Rhynchotalona latens ( Sarmaja-Korjonen, Hakojärvi et Korhola, 2000 )

Sinev, Artem Y. & Dadykin, Ivan A., 2022, Notes on morphology and ecology of an elusive water flea Rhynchotalona latens (Sarmaja-Korjonen, Hakojärvi & Korhola, 2000) (Crustacea: Cladocera), Zootaxa 5200 (3), pp. 260-270 : 261-267

publication ID	https://doi.org/ 10.11646/zootaxa.5200.3.4
publication LSID	lsid:zoobank.org:pub:89E647E5-9C5C-416C-A633-775DE19AEA6D
DOI	https://doi.org/10.5281/zenodo.7262176
persistent identifier	https://treatment.plazi.org/id/732987E2-F22F-A356-FF74-3E80136BA3FA
treatment provided by	Plazi
scientific name	Rhynchotalona latens ( Sarmaja-Korjonen, Hakojärvi et Korhola, 2000 )
status

Treatment

Rhynchotalona latens ( Sarmaja-Korjonen, Hakojärvi et Korhola, 2000)

Sarmaja-Korjonen et al., 2000: 166–167, Fig. 2 View FIGURE 2 ( Unapertura ); Van Damme & Nevalainen, 2019: 465–466, Fig. 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 ; Korovchinsky et al., 2021: 386, Fig. 117:10–13.

Description. Parthenogentic female. Body oval in lateral view, low in juveniles ( Figs. 1A,B View FIGURE 1 ), of moderate height in adults ( Figs. 1C,D View FIGURE 1 , 2A–F View FIGURE 2 ); maximum height at body middle or behind it; body moderately compressed laterally. Height-length ratio about 0.6 in adults. Dorsal margin strongly convex, postero-dorsal and postero-ventral angles broadly rounded. Posterior margin strongly convex, ventral margin almost straight, antero-ventral angle rounded. Ventral margin ( Fig. 1E View FIGURE 1 ) with about 40 setae; about 10–15 anterior setae moderately long, 5–7 middle setae very short, posterior 20 setae ( Figs. 3E–F View FIGURE 3 ) very long, longer than setae of anterior group, posteriormost setae in the group being longest, located at postero-ventral angle. Postero-ventral angle ( Fig. 1F View FIGURE 1 ) without denticles, with 15–20 moderately long setulae not reaching to the margin of the valves. A row of about 100 very short setulae along posterior margin on inner side of carapace. Sculpture of valves variable, with well or weakly developed longitudinal lines ( Figs. 2A–C, F View FIGURE 2 ), or with tubercles ( Figs. 2D,E View FIGURE 2 ). Unlike most Aloninae , adult female always bears a single parthenogenetic egg in the brood chamber; length of the egg slightly less than half-length of specimen.

Head relatively small, triangular in lateral view. In lateral view rostrum moderately long, about two lengths of antennule, weakly curved posteriorly. Ocellus smaller than eye. Distance from tip of rostrum to ocellus about 2–2.5 times of that between ocellus and eye.

Head shield with shape typical for the alonines, with maximum width behind maxillar articulation, smooth in specimens with linear sculpture of valves, tuberculated in specimens with tuberculated valves. Rostrum elongated, narrow triangular in dorsal view, with rounded tip ( Figs. 1G View FIGURE 1 , 3A View FIGURE 3 ). Posterior margin of head shield rounded ( Figs. 4A,B View FIGURE 4 ). Main head pore ( Figs. 4A,B View FIGURE 4 ) as oval rimmed field; distance from pore to the posterior end of head shield slightly greater than pore length. Lateral head pores minute, located at about two lengths of main head pore from midline, at the level of anterior margin of main head pore.

Labrum ( Figs. 1H–J View FIGURE 1 ) small, without lateral projections. Labral keel of narrow, height about 2 times width. Anterior margin of keel evenly convex, apex broadly rounded, posterior margin weakly convex, without clusters of setules.

Thorax two times longer than abdomen. Dorsal surface of abdominal segments not saddle-shaped.

Postabdomen short and narrow, with almost parallel margins ( Figs. 1K View FIGURE 1 , 4C–E View FIGURE 4 ). Distal angle obtuse, rounded. Dorsal margin with anal portion longer than postanal one. Preanal and postanal angles weakly defined. Preanal margin without notches. Postanal margin with 5–6 large single marginal denticles, length of largest of them 1.5 greater than width of postabdominal claw base. Anal margin with 2–3 groups of small spinules. Postanal part with about 3–4 well-developed lateral fascicles of 3–6 thick setules, shorter than marginal denticles; in anal portion 4–5 groups of 7–15 thinner, shorter setulae. Postabdominal claw ( Figs. 1L View FIGURE 1 , 4F View FIGURE 4 ) weakly curved, shorter than preanal portion of postabdomen. Basal spine short, slightly curved, about 0.15–0.2 length of claw.

Antennule long, truncated at the end ( Figs. 1M View FIGURE 1 , 3B View FIGURE 3 ). Antennular sensory seta slender, about half length of antennule, arising at 2/3 distance from the base. Nine terminal aesthetascs, two longest only slightly shorter than antennule.

Antenna of moderate length ( Figs. 3C,D View FIGURE 3 , 5A View FIGURE 5 ), antennal formula: setae 0-0-3/0-0-2; spines 1-0-1/0-0-1. Basipodite robust, with small seta between branches. Branches with segments of similar length. Spine on basal segment of exopodite about 1/3 length of middle segment. Spine on apical segment of endopodite significantly longer than apical segment. Spine on apical segment of exopodite as long as apical segment. One seta on apical segment of exopodite two times shorter and thinner than two others; longer setae on apical segment of exopodite of similar length and thickness than apical setae of endopodite.

Thoracic limbs: six pairs.

Limb I ( Figs. 5B–C View FIGURE 5 ). Epipodite rounded, with a process 2–2.5 times longer than epipodite itself. Accessory seta moderately long, two times shorter than ODL single seta. IDL with two setae, seta 1 not found, setae 2 and 3 moderately thick, armed distally with thin setules, seta 2 slightly shorter than ODL seta, seta 3 slightly longer than ODL seta. Setae of endite 3 increasing in length from behind; seta c of same morphology as setae e–f of endite 2. Inner seta (2) on endite 2 not found; seta b much shorter than setae e–f; setae e and f of similar length, two times shorter than limb itself. Endite 1 with short inner seta (3), two two-segmented setae (g–h) and a short flat seta (i). Ventral face of limb with 5–6 clusters of setae. Maxillar process short, with a single seta.

Limb II ( Figs. 5D,E View FIGURE 5 ). Exopodite elongated, with a long seta. Eight scraping spines decreasing in length proximally, armed with thin setules. Distal armature of gnathobase with four elements. Filter plate with seven setae, posteriormost seta remarkably shorter than others.

Limb III ( Figs. 5F,G View FIGURE 5 ). Epipodite without a process. Exopodite subquadrangular, with seven setae. Setae 1–5 plumose, setae 6–7 slender, thin, without long setules. Seta 3 being longest, seta 6 slightly shorter than seta 3, setae 1, 4 and 7 about half length of seta 3, setae 2 and 5 of 1/4 and 1/3 length of seta 3 respectively. Distal endite with three long setae, two distal setae (1–2) long and thin adapted for scraping, small sensillum (s) located between their bases, seta 3 flattened, with long setules. Basal endite with four stiff setae (a–d) of similar size. Four soft setae increasing in size proximally, small sensillum near the base of distalmost seta. Distal armature of gnathobase with four elements: an elongated, cylindrical sensillum; large geniculated seta; and two spines with fused bases. Filter plate of seven setae.

Limb IV ( Figs. 5H,I View FIGURE 5 ). Pre-epipodite setulated; epipodite with a process longer than epipodite itself. Exopodite subrectangular, with six setae Setae 1–4 flattened, plumose, setae 5–6 slender, thin, naked. Seta 1 being longest, setae 2–3 slightly shorter thans seta 1, seta 4 about 1/3 length of seta 1, setae 5–6 about 2/3 length of seta 1. Inner portion of limb IV with four setae and small elongated sensillum (s) (see Fig. 5I View FIGURE 5 ). Scraping seta (1) moderately long, three flaming-torch setae elongated, of similar length, first of them (2) armed with long, thick setulae, two other—with short thin setulae. Three soft setae (a–c) increasing in length basally. Gnathobase with two-segmented seta and a small hillock distally. Filter plate with five setae.

Limb V ( Figs. 5J,K View FIGURE 5 ). Pre-epipodite setulated. Epipodite with a process longer than epipodite itself. Exopodite clearly bilobed of moderate size, with four plumose setae decreasing in length basally; seta 4 about half length of seta 1. Inner limb portion as elongated oval lobe with setulated inner margin. At inner face, two short setae of similar length. Filter plate with three small setae and a large triangular sensillum (s).

Limb VI as oval setulated lobe ( Fig. 5L View FIGURE 5 ).

Ephippial female and male unknown.

Size. Single found juvenile female of instar I length 0.21 mm, height 0.11 mm Juvenile females of instar II length 0.26–0.27 mm, height 0.14–0.15 mm. Adult parthenogenetic female length 0.27–0.33 mm, height 0.17–0.2 mm.

Differential diаgnosis. Rhynchotalona latens differs from all other species of the genus in shape of head shield, which has a broadly rounded posterior angle, and in lateral pores located at significant distance from midline. All other species of the genus (see Røen 1973; Sinev & Kotov 2014) have the posterior portion of the head shield as a broad elongated protrusion with rounded tip, and lateral head pores located very close to the main pore. Habitually, R. latens is most similar to Greenland endemic R. kistarae Røen, 1973 , these species differ in shape of the main head pores, which is elongated in R. kistarae . R. latens clearly differs from species of falcata -clade ( R. falcata (Sars. 1862) , R. weiri Sinev & Kotov, 2014 , and R. longiseta Sinev & Kotov, 2014 ) in rounded anteroventral corner of valves and armament of the postabdomen. In species of the falcata -clade the anteroventral corner of the valves forms an elongated protrusion with rounded tip ( Sinev & Kotov 2014). Other differences between R. latens , R. kistarae and species of the falcata -clade are summarized in Table 1 View TABLE 1 .

Distribution and ecology. Recent findings belong to Finland, North Karelia and Pechora River Delta. This is an interstitial species, dwelling within patches of the waterlogged Sphagnum mosses at lakes shores; some specimens are accidentally found in littoral zone of such lakes. Water conditions measurements, taken on 26 June 2022 at Krugloe Lake, reveal significant differences between littoral zone of the lake and the patches of Sphagnum mosses. Water squeezed from Sphagnum had a temperature 24.9°C, pH 3.4, oxygen concentration 3.4 mg /l (saturation 41%) and conductivity 34.1 µS/cm, while littoral lake water at 10–15 cm from the coast had temperature 22.1°C, pH 5.3, oxygen concentration 8.8 mg /l (saturation 100%), and conductivity 24 µS/cm. In the studied material, only parthenogenetic females were present from late June to the middle of September. R. latens seems to be a slow and rather reluctant swimmer; observed specimens swim rather slow, unsteady, and for short periods, usually less than 10–15 seconds, preferring crawling on substrate using antennae and postabdomen.