Separation of Pleuroxus laevis Sars, 1861 from two resembling species in North America: Pleuroxus straminius Birge, 1879 and Pleuroxus chiangi n.sp. (Cladocera, Chydoridae)

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Abstract
E. A. Birge, in his Ph.D. dissertation (1878. Harvard University, Cambridge, MA) and in his first published paper (E. A. Birge. 1879. Trans. Wis. Acad. Sci. Arts. Lett. 4: 77–109), described a species of Pleuroxus from North America and named it P. straminius, then later (E. A. Birge. 1893. Trans. Wis. Acad. Sci. Arts. Lett. 9: 275–317) decided the organism was the same as P. laevis of Scandinavia and hence gave it that name, or rather the invalid, newer name P. hastatus. Detailed study of these two taxa has shown that they are completely different from each other in size, shape, surface ornamentation, nature and color of the carapace, structure of the posterior-ventral corner of the shell, shape and denticulation of the postabdomen, and especially armament of the male postabdomen, that of P. straminius having essentially the same denticulation as the female, whereas in P. laevis all of the marginal denticles are replaced by oblique short rows of spines. Perhaps more significant is the occurrence of a second species in North America, Pleuroxus chiangi n.sp., which is much more closely related to P. laevis than is P. straminius. It differs somewhat vaguely in the relative size of various parts, but it also differs in a number of specific characters, all of which demonstrate that it is a valid separate species. Thus, the posterior-ventral angle of the shell is rounded, with the shell tooth some distance anterior and with several setae immediately posterior being long and projecting well beyond the margin; the postanal portion of the postabdomen is shorter and has about two fewer denticles; and, most important, the male postabdomen retains the two large distalmost denticles and has the oblique rows of setae substituting for the others. Pleuroxus straminius is widely distributed in glaciated North America from Newfoundland to northern Wisconsin and Minnesota, then south along the Atlantic Coast into northern Florida. One suspects that because of species differences already demonstrated between northern and southern populations of Eurycercus and of Pleuroxus denticulatus, P. straminius might also be so differentiated. However, no decision can be made without having males and ephippial females from the southern populations. Pleuroxus chiangi presently seems much less frequent and less abundant than P. straminius. It occurs from Nova Scotia westward to British Columbia, including northern Wisconsin and possibly northern Minnesota, and northward into the Northwest Territories. It seems similar to a number of species, such as Alonopsis americana and Chydorus canadensis, whose distributions barely reach the United States at all but which are well distributed northward in eastern Canada. We need many more data on the occurrence of various taxa in the north. Thus, here is still another example of noncosmopolitanism in the chydorids. The presence of closely related species in North America and Europe might have derived vicariously from the splitting of North America from Europe as the Atlantic Ocean developed. The other species, P. straminius, does not seem to have any counterpart in Europe, although possibly P. sinkiangensis of China is related to it. This and other taxa presently called P. laevis must be studied to work out the degrees of relationship and to develop ideas as to when and how the species evolved. Neotypes have been selected for P. laevis and for P. straminius. These and diverse samples of all three taxa have been deposited in major museums in North America and Europe.

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