Molecular phylogeny of tubificid oligochaetes with special emphasis on Tubifex tubifex (Tubificidae).

Tubifex tubifex is a cosmopolitan freshwater oligochaete whose presence has been studied as a health indicator of the aquatic environment and as a host for several myxozoan parasites of fish. Unfortunately, current morphological criteria used to distinguish Tubifex spp. (Tubificidae) are inadequate. We therefore developed mitochondrial 16S ribosomal DNA markers to examine phylogenetic relationships among aquatic oligochaetes and to distinguish species of Tubifex that might serve as hosts for a particular myxozoan parasite, Myxobolus cerebralis. Our phylogenetic analyses of oligochaetes based on a 378-bp segment yielded one most parsimonious tree with three major groups that corresponded to the families Lumbricidae, Sparganophilidae, and Tubificidae. T. tubifex and T. ignotus formed a monophyletic assemblage, and a sister relationship between the genera Tubifex and Limnodrilus was strongly supported. A second analysis of the relationship within the genus Tubifex identified six genetically distinct lineages of T. tubifex from North America and Europe that were separated by genetic distances comparable to those found for "well-defined" species of Limnodrilus. Therefore, the existence of several morphologically indistinguishable, thus cryptic, species of Tubifex in North America and Europe is suggested.