Bythotrephes longimanus austriacus, Korovchinsky, 2023

Korovchinsky, Nikolai M., 2023, Unexpected high species richness of Bythotrephes Leydig, 1860 (Branchiopoda: Cladocera: Cercopagididae) in subalpine Austrian lakes, with the description of new taxa, Zootaxa 5264 (1), pp. 77-93 : 84-87

Bythotrephes longimanus austriacus ssp. nov.

( Figs. 3 , 4 )

B. longimanus Leydig : Steuer, 1897: p. 520 (Lake Millstättersee).

B. longimanus v. carnica Ischreyt, 1939: 125–126 , 128, Abb. 6.

B. longimanus longimanus Leydig, 1860 : Flössner, 1972: 405–406, Abb. 190A; 2000: 375

B. longimanus Leydig, 1960 : Korovchinsky, 2015: 20 , Figs. 8E, 8F, 8I, 8K, 8P, 8Q, 8R, 8X.

Etymology. The name of the species denotes the occurrence of the taxon in Austrian subalpine lakes.

Type locality. Lake Erlaufsee , Styria, Austria (47°48′51′′N; 15°17′43′′E), 827 m a.s.l.

Holotype. A female with body length 2.08 mm deposited in Zoological Museum of Moscow State University (№ MGU Ml 264).

Paratypes. Three females from the same sample deposited in the same museum (№ MGU Ml 265).

Material examined. Austria: 1) Lake Erlaufsee (Styria), 18.9.2018, 20 ad, 5 juv, coll. R. Ptáčniková; 2) Lake Irrsee (Upper Austria), 27.8.2012, 7 ad, coll. R. Ptáčniková ; 3) Lake Wolfgangsee (Upper Austria), 28.8.2012, 9 ad, coll. R. Ptáčniková ; 4) Lake Grundlsee (Styria), 18.7.2007, 20 ad, leg. M. Luger ; 5) Lake Millstättersee (Carinthia), 8.11.2011, 2 ad, 1 male, 1 juv. leg. G. Santner ; 6) Lake Mondsee (Upper Austria), 7.1993, 5 ad, leg. H. Löffler and S. Gaviria-Melo. (specimens from the localities 4-6 were investigated earlier (see Korovchinsky 2015) but here after adding new individuals their diagnostic features were revised anew) .

Data on body and body parts measurements of specimens from Erlaufsee and Grundlsee are presented in Table 1 (see also Korovchinsky 2015: Table 2 ).

Description. Female. The general body structure and its parts as in other representatives of the genus.

Thoracic limbs: four pairs ( Figs. 2 , 3 )

First pair of limbs (tl I) are especially long and strong ( Fig. 3A ) (78.9–129.6 % of body length). The first segment of the endopodite is long and bears 4–8 (mostly 5–8) anterior lateral setae with rows of spines and fine setules. Distally, this segment bears shorter anterior seta of the same type and long posterior finely setulated seta ( Figs. 3B–E ). The second segment of the endopodite is conspicuously shorter and bears only two apical either short or rudimentary setae ( Fig. 3F–R ). The terminal third segment of the endopodite slightly varies in length, being either shorter or slightly longer than the proximal first endopodital segment (71.4–108.0 % of body length) and always bearing apically four long roughly spinulated setae, two of them terminally and two subterminally.

Second pair of limbs (tl II) are considerably shorter, their protopodite, again externally, is conspicuously longer and bears a conical outgrowth. The first basal segment of their endopodite bears a row of 4–8 (mostly 5–8) rather long anterior lateral setae (their number can vary in one individual). There are 1–3 posterior lateral setae on this segment. Internally, this segment bears a stout cylindrical pseudognathobasic process, possessing some prominences of different size and one small, thin seta. The second segment of the endopodite is short with only two setae, the anterior one of which is similar to the anterior terminal seta of previous segment, whereas the posterior seta is longer and finely setulated. The distal, third segment of endopodite of the limb bears four setae, two terminal and two subterminal ones.

Third pair of limbs (tl III) are generally similar to those of the previous ones, differing in some details. The external outgrowth of their protopodite is conspicuously larger and lateral anterior and posterior setae (if present) of first segment of endopodite are fewer (4–6 and 1–2, respectively). The pseudognathobasic process is also similar to that of tl II. Terminal and subterminal setae of the third segment are similar to those of tl II but slightly shorter and bear fewer denticles.

Fourth pair of limbs (tl IV) are considerably reduced; their protopodite bears slightly spinulated seta sited on a short cylindrical base. The only segment of the endopodite has two rows of comparatively short spine-like setae. The external row (group) ( Fig. 2C : as) always consists of two setae, and the internal row of 5–8 (mostly 6–7) setae, which differ in their appearance and armament.

The “ postabdomen” actually consists of two parts: the last small abdominal segment and the postabdomen per se (see Korovchinsky (2015) for more details), which is comparatively small, with the anal opening situated between the postabdominal claws. These claws are comparatively small (2.9–7.8 % of body length) and clearly directed backwards ( Fig. 4A–H ).

Caudal process is directly connected with the postabdomen and proceeds as a very long, proximally rather thick and curved, then straight spine-like structure, variable in its length (159.0–274.0 % of body length), thus exceeding the body length by about 1.6–2.7 times. Basally, the caudal process bears one pair of claws similar to those of the postabdomen but usually larger (3.8–9.5% of body length). Pairs of claws sit moderately distantly, interclaw distance usually constitutes 8.1–22.9%. Between the claws, the thickness of the structure constitutes 4.1–9.6 %. Borders separating old molted integuments of caudal process with claws either are conspicuous or invisible.

Size. 1.41–2.42 mm.

Gamogenetic females have not been detected.

Males. The only male we found did not differ, as usual, from males of the nominotypical subspecies.

Intrapopulation variability. According to Table 1 , the length of claws of postabdomen and caudal process is especially variable (CV = 12.7–21.2 and 14.4–18.7, respectively), the interclaw distance and interclaw thickness follow them in this respect (CV = 13.1–14.4 and 11.5–16.5, respectively). Of other structures, the comparative length of thoracic limbs of the first pair and that of caudal process are also rather variable (CV = 6.9–7.2 and 9.9–12.9, respectively). Also, the apical setae armament of a second endopodital segment of tl I ( Fig. 3 ) and shape of claws of postabdomen and caudal process ( Fig. 4 ) are variable as well.

Differential diagnosis. The representatives of Bythotrephes longimanus austriacus ssp. nov. have generally a relatively smaller body size (up to 2.42 mm vs. 3.1 mm in B. longimanus longimanus ) but they mostly differ from those of the B. l. longimanus by relatively smaller claws of postabdomen and caudal process ( Table 2 ) (p <0.001) which both are directed backwards, not downwards as in the nominotypical subspecies.

Remarks. The present study confirmed the past preliminary observation that “Small size of claws of postabdomen and caudal process was the most pronounced common feature of all Austrian specimens” of B. longimanus ( Korovchinsky 2015: p. 20) .

B. longimanus v. carnica was described by Ischreyt (1939) from Lake Millstättersee (Carinthia) with indication of doubtful diagnostic features, why a taxon can be considered incertae sedis (ICZN 67.2.5). Morphological traits of specimens from this lake certainly do not differ from those of B. l. austriacus occurring in some other adjacent lakes.

Geographical distribution. T he representatives of the new subspecies are known to occur in some deep subalpine lakes in the central part of Austria.

Figures

FIGURE 2. Bythotrephes inexpectatus sp. nov., females, Lake Toplitzsee (A–C, D, F, I) and Lake Altaussee (E, G, H). A–C, thoracic limbs of second (tl II), third (tl III), and fourth (tl IV) pairs, respectively. D–I, claws of postabdomen and caudal process.

FIGURE 3. Bythotrephes longimanus austriacus ssp. nov., females: Lake Irrsee (A, F–I, K, L), Lake Erlaufsee (C, D, J, M, N), Lake Wolfgangsee (E, O–R). A, endopodite of thoracic limb of first pair (tl I). B–E, setae of distal part of first endopodital segment of tl I. F–R, setae of distal part of second endopodital segment of tl I.

FIGURE 4. Bythotrepheslongimanus austriacus ssp. nov., females:Lake Irrsee(A–C), Lake Erlaufsee(D–F), LakeWolfgangsee (G, H). A–H, claws of postabdomen and caudal process.