Extremalona, Sinev & Shiel, 2012

Sinev, Artem Y. & Shiel, Russell J., 2012, Extremalona timmsi gen. nov., sp. nov., a new cladoceran (Cladocera: Anomopoda: Chydoridae) from an acid saline lake in southwest Western Australia, Journal of Natural History (J. Nat. Hist.) 46 (45 - 46), pp. 2845-2864 : 2847-2849

Extremalona gen. nov.

Type species Extremalona timmsi sp. nov.

Etymology

The name reflects the ability of the genus to inhabit an extreme habitat for Cladocera – a highly acidic, saline lake.

Diagnosis

Female. Aloninae of moderate size. Body oval, valves and head shield without keel. Posteroventral angle low, being the posteriormost extremity of the body. Dorsal and ventral margins of valves evenly curved, posterodorsal, and anterioventral angles of valves broadly rounded. Posteroventral angle of valves without denticles, with about 25 small setules not organized into groups.

Head is set relatively high for Aloninae , tip of rostrum located at half height of the body. In lateral view head small, low triangular-round, rostrum short, directed forward and downward. Eye usually larger than ocellus. Head shield narrow, with short, broadly rounded rostrum, posterior margin of head shield broadly rounded. Three narrowly connected major head pores of the same size. Lateral head pores elongated. Labrum with narrow oval keel, keel apex rounded, posterior margin of keel without clusters of setae.

Thorax twice as long as abdomen. Abdomen without abdominal joint. Postabdomen relatively short, slightly curved, narrowing distally, with broadly rounded dorsodistal angle. Distal margin almost straight. Dorsal margin with distal part about 1.3–1.5 times longer than preanal one, with postanal portion half the length of the anal portion. Postanal portion of distal margin weakly convex, anal portion concave. Preanal angle well-defined, prominent, postanal angle not defined. Preanal margin slightly concave. Provided with 10–12 clusters of marginal setules, even in preanal portion these clusters are similar to lateral groups of setules. Lateral fascicles of setules numerous, spaced irregularly; main row consists of 9–11 broad fascicles. First setules in postanal fascicles thicker than others. Postabdominal claw curved, about two-thirds length of preanal portion of postabdomen. Basal spine thin, about 0.3 times length of claw.

Antennule almost reaching the tip of rostrum. Antennular sensory seta slender, about two-thirds length of antennule, arising at two-thirds distance from the base. Nine terminal aesthetascs, three of them longer than others, as long as antennule. Antenna with antennal setae formula 0-0-3/1-1-3, spines 1-0-1/0-0-1. Basal segment robust, branches short and stout. Seta of the basal endopodite segment thin, as long as endopodite itself. Seta of the middle endopodite segment and apical setae of same morphology. Spine on basal segment of exopodite about two-thirds length of middle segment. Spines of apical segments significantly shorter than these segments.

Five pairs of thoracic limbs. Epipodites of all limbs indistinctly separated from the main body of the limb.

Limb I with accessory seta four times shorter than ODL seta. IDL with three setae, seta 3 longest, about two-thirds length of ODL seta, seta 1 quite large, about halflength of seta 2, setae 2 and 3 with thin setules in distal part. Endite 3 with four setae. Endite 2 with three setae. Endite 1 with two two-segmented setae, and a long seta directed to the base of the limb. No naked setae on anterior face of endites 1–2. Six rows of long setules on ventral face of limb. Two ejector hooks, one a little larger than the other.

Limb II: exopodite narrow, elongated, with very small seta. Inner portion of limb with eight scraping spines, scrapers 5 and 6 bear larger denticles than others. Small naked seta located near the base of scraper 1, small sensillum between bases of scrapers 3 and 4. Distal armature of gnathobase with four elements. Filter plate II with seven setae, the posteriormost seta considerably shorter than others.

Limb III: exopodite six setae, seta 3 being longest, setae 4–6 long, of similar size and shape, armed with thick setules in distal part. Distal endite with three setae, two distalmost members long, slender, sharp, with long denticles; basalmost seta shorter, geniculate, with long setules. Basal endite with four setae. Four inner setae increasing in size basally. Distal armature of gnathobase with four elements – elongated, cylindrical sensillum, thin, geniculated seta, and two short acute spines. Filter plate III with seven setae.

Limb IV: exopodite broad oval, with six setae decreasing in size basally, setae 5–6 plumose as others. Inner lobe of limb IV with four setae and ovoid sensillum. Scraping seta short and broad, flaming torch setae of similar morphology, decreasing in size basally. Three inner setae increasing in size basally. Gnathobase with short two-segmented seta and a blunt process near it. Filter plate IV with five setae.

Limb V: exopodite oval, not separated into two lobes, with four plumose setae decreasing in size basally. Inner limb portion as elongated lobe. Setae of inner face of same size. Filter plate as a single seta.

Male. Body low oval, maximum height before the middle of the body. Ocellus much smaller than eye. Postabdomen short, with almost parallel margins in distal portion, dorsodistal angle broadly rounded. Preanal angle not defined, postanal angle welldefined. Distal part of postabdomens 1.2–1.3 times longer than preanal. Sperm duct openings located almost at the end of postabdomen. Armament of postabdomen similar to that of female. Postabdominal claw short, about two-thirds of preanal margin length, basal spine same size as in female. Antennule much broader than in female, with nine terminal and six lateral aesthetascs. Male seta arising at one-third antenna length from tip, exceeding one-third of antennule length. Limb I with U-shaped copulatory hook. Row of about 40 long, dense setules on ventral face of limb under copulatory brush. IDL retains seta 1, setae 2 and 3 much thinner and shorter than in female, male seta thick, curved, as long as seta 2.

Differential diagnosis

Extremalona gen. nov. belongs to the Coronatella -branch of Alona -like Aloninae (genera Coronatella Dybowski and Grochowski, 1894 , Karualona Dumont and Silva- Briano, 2000, Leberis Smirnov, 1989 , Celsinotum Frey, 1991 , Anthalona Van Damme, Sinev and Dumont, 2011 , and elegans- group of Alona s.l.), sharing several main diagnostic features of the group, including six setae on exopodite III, reduction of filter plate on limb VI and reduction of limb VI; these features clearly separate the genus from genera of the Hexalona -branch of Aloninae . Extremalona gen. nov. differs from all members of the group by the oval body with small high-set head (tip of rostrum located at half-height of the body), the exopodite III with uniform, well-developed setae 4–6 and male antennule with six lateral and nine terminal aesthetascs. Also, Extremalona gen. nov. clearly differs from Coronatella , Leberis , Celsinotum , Anthalona and elegans -group by the presence of well-developed IDL seta 1; from Anthalona and Karualona by three major head pores; from Leberis and Celsinotum by the rounded rostrum and long apical spines of antenna. Other differences between these groups are separated in Table 1.