The North American Chydorus faviformis (Cladocera, Chydoridae) and the Honeycombed Taxa of Other Continents

The taxon Chydorus faviformis, described by Birge from North America in 1893, has been considered to occur also in Asia, Australia, and South America. However, careful study of populations from these regions has revealed that all represent different species, none of which is closely related to C. faviformis. The taxa described here are C. obscurirostris and C. opacus from Australia, C. obscurirostris tasekberae from Malaysia, C. sinensis from China, C.angustirostris from India, and C. parvireticulatus from South America. The taxon in Malaysia differs somewhat from the corresponding taxon in Australia, but cannot be characterized more closely until males and ephippial females become available. The taxa differ among themselves in number of meshes on the shell of parthenogenetic females, surface patterning within the meshes, shape of the rostrum and height of the mesh walls along the edge and near the tip of the rostrum, stoutness and length of the major seta on the inner distal lobe of trunklimb I, shape of the labral plate, and shape and armament of the postabdomen. Ephippial females all have a single resting egg. They differ in the extent of secondary thickenings of the surface network within the shell meshes and in the amount of pigment deposited in the region of the egg locule. Males are most important for separating the taxa, indicating how necessary they are for working out evolutionary similarities and differences. Unfortunately no males of the taxa from Malaysia, India, and South America have been available. For the others, C.faviformis sens. str. is unique in that it is the only taxon in which the male loses its honeycomb (that is, the raised walls of the meshes) on reaching maturity. It also has a sharp pre-anal angle and a marked narrowing of the postabdomen distad from here, which is the pattern typical of species in the Chydorus sphaericus complex. None of the other faviformis-like species share this characteristic. Because of the marked differences in morphology and in geographical distribution of the species in North America and in South America, it is certain that even during the glacial ages, when the northern C. faviformis would have been displaced farthest southward, there was no exchange of either taxon to the other continent. The taxon from Manáos, Brazil listed as C.faviformis in the Birge collection is the C. parvireticulatus reported from much farther south in Brazil and Argentina. In Australia and Asia, except for the uncertain disti