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The scientific name Cottus kazika has been used for over a century as valid for the catadromous fourspine sculpin endemic to Japan. However, recent DNA sequence analyses have indicated a sister relationship between two catadromous species, so-called C. kazika and Trachidermus fasciatus with a great phylogenetic divergence, separated from a monophyletic freshwater spawning clade including the other Cottus species and Baikalian sculpins. Morphological, life-historical, ecological, and karyological characteristics have also suggested that the fourspine sculpin so-called C. kazika is neither a member of Cottus nor Trachidermus. Therefore, the fourspine sculpin is transferred to the genus Rheopresbe. The genus and R. kazika are re-described. The genus Rheopresbe is discriminated from related genus Trachidermus by not having bony ridges on both nape and cheeks and also by presence of entopterygoid, and from the genus Cottus by having teeth on palatines and four spines on preopercle ridges.

The original version of this article unfortunately contained mistakes introduced during the production process. Tables 1 and 2 were presented incorrectly. The original article has been corrected.

Freshwater sculpins, grouped by their common ecological characteristic of freshwater habitats, comprise the species from the genera Cottus, Trachidermus, Mesocottus and Myoxocephalus as well as the Baikal sculpins. These fishes are typically coldwater-adapted, having probably originated polyphyletically from ancestral species of marine sculpins. Gottus, the most speciose taxonomic group, includes at least 64 species and is distributed throughout the fresh waters of the northern part of the Northern Hemisphere. Indinidual species have diverse life histories, such as fluvial, lacustrine, catadromous, and freshwater amphidromous. The second most abundant group, the Baikal sculpins, includes 33 species in 12 genera representing 3 families, and comprises many benthic, and a few bentho-pelagic and pelagic species. The freshwater sculpins belonging to Trachidermus, Mesocottus and Myoxocephalus include only one or two species in each genus. Recent molecular phylogenetic analyses of Cottus species, Baikal sculpins and their relatives have demonstrated mainly that (1) Cottus kazika is a sister taxon to Trachidermus fasciatus (designated lineage A), (2) Cottus species, except for C. kazika, and the Baikal sculpins are monophyletic, (3) on the basis of (1) and (2), Cottus as presently recognized is not monophyletic, and (4) the Major monophyletic lineages include 7 lineages: lineage B from Eurasia, lineages C and D from East Asia, the Cottopsis clade (sensu Copeia 2005:303–311, 2005) from the west coast of North America, the Cottus clade from the Circum-Arctic sea, the monophyletic Baikal sculpins, and the Uranidea clade. These findings suggested that the monophyletic freshwater sculpins that comprise the lineage A and the 7 other lineages may undergone two major radiations, one having occurred in the fresh water Cottus species in the northern part of the Northern Hemisphere, and the other in the Baikal sculpins in the Lake Baikal, the world’s oldest freshwater lake. Through these adaptive radiations, a tremendous diversity of morphological, ecological, physiological and life historical traits now exists in the freshwater sculpins.

Information on migration ecology is important for effective conservation and management of fishery resources, particularly diadromous animals. We investigated the genetic population structure and gene flow pattern of anadromous Arctic lamprey Lethenteron camtschaticum by analyzing polymorphic microsatellite loci. Bayesian population structure analysis was used to estimate the existence of two genetically divergent ancestral sources and latitudinal gradual changes of their frequencies among the present populations. When hypotheses of historical migration models were tested using coalescent-based methods, the best model was found to be an n-island model assuming equal population size and reciprocal migration among all populations. These results suggest that L. camtschaticum has considerable ability to migrate long distances in the sea and has lower homing abilities. For resource management of L. camtschaticum, unconstrained gene flow across all populations should be maintained. The present findings might be beneficial for conservation and contribute the sustainable use of L. camtschaticum resource.

Freshwater sculpin dwelling in Lake Chuzenji, Nikko, Kanto District, Japan, was identified as the lacustrine type of Cottus reinii, Utsusemikajika, by using a combination of pectoral fin ray counts (15–17, mode 16), mitochondrial DNA variations and egg size (1.33 ± 0.06 mm). We suppose that C. reinii in Lake Chuzenji was introduced unintentionally from Lake Biwa along with either Oncorhynchus masou subsp. or Sarcocheilichthys variegatus microoculus during the late nineteenth to early twentieth centuries. This is the first report of an establishment of the lacustrine type of C. reinii outside of Lake Biwa.

Artificial introduction and habitat fragmentation affect the indigenous gene pools of fluvial animals. To investigate the effect of human activities on the genetic population structure of vulnerable brook lamprey Lethenteron sp. S in a single river system, samples from 12 tributaries of the Jinzu River, Japan, were analyzed using mitochondrial and microsatellite DNA markers. Exogenous Lake Biwa (Japan) haplotypes and alleles were detected in lampreys from several Jinzu River tributaries. Since Lake Biwa is the source of the commercial ayu fish Plecoglossus altivelis that is introduced in the Jinzu River, the exogenous Lake Biwa lamprey genotypes in the Jinzu River probably originated from the Lake Biwa lampreys that were unintentionally introduced along with the ayu fish. Bayesian admixture and mitochondrial DNA analyses revealed various genetic disturbance phases of the exogenous genotypes in the Jinzu River, such as the six indigenous populations, four admixed populations with low frequencies (average admixture proportion = 0.02–0.04; exogenous haplotype proportion

Specimens of the Cottus pollux species' group collected from the upper part of the Honmyo River, Nagasaki Prefecture, Japan, were subjected to morphological and allozyme analyses to place them into one of the recognized valid taxa, viz. small-egg type (SE type), middle-egg type (ME type), or large-egg type (LE type). They were identified as ME type on the basis of specific morphological characteristics, such as laterally depressed cross-sectional shape at posterior half of the body and deep caudal peduncle, and by having a diagnostic allozyme allele (MEP * 54). This ME-type population is the first recorded from Kyushu Island, Japan. An ecological survey of the population revealed that females spawned larger eggs (2.8–3.2 mm in diameter) than those of other amphidromous populations, from which well-developed yolk-sac larvae of about 8.0 mm TL were hatched out. In addition, ME-type specimens collected on 25 and 26 May 2001 included 15 sex-unknown juveniles ranging from 18.8 to 30.2 mm SL, suggesting that they represented larvae hatched out on a nearby spawning ground, with no experience of downstream migration into Isahaya Bay. This observation strongly suggests that the ME-type population in the Honmyo River has a fluvial lifestyle, being different from other amphidromous populations. The former population may have arisen from an amphidromous ancestor through changes in egg size and early ontogenetic development.

The fluvial sculpin, Cottusnozawae, is a coldwater-adapted fish distributed in Hokkaido Island and the northeastern part of Honshu Island (Tohoku District), Japan. Mitochondrial DNA (mtDNA) control region sequencing was used to investigate the geographic distribution of genetic variation and phylogeography of C. nozawae. Most populations possessed unique haplotypes, few being shared across river systems. Phylogenetic analysis of the sequences of the mtDNA control region and adjacent regions of C. nozawae revealed three distinct phylogenetic groups that differed by 3.05% to 3.11%, corresponding to distinct geographic regions, Hokkaido Island, northern Tohoku District, and Yamagata Prefecture (southwestern Tohoku District), respectively. The divergence times of three groups were estimated to be about 1.5 million years ago by applying a general rate for mtDNA, suggesting that the divergence among them might have occurred in the early Pleistocene. Divergence among the haplotypes within the group from the northern Tohoku District was also high (1.84%), no haplotypes being shared by local populations in different river systems in this region. Local populations from a single river system in this region comprise a distinct lineage that differed from other river systems. Such genetically divergent population structures among the different regions and river systems are considered to have resulted mainly from long-term isolation and restricted gene flow among river systems, probably promoted by the fluvial benthic life history and low dispersal ability of this species.