Morphology of skin armament of the diadromous and freshwater sculpins (Actinopterygii, Scorpaeniformes, Cottidae) of Eurasia

The morphology of skin armament has long been studied in some of freshwater sculpins, but has not found wide use in the taxonomy of this group. To assess the taxonomic value and determine the patterns of variability of this character, alizarin-stained skin samples of 19 diadromous and freshwater sculpins from Eurasia and one ancient North American species, Leptocottus armatus, were examined for the morphology of prickles. Three morphological types of prickles were ascertained. The largest prickles with a wide basal plate having an uneven anterior edge and a short, strong spine should be considered as the ancestral state. On a series of prickles examined a successive process of reduction was revealed from the ancestral type to the prickles that represent a thin, elongated, fusiform spine extending from a barely visible rounded basal plate with a smooth anterior edge, usually specific for younger freshwater species. The types of skin armament are not strictly consistent with the current concepts on the phylogeny of the studied freshwater Cottidae in view of the phenotypic parallelisms in this group. It is tentatively indicated insignificant intraspecific variation of the prickles shape in Mesocottus haitej, Cottus amblystomopsis, C. czerskii, and C. volki. Identification of most species studied using a single prickle is quite a challenge, although some species are easily identified this way. The species-specificity of prickles is shown to be the greatest in deeply divergent lineages, providing the opportunity to use these structures for diagnosing species and revealing hidden species richness. The skin armament morphology may also become an accessible and useful tool for testing taxonomic, biogeographical, and evolutionary hypotheses.