Bythotrephes cederströmii Schödler, 1877, Schodl., Esrum So, P. E. Muller, 1877

Korovchinsky, Nikolai M., 2020, Description of a new species in the genus Bythotrephes Leydig, 1860 (Crustacea Cladocera: Onychopoda), supplements to selected species, and concluding remarks on the genus, Zootaxa 4789 (2), pp. 441-465 : 450-451

Bythotrephes cederströmii Schödler, 1877

Material examined: Sweden, Norrbotten: 1) sample SMNH N 156547 labeled “ Bythotrephes longimanus , Sweden, Kvikkjokk, Quickjock, 66° 57’ N; 17° 43’ E ”, 8 ad; 2) sample SMNH N 158527 labeled “ Bythotrephes longimanus ” (old label “ Bythotrephes longimanus Leydig , Sweden, Lappland, Kvikkjokk, Carexhårren, 29/7/1946, N. Odhner coll. et det. Bythotrephes cederströmii v. robusta Lilljeborg ”), 2 gam; 3) sample SMNH N 158508 labeled “ Bythotrephes cederstroemi , lake Kaarevuopio” (old label: “Karesuando, isjön, 25.7.1946, N. Odhner, Bythotrephes cederströmi v. robusta Lilljeborg ”), 1 ad, 1 gam; Jämtland: 4) sample SMNH N 156573 labeled “Sweden, lake Revsundssjön, 62° 53’ N; 15° 08’ E ” (old label: ”N 2470, Jämtland, Refsunden, Bythotrephes n. sp., Lilljeborg”), 1 ad, 2 males; Värmland: 5) sample SMNH N156575 labeled “ Bythotrephes cederstroemi , Sweden, lake Saxen, 59° 46’ N; 14° 25’ E, Leg. Cederström 18 July 1863 ” (old label: “N 1381,Värmland, sjön Saxen, Cederström, 18/7 1863, Bythotrephes cederströmi , det. Schwartz”), 8 ad.

Norway, Finmark: 6) sample SMNH N158534 labeled “ Bythotrephes longimanus , Norway, Pasvikälv, Svanvik, Sörvaranger, 25/8 1948, N. Odhner ”, 3 gam .

Russia, Murmansk region: 7) sample SMNH N156572 labeled “Russia, Kola Peninsula, Lake Imandra, 67° 40’ N; 33°00’ E, Leg. Sandebergska Exp. 1877, sts. 10” (old label: “N 2469, Sandeberg Exp., Imandra, 9/7 1877, Trybom, Bythotrephes n. sp. ”), 9 ad of first generation. Novosibirskaya region : 8) Novosibirskoe Reservoir, 2014, leg. N.I. Ermolaeva, 12 ad.

Remarks. Among the samples of the species, there is one (locality 5) from the Swedish Lake Saxen which actually represents its type locality (see Korovchinsky 2015, 2018). In fact, it is not known whether Lilljeborg saw these specimens because there was a rather tense relationship between him and G. Cederström, a collector of the material (see Lieder 1988). In any case, the label indicates that the specimens of the sample were identified by Schwartz, not Schödler, the author of the original description of the species under consideration, and for this reason they formally cannot be considered type specimens (ICZN 72.4.1.1).

In total, this sample contained eight only slightly deformed specimens which are large and possess quite species-specific morphological traits, i.e. very long caudal process having denticulated bend and large distantly situated claws. Their morphometric traits shown in Table 2 are closer not to those of type specimens from the same Lake Saxen kept in Museum für Naturkunde (Berlin, Germany), used for the original redescription of the species ( Korovchinsky 2015) but, on the contrary, to those from other studied populations (see Korovchinsky 2018). It became clearer that quite deformed features of the former ones hindered the proper evaluation of diagnostic features of the species. It is also appropriate to note that judging by an old label of sample 4, Lilljeborg designated the taxon under consideration as a new species probably not being aware of Schödler’s (1877) description.

Specimens from other populations were sometimes not so typical, e.g., some of them from the localities 1, 4, and 6 had caudal bend either poorly developed or lacking altogether. In locality 4, the species under consideration co-occurred with B. brevimanus Lilljeborg. In some others it was represented by few specimens that did not allow estimating the degree of morphological variability, thus the presence of interspecific hybrids instead of B. ceder- strömii s.str. (see Korovchinsky 2019) in local populations cannot be excluded. At the same time, it may be noted that no specimens with intermediate morphological traits characteristic of hybrid forms were found in the whole set of samples studied.

Sample 7 from Lake Imandra (Kola Peninsula, north-west of European Russia) contained females indicated as being of the first generation hatched from resting eggs, which are described herein for B. cederströmii for the first time (these unusual specimens were evaluated by Lilljeborg as representing a new species). Five of these specimens had three pairs of claws and four other ones, four pairs of such claws with the apical end clearly curved forward ( Fig. 5B ). This probably means that the additional claws are added in subsequent instars. The morphometric measurement of two such specimens has revealed the following parameters: body length, 2.30 and 2.55 mm; length of caudal process, 113 and 207 % of body length (the former and the latter ones are those with three and four pairs of claws, respectively); length of claws, 2.7–5.2 % of body length; interclaw distance, 23.8–54.3 % of body length. Thoracic limbs of first pair (tl I) were comparatively short (60.4 and 67.4 % of body length, respectively) with short distal endopodital segment (55.6 and 63.8 % of body length, respectively) ( Fig. 5 A-upper).

The finding of the species under consideration in the Novosibirskoe Reservoir is its first record for Western Siberia.

Figures

FIGURE 5. Bythotrephes cederströmii, females of first generation hatched from resting egg, Lake Imandra (Kola Peninsula, Russia) (A-upper, B); B. lilljeborgi, females, Ålsjön (Kragered, Halland, Sweden) (A-lower, C–F). A, distal segments of tl I. B, postabdomen and caudal process. C–F, claws of postabdomen and caudal process.