Kelp flies and species concepts -: the case of Coelopa frigida (Fabricius, 1805) and C-nebularum Aldrich, 1929 (Diptera: Coelopidae)

The beaches of the North Atlantic and North Pacific are home to kelp flies of the Coelopa frigida/nebularum complex, which consists of one to three different species depending on whether the two nominal species are accepted and a cryptic species proposed by Remmert is counted. The morphological differences between two populations of C. frigida (Fabricius, 1805) from the North Sea and the Baltic Region and two populations of C. nebularum Aldrich, 1929 from Alaska and Japan are described and discussed for small, medium, and large specimens. Crossing experiments are used to demonstrate that, under laboratory conditions, no isolation mechanisms between either population exist. Coelopa frigida and C. nebularum are therefore regarded as a single biological species, a conclusion that is congruent with the observation that the genetic distances based on Ef 1-alpha and 16S rDNA indicate lower levels of differentiation within C. frigida/nebularum than between undisputed Coelopa species. The substantial morphological, breeding and genetic information on the C. frigida/nebularum species complex is then applied to six different species concepts popular in the modern systematic literature. According to the Biological, Hennigian and Recognition Species Concepts, only a single species would be recognized. The Evolutionary Species Concept is too vague to be applicable and under two variants of the Phylogenetic Species Concept, C. frigida and C. nebularum would constitute separate species. This result confirms that Phylogenetic Species Concepts lead to a higher species number than concepts based on reproductive isolation. Practical and theoretical problems with the various species concepts are briefly discussed.