Bythotrephes, Leydig, 1860

Korovchinsky, Nikolai M., 2019, Morphological assessment of the North Eurasian interspecific hybrid forms of the genus Bythotrephes Leydig, 1860 (Crustacea: Cladocera: Cercopagididae), Zootaxa 4550 (3), pp. 340-356 : 346-353

Hybrid form Bythotrephes sp. x B. cederströmii

( Figs. 6 H–6M)

Material examined: Tyva (south of East Siberia, Russia): Lake Borzu-Khol’ . 27.7.2004, coll. E.I. Zuykova, 6 ad; Lake Todzha (Azas), 5.7.2004, coll. E.I. Zuykova, 6 ad.

In the mountainous area in the south of East Siberia, there have been found representatives of the genus Bythotrephes which seem to represent a new species of the genus to be described (Korovchinsky, unpublished). In the same area but in the neighboring lakes Borzu-Khol’ and Todzha, there have been encountered similar specimens bearing a caudal process with a denticulated bend resembling that of B. cederströmii . For this reason, it has been suggested that these specimens may represent a new form of interspecific hybrids.

Diagnosis. General body appearance and structure as in parental Bythotrephes species with length 1.86–2.48 mm. The same is true for the body appendages, antennules, swimming antennae, mouth parts, and carapace. Thoracic limbs of first pair (tl I) long (73.5–90.5 % of body length), with long and thin endopodital segments (distal / proximal segment = 80.0–89.0 %) bearing comparatively short apical setae of second segment ( Fig. 6I ) (in one individual, these setae are of almost equal length). Postabdominal claws comparatively short (5.3–8.5 %) directed down and either slightly forward or backwards ( Figs. 6J, 6K ). Caudal process not long (205–240 %) and either straight or more often (in lake Borzu-Khol’) with more or less developed denticulated bend ( Figs. 6L, 6M ) situated in the middle of the process or closer to trunk (in Lake Todzha, only one individual had such a bend). Basally, the caudal process bears one pair of claws similar to those of postabdomen (5.6–8.9 % of body length) ( Figs. 6L, 6M ) and apically, a minute postabdominal setae. Pairs of claws sit rather close to each other (distance between postabdominal claws and claws of caudal process (interclaw distance) constitutes 11.6–16.2 % of body length). Between them, the thickness of the caudal process constitutes 3.5–4.3 % of body length. Borders separating old molted integuments of caudal process with claws normally are quite conspicuous.

Figures

FIGURE 6. Hybrid form Bythotrephes brevimanus x B. cederströmii, females (A–G). Bythotrephes sp. x B. cederströmii, females (H–M). (A–F, Lake Gusinoe; G, Lake Zaysan; H–M, Lake Borzu-Khol’). A–E, J, K, postabdominal and caudal claws. F, L, M, bend of caudal process. G, I, setae of distal end of second endopodital segment of tl I. H, the same of first endopodital segment of tl I.