Molecular population structure of the marine benthic copepod Microarthridion littorale along the southeastern and Gulf coasts of the USA

Relationships among populations (southeast Atlantic and northern Gulf coast , USA) of a ubiquitous, estuarine, harpacticoid copepod (Microarthridion li ttorale Poppe) were estimated using sequence data from two loci: one mitoch ondrial [cytochrome b (Cyt b)] and one nuclear [first internal transcribed spacer of ribosomal DNA (ITS-1)]. Copepods were collected from seven estuar ies in 1997/98. Allelic phylogenies based on both genes were generally conc ordant, and suggested that M. littorale populations are structured over lar ge geographic scales (hundreds of kilometers). Three well-supported groups were found in both gene trees comprising clades of alleles sampled from Sou th Carolina, Florida, and Louisiana. Alleles from the Savannah, Georgia sam ple formed a monophyletic group using the Cyt b data, but this clade was no t distinguishable with comparable ITS-1 data. A single specimen from Louisi ana was classified in different clades depending on the locus assayed.