Reconciling molecular systematics and traditional taxonomy in a species-rich clade of sea stars (Leptasterias subgenus Hexasterias)

Recent or incomplete speciation events can often lead to conflicting patter ns of genetic and morphological variation, due to a combination of hybridiz ation, incomplete lineage sorting and limited morphological divergence amon g genetically distinct species. This situation complicates the reconciliati on of molecular systematic results with morphology-based taxonomy. For exam ple, six-rayed sea stars of the genus Leptasterias have been the subject of considerable taxonomic controversy. Prior molecular studies established th at Leptasterias subgenus Hexasterias consists of several genetically discre te clades, but extensive morphological variability has prevented the applic ation of formal taxonomic nomenclature to these groups. In the present stud y, molecular and morphological approaches were combined to reconcile, where possible, the traditional taxonomy of Fisher (1930: US Nat Mus Bull 76:1-3 56) with recent molecular systematic results. Partial mitochondrial DNA con trol region sequences were collected from 29 Fisher-identified museum speci mens representing seven nominal species of Leptasterias subgenus Hexasteria s, to establish the relationship between traditional taxa and molecular lin eages. Additional DNA sequences were obtained from 52 recently collected in dividuals representing the L. aleutica (Fisher, 1930)/L. camtschatica (Bran dt, 1835) complex; these sea stars were collected on Kanaga Island, Alaska, in August 1997. Morphological data were collected for 227 museum specimens collected before 1930, and 309 specimens collected between 1988 and 1998 f rom locations in the eastern North Pacific. The analyses indicate that thre e nominal species within the Camtschatica section of subgenus Hexasterias, L. alaskensis (Verrill, 1914), L. hexactis (Stimpson, 1862), and L. leptodo ma (Fisher, 1930), correspond to separate major clades of a molecular phylo geny, permitting the re-description of these taxa using molecular character s. Extensive sharing of mitochondrial DNA sequences by L. aleutica and L. c amtschatica suggests recent genetic divergence of these two species and pos sible incomplete lineage sorting or frequent hybridization. The lack of dia gnostic molecular differences between these two nominal species means that they are currently identifiable only by morphological criteria. Determining the status of three other nominal species, L. aequalis (Stimpson, 1862), L . asteira (Fisher, 1930) and L. pusilla (Fisher, 1930) will require additio nal molecular and morphological data.