Molecular insights into geographic and morphological variation within the Eumicrotremus asperrimus species complex (Cottoidei: Cyclopteridae)

Abstract

A molecular phylogeny of lumpsukers, Eumicrotremus asperrimus and related species (family Cyclopteridae), is presented on the basis of sequence variations in the cytochrome b and cytochrome c oxidase subunit 1 genes (1,659 base pairs) of mitochondrial DNA using specimens collected from across the North Pacific, including the Sea of Japan, Sea of Okhotsk, Bering Sea, and Gulf of Alaska. Specimens identified as Eumicrotremus phrynoides, Cyclopteropsis bergi, Cyclopteropsis lindbergi, and Lethotremus muticus on the basis of the presence or absence of spiny tubercles and height of the first dorsal fin did not exhibit reciprocal monophyly, but were randomly clustered with E. asperrimus. This collection of forms is therefore referred to as the “E. asperrimus species complex.” DNA sequence data presented here divided the E. asperrimus species complex into two distinct clades, corresponding to the eastern North Pacific (the Bering Sea, Aleutian Islands, and Gulf of Alaska) and the western North Pacific (the Seas of Japan and Okhotsk) regions. Slight morphometric differences between eastern and western clades were also evident, indicating that they represent at least two different species. The genetic level of divergence between the two clades suggests that the speciation event occurred during the early Pleistocene to late Pliocene. Although the presence and morphology of tubercles have been used extensively for species discrimination in Cyclopteridae, our results suggest that this character complex is confounded by intraspecific variation. Examined samples showed some sexual dimorphism in the relative development of the tubercles, although the pattern was different between the eastern and western North Pacific clades. These results underscore the need for a thorough re-examination of the taxonomy of Pacific cyclopterids, using molecular data to supplement potentially misleading tubercle morphology.