Speciation of two salinity-associated size forms of Oithona dissimilis (Copepoda: Cyclopoida) in estuaries

Abstract

The cyclopoid copepod Oithona dissimilis in Okinawa, Japan, has two size forms; the smaller occurs in upper estuaries and the larger in lower estuaries and coastal lagoons. Morphological difference between the forms is seen in the length/width ratio of the female caudal ramus. Genetically they differ by 1.84–2.21% from nuclear LSU rRNA (the large subunit ribosomal RNA gene) and 20.85–21.34% from the mitochondrial cytochrome c oxidase subunit I gene (mtCOI), which, in contrast, vary within a form by 0.00–0.18% and 0.00–1.34%, respectively. This genealogical concordance indicates that they are reproductively isolated species. Their habitats suggest speciation associated with different salinity regimes. The subspecies O. dissimilis dissimilis and O. dissimilis oceanica from India are also probably independent species because they are comparable to the two forms from Okinawa. However our specimens were notably different from previous descriptions of the species with respect to some micromorphology, suggesting that O. dissimilis is a species complex containing more than two cryptic species.

Keywords:

• Oithona dissimilis
• species complex
• genealogical concordance
• estuary
• Okinawa

Acknowledgements

We are grateful to Dr Frank D Ferrari and Dr Ryuji J Machida of the Smithsonian Institution for their useful suggestions, Dr Shinsuke Saito of Sessile Research Corp. Co. Ltd., Japan for his help in microscopic examination, and Dr Brian Ortman of the University of British Columbia and Dr Todd M Miller of Ehime University for kindly reviewing the English.