Taxonomic implications for diaptomid copepods based on contrasting patterns of mitochondrial DNA sequence divergences in four morphospecies

Abstract

Morphological identification methods do not provide reliable and meaningful species identifications for taxa where morphological differences among distinct species are either absent or overlooked (i.e., cryptic species). For example, due to the minute nature of the morphological characters used to delineate diaptomid copepod species and the apparent potential for copepod speciation to occur with little or no morphological change (i.e., morphological stasis), morphological identifications of diaptomid species may not adequately capture their true species diversity. Here, we present results from a geographic survey of mtDNA sequences from populations across the geographic ranges of four North American diaptomid species—Leptodiaptomus minutus, Skistodiaptomus pallidus, Skistodiaptomus reighardi, and Onychodiaptomus sanguineus. Shallow mitochondrial DNA sequence divergences (maximum of 1.1%) among haplotypes of L. minutus from across its geographic range suggest that current morphological identification techniques reliably identify this species. In contrast, we found large mitochondrial DNA sequence divergences (14–22%) among populations within the currently recognized morphospecies of S. pallidus, S. reighardi, and O. sanguineus. However, pairwise sequence divergences within four distinct S. pallidus clades and within populations of S. reighardi and O. sanguineus were similarly low (maximum of 1.5%) as found within L. minutus as a whole. Thus, the S. pallidus, S. reighardi, and O. sanguineus morphospecies may be considered best as cryptic species complexes. Our study therefore indicates that morphological identifications, while sufficient for some species, likely underestimate the true species diversity of diaptomid copepods. As such, we stress the need for extensive taxonomic revision that integrates genetic, morphological, reproductive, and ecological analyses of this diverse and important group of freshwater zooplankton. Furthermore, we believe an extensive taxonomic revision will shed important insight into major questions regarding the roles of geography, phylogeny, and habitat on the frequency of cryptic species on earth.