Morphological variation and systematics in the Scyphozoa:: Mastigias (Rhizostomeae, Mastigiidae) -: a golden unstandard?

Vagarious descriptions of species boundaries in jellyfishes have been attributed to inconsistent phenotypic variation between individuals, size-classes, populations, and species. However, the historical predominance of subjective and largely qualitative analyses of geographic variation has made it difficult to know where, if not in the analyses themselves, the real problems lie. Statistical analyses of morphological variation provide more objective and quantitative datasets. They also can be integrated with, for example, molecular genetics, geography, and paleoclimatology to provide an evolutionary perspective on biodiversity. Here, I illustrate some of the benefits of integrative statistical analyses of morphological variation in the golden jellyfish, Mastigias L. Agassiz, that inhabit lagoon and marine lake ecosystems in Palau, Micronesia. The morphology of Mastigias varies considerably between medusae, size-classes, populations, and environments and, although medusae generally showed location-specific morphologies, none of the variable features measured diagnosed all medusae from any location. DNA sequence data from cytochrome c oxidase subunit I and internal transcribed spacer one showed little variation and also did not reliably distinguish medusae from different locations. These results are consistent with post-glacial changes in sea-level and topography that suggest recent evolution of marine lake populations from an ancestral lagoonal form. Remarkably, many morphological features show greater variety in Mastigias in Palau than in all other members of the genus described from eastern Africa to the tropical South Pacific. Their morphological similarity, however, may mask considerable genetic divergence, as is the case for lagoonal forms in Palau and Papua New Guinea. There is, therefore, considerable heterogeneity in evolutionary process and morphological variation may be decoupled from variation in commonly used molecular markers. These results contribute to our understanding of inconsistencies in the taxonomy of scyphozoans and confirm that there is no widely applicable taxonomic standard for divining species. An evolutionary approach, however, provides a diverse set of tools for satisfactorily interpreting geographic variation for systematic purposes.