Taxonomy of Ceriodaphnia (Crustacea: Cladocera) in U.S. Environmental Protection Agency Cultures

This study investigated the taxonomy of three groups of the cladoceran genus (Ceriodaphnia) in cultures being used by the U.S. Environmental Protection Agency. One taxonomic group, having heavy, triangular denticles in a pecten on the postabdominal claw and very short male antennules, was identified as C. reticulata (Jurine 1820). The second group, with a heavy, setulated pecten on the claw and long male antennules was identified as C. dubia Richard 1894. The third group was taxonomically nearly identical to C. dubia except that the claw pecten of females sometimes had ovate, sharp teeth rather than comb-like setules, depending upon culture conditions. This was determined to be a hitherto undescribed phenotypic variant of C. dubia, and is designated as C. dubia, toothed-pecten variety. Specimens of this form have been found in populations of C. dubia collected in the U.S. west of the Mississippi River. Similarities in the general morphology, postabdomens, and ephibbia of C. reticulata and C. dubia suggest that they are evolutionarily closely related and might be able to hybridize and produce offspring having an ovate-toothed pecten like that of the C. dubia variant. Experiments designed to test this possibility were inconclusive although two successful interspecific matings were observed. It is suggested that the relationship between these two Ceriodaphnia could be further elucidated by study of more extensive field samples, and by interspecific breeding experiments that include hatching of hybrid young from ephippia and study of their taxonomy and fertility.