennal formula is present only in Bryospilus Frey, 1980 , consisting of non-swimming animals inhabiting mosses in wet forests. The third distinctive feature is exopodite V with three, instead of four, setae. Such a situation has not been recorded in any other Aloninae . Head shield of Acanthalona gen. nov. with shortened distal portion is also unusual; a somewhat similar shield is present in genus Monospilus Sars, 1962 , but in the latter distal portion of shield is even more reduced. Absence of lateral head pores is also uncommon for Aloninae , it is recorded for genus Monospilus and for a single species of Alona s. l., A. dentifera (Sars, 1901) . Other characteristic features of the new genus include reduction of anterior group of setae on valves, two connected major head pores, presence of thorachic limb VI, and differentiated flaming-torch setae of limb IV.