A contribution to the taxonomy ofC. sphaericus sens. lat. (Cladocera, Chydoridae)

Two distinct morphotypes ofChydorus sphaericus are recognised in Ireland. A reference collection of scanning electron micrographs of male and femaleC. sphaericus sens. str. has been compiled. This collection is used to compare the varying morphological features of Danish and Irish material to determine their taxonomic significance. However, no taxonomic decisions can be made about the Irish morphotypes until mature males and ephippial females become available. Comparison of population structures and stages of reproduction are accomplished by size-frequency distribution. This study illustrates the ever increasing value of scanning electron microscopy (SEM) in chydorid taxonomy, where characters such as shell-reticulation features may easily be misinterpreted using only light microscopy.