Acanthalona willisi ( Smirnov, 1989 )

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 : 46-53

publication ID	https://doi.org/ 10.5281/zenodo.281766
DOI	https://doi.org/10.5281/zenodo.5695068
persistent identifier	https://treatment.plazi.org/id/03DCAF27-FFED-FF9D-FF4C-70CFCFE7FC5D
treatment provided by	Plazi
scientific name	Acanthalona willisi ( Smirnov, 1989 )
status

Treatment

Acanthalona willisi ( Smirnov, 1989)

( Figs 1–5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 )

Smirnov, 1989: 137–138, fig.2А–Г ( Biapertura ); Van Damme, Kotov & Dumont, 2010: 46 ( Alona ).

Type locality: Lake Wicheura, via Bamaga, Cape York, Queensland, Australia, 10°46' S, 142°34' E.

Studied material: 27 parthenogenetic females from Jibbon Lagoon, Royal National Park, New South Wales, Australia, 34°05'12''S, 151°09'53''E.

Redescription. Parthenogenetic female. In lateral view body regular oval ( Figs.1 View FIGURE 1 A–C, 2A, C–H), moderately high, maximum height at middle of body, a height-to-length ratio of about 0.7 in adults, juvenile females with slightly narrower body than adult females. Body only weakly compressed laterally ( Figs. 2 View FIGURE 2 B, F). Dorsal margin uniformly curved; postero-dorsal and postero-ventral angles broadly rounded; posterior margin uniformly curved; ventral margin almost straight; antero-ventral angle rounded. Valves with moderately developed linear sculpture in dorsal and posterior portions, numerous minute dot-like depressions located between lines ( Fig. 3 View FIGURE 3 A). Antero-ventral corner of valves without setae, ventral setae start at some distance from it. About 45–50 ventral setae starting at some distance from antero-ventral angle ( Fig. 1 View FIGURE 1 D), about 20 anterior setae very short, other setae of moderate size, reaching to postero-ventral corner of valves ( Fig. 3 View FIGURE 3 B). Postero-ventral angle ( Fig. 1 View FIGURE 1 E) bears about 40 short setules of similar length, not organised in groups. A row of about 100 setules of unequal length, with very long setules separated by shorter ones, along the posterior margin on inner side of valve.

Head of moderate size, narrow in lateral view, rostrum elongated, pointing downward ( Fig. 3 View FIGURE 3 C). Eye and ocellus large, of similar size. Distance between tip of rostrum and ocellus slightly greater than that between ocellus and eye in adults.

Head shield ( Figs. 1 View FIGURE 1 F–G, 2H) wide, with quite small distal portion (behind the point of mandibular articulation), somewhat similar to that of Monospilus , 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 a narrow connection between them, PP about 1.4–1.8 IP ( Figs. 1 View FIGURE 1 H–I, 2I). Lateral head pores absent. Labrum ( Figs. 4 View FIGURE 4 A–B) of moderate size; labral keel moderately wide (height-to-width ratio of about 1.5), with rounded apex; anterior margin of keel unevenly convex, posterior margin without any setules (Fig. 19I).

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 ( Figs. 3 View FIGURE 3 В–E, 4C–D). Ventral straight in posterior portion and convex near the end of postabdomen or evenly convex. Basis of claws separated from distal margin by clear incision. 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 portion. Postanal portion of distal margin almost straight, anal portion strongly concave. Preanal angle prominent, postanal angle well-defined. 11–12 well-developed 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 times longer than postabdominal claw base, other denticles in group at least two times shorter. In some specimens ( Figs. 4 View FIGURE 4 D–E) these denticles are curved irregularly, perhaps having been deformed during or after molt. In anal groups, denticles shorter, size of denticles in groups decreases anteriorly, number of denticles in group increases anteriorly. 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. In anal portion, fascicles moderately wide, several additional fascicles above the main row. Postabdominal claw short, strongly curved, 1.5 times shorter than preanal portion of postabdomen. Basal spine about one-third of length of claw, several small spines located near its base.

Antennule ( Fig. 4 View FIGURE 4 F) elongated with a length-to-width ratio of about 2.5, and unlike most Aloninae , very narrow in lateral view (see Fig. 3 View FIGURE 3 C) without any clusters of setules at anterior face. 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 ( Figs. 3 View FIGURE 3 F, 4D). Antennal formula, setae 0–0–3/0–1–2, spines 1–0–1/0–0–1. Basal segment robust, branches long and narrow, length of segments evenly decreases distally in both branches. Seta arising from middle segment of endopodite as long as endopodite. One of apical setae of endopodite thicker than other, apical setae of exopodite of similar thickness with the latter. All apical setae with very long thin spinules at the point of articulation. Spine on basal segment of exopodite about half length of the middle segment. 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).

Maxillule small, with three short setulated setae ( Figs. 5 View FIGURE 5 A)

Six pairs of thoracic limbs.

Limb I ( Fig. 5 View FIGURE 5 B) of moderate size. Epipodite ovoid. Accessory seta very short, about 1/5 length of ODL seta. ODL with a single seta, armed with long setules in distal part. IDL ( Fig. 5 View FIGURE 5 C) with three setae and two-three clusters of small setules, setae 2 and 3 only slightly shorter than ODL seta, armed with long setules in distal part, seta 1 sharp and narrow, of about 1/3 length of seta 3. Endite 3 with four setae, anterior seta (1) being shortest, seta c longer and much thicker than two others. Endite 2 with three setae (d–f), anterior naked seta 2 and small sensillae (s1) ( Fig. 5 View FIGURE 5 E); setae f and e very long, 1.2–1.3 times longer than ODL seta, with complex setulation in distal part. Setae b, c and d ( Fig. 5 View FIGURE 5 D) with asymmetric setulation, unlike these in most Aloninae . Endite 1 with two 2-segmented setae (g–h), both setulated in distal part, flat seta (i), anterior naked seta (3) 2 times longer than seta 2, and small sensillum near its base (s3). Seta i large in comparison with other Aloninae , armed with long, thick setules. Six rows of thin long setules on ventral face of limb. Two ejector hooks of similar size, armed with long setules. Maxillar process long and narrow, with a short seta at the end.

Limb II ( Figs. 5 View FIGURE 5 F–G). Exopodite elongated, with a single naked seta of about 1/2 length of exopodite. Eight scraping setae (1–8), setae 1–5 long, decreasing in length basally, seta 3 thicker than others and armed with larger spinules; setae 6–8 short, of similar size, armed with about ten large, thick spinules. Distal armature of gnathobase with four elements. Filter plate with seven setae, the posteriormost much shorter than others.

Limb III. Epipodite oval; exopodite elongated, narrow, somewhat folded, with six setae ( Fig. 5 View FIGURE 5 H). 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 ( Fig. 5 View FIGURE 5 I), two distalmost members very long, about half length of exopodite seta 3, slender and sharp, with distal parts unilaterally armed with sharp denticles; basalmost seta much shorter, bilaterally armed with setules. Two small sensillae (s1–2) located between their bases. Basal endite ( Fig. 5 View FIGURE 5 J) with four stiff setae, increasing in size toward the base, a small sensillum (s3) near the base of distalmost seta. Four soft setae increasing in size basally (a–d). Gnathobase not clearly separated from basal endite. Distal armature of gnathobase with four elements: an elongated, cylindrical sensillum (s4); thin, bent seta; and two spines with fused bases. Filter plate III with seven setae.

Limb IV. Pre-epipodite setulated, epipodite without process. Exopodite almost quadrangular, with six setae ( Fig. 5 View FIGURE 5 K). 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. Inner portion of limb IV ( Figs. 5 View FIGURE 5 L–M) with four setae and elongated sensillum (s1). Scraping seta (1) large, almost as long as exopodite seta 6, with small denticles in distal portion. The first flamingtorch seta (2) broad and massive, of characteristic shape, armed with about ten long setules with thickened basal portion. Two other flaming torch setae (3–4) of similar shape, thin and elongated, armed with very thin, short setules. Small sensillae (s2) located between bases of basalmost flaming-torch setae. Three soft setae of similar size with filter plate setae, increasing in size basally. Gnathobase with a 2-segmented seta, an elongated process, and large sensillum (s3). Filter plate with five setae.

Limb V. Pre-epipodite setulated, epipodite oval, without any process. Exopodite ( Fig. 5 View FIGURE 5 N) subrectangular, not divided into two lobes, with three plumose setae, slightly decreasing in size basally, basalmost seta (4) absent. Inner limb portion ( Fig. 5 View FIGURE 5 O) as narrow oval lobe, with setulated inner margin. 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 ( Fig. 5 View FIGURE 5 P) a setulated rounded lobe with wavy margin, length about 1.5 widths.

Ephippial female and male unknown.

Size: in juvenile female of instar I: length: 0.34–0.36 mm, height: 0.22–0.24 mm; in juvenile female of instar II: length: 0.38–0.40 mm, height: 0.25–0.26 mm; in adult female: length: 0.40–0.52 mm, height 0.29–0.36 mm.

Water quality and food of A. willisi

Physico-chemical analyses indicated that Jibbon Lagoon water was fresh, acidic, with low levels of turbidity and nutrients. Water samples collected on 9 September (n=1) and 16 November 2011 (n=3) had the following overall range of conditions: water temperature: 21.3–23.7 °C; dissolved oxygen: 9.0– 9.5 mg l -1; conductivity: 337–382 µS cm -1; pH: 6.1–6.3; turbidity: 1.6–2.2 NTU; total nitrogen: 399–503 µg l -1; and total phosphorus: 7–8 µg l -1. An analysis of gut contents of A. willisi (n=10) showed that the species ingested algae such as Oocystis , Scenedesmus , Staurastrum and pennate diatoms, and detrital particles as food items.