LARGE BRANCHIOPOD ASSEMBLAGES COMMON TO MEXICO AND THE UNITED-STATES

We present observations on the frequency of large branchiopod associat ions found in north-central Mexico, and in Arizona, USA. Of a total of 25 species involved in these assemblages, 12 were common in both area s. Fifty-eight (43.3%) of-the ponds in Mexico, and seventy-eight (47%) of those in Arizona had two or more species present. The combinations of species which occurred with highest frequency were Streptocephalus mackini with Thamnocephalus platyurus for Arizona, and T. platyurus, Triops sp., and Leptestheria compleximanus for Mexico. In Mexico, and Arizona, as in many parts of the world, multispecies assemblages of la rge branchiopods are a common phenomenon. Therefore, the 'common rule' of 'one-phyllopod-per-habitat' advanced by Weise (1964) is inconsiste nt with field observations. Furthermore, since cases of co-occurrence of two or more congeneric species of Anostraca and Notostraca are not uncommon, generalizations about congeners not coexisting remain applic able only for the Spinicaudata and Laevicaudata. On the basis of the l iterature; and of observations in the field and in the laboratory, we List potential factors contributing to the co-occurrence of several sp ecies within a pond.