The phylum Cnidaria -- and particularly the Class Hydrozoa, are famous for their incredible diversity of life cycles, alternating between a benthic polyp stage, and a pelagic medusa, or jellyfish stage. The extreme morphological differences between these stages have made taxonomy very difficult, with independent Families being erected for the polyp and medusoid stage. Until recently, the only way to tie together the stages was to directly observe the life cycle stages in laboratory culture.

Other difficulties plague hydrozoan taxonomy. Reproductively isolated cryptic species that are morphologically identical are common, leading us to undercount species. Conversely, distinguishing among similar morphologies is a large problem, the extreme plasticity of hydrozoan morphology has led some workers to over-diagnose species. For example, the major works on American hydrozoans were written by E. Fraser (1937; 1944). Fraser was a notorious "splitter," often naming new species from singular occurrences on novel substrata. The Emperor Hirohito of Japan, an eminent hydrozoan taxonomist, was also known to name new species from singular occurrences on fish! As with the problem of linking polyps and medusae, a database of 16S sequences will help link together disparate morphotypes of the same species.

In sum, the occurrence of cryptic species leads us to under-count species, and morphological plasticity and broad host-range leads us to over-count species. This means that DNA sequencing (together with careful morphological analysis) must play a central role not only in the monographs we will produce, but in our linked databases.