Decoupling of molecular and morphological evolution in deep lineages of a meiobenthic harpacticoid copepod

Molecular and biochemical genetic analyses have revealed that many marine i nvertebrate taxa, including some well-studied and presumably cosmopolitan s pecies, are actually complexes of sibling species. When morphological diffe rences are slight and estimated divergence times are old, data suggest eith er unusually high rates of sequence evolution or long-term morphological st asis. Here, five gene regions (mitochondrial cytochrome oxidase subunit I a nd large-subunit ribosomal 16S rDNA and nuclear ITS1, 5.8S rDNA, and ITS2) were analyzed in four geographic samples of the meiobenthic harpacticoid co pepod Cletocamptus deitersi. Molecular sequences revealed four extremely di fferentiated molecular lineages with unalignable nuclear intergenic spacers and mitochondrial uncorrected divergences reaching 25% (cytochrome oxidase ) and 36% (16S rDNA). These levels of divergence are greater than those rep orted previously for congeneric species in diverse invertebrate taxa, inclu ding crustaceans. The nominally intraspecific divergence matches or exceeds the corresponding divergence from a known congener (Cletocamptus helobius) . A molecular clock applied to the cytochrome oxidase subunit I data sugges ts that these lineages split in the Miocene, consistent with the fossil rec ord of a North American Cletocamptus from the same period. Morphological di fferences among the major lineages are subtle but congruent with the patter ns of genetic differentiation. Our conclusion, based on concordant patterns of variation in two mitochondrial and three nuclear gene regions, as well as morphological observations, is that C. deitersi in North America is comp osed of at least four separate species by the genealogical concordance, phy logenetic, and morphological-species criteria. Alternative explanations for the deep phylogenetic nodes and apparent morphological stasis, including h igh rates of sequence evolution, balancing selection, and genetic signature s of historical events, are considered unlikely.