Redescription of Bythotrephes longimanus Leydig, 1860 and B-cederstromii Schodler, 1877 (Crustacea: Cladocera: Onychopoda), with notes on the morphology and systematics of the genus Bythotrephes Leydig, 1860

摘要

Two core species of the genus Bythotrephes Leydig, B. longimanus Leydig and B. cederstromii Schodler, are redescribed on the basis of the type material, topotypic material, and other materials from Western and Northern Europe. They were investigated with an application of detailed morphological analysis, including intra-and interpopulation variability and the original scheme of morphometric measurements. The neotype of B. longimanus was selected from the topotypic population of the species from Bodensee (Switzerland-Germany). The taxonomy of B. cederstromii and of the whole genus is complicated by the presence of a variety of similar forms with intermediate morphology supposed to be interspecific hybrids, which much confused previous researches. The evolutionary transformations of some morphological structures, such as thoracic limbs, abdomen, and caudal process, are analyzed and discussed. In particular, it is suggested that the internal side of the endopodites of the thoracic limbs of the second-fourth pairs grew in proximal direction, occupying the part of protopodite with the "gnathobasic" process, which remains to be homologous to those of the thoracic limbs of the first pair. The strongly reduced thoracic limbs of the fourth pair lost two terminal segments and have proved to be represented by three segments, two proximal of which are those of protopodite, while the third terminal segment is composed of the fused distal part of the protopodite and the first proximal endopodital segment, bearing transformed and specifically arranged setae. The specificity of the "gnathobasic" processes of Bythotrephes is that they are the derivatives of the second, not of the first, inner endite of the thoracic limb and for this reason they are not homologues to the gnathobases of other cladocerans and should be called pseudognathobases.