Marine lakes in the Palau Islands are known to harbor unique marine fauna that have remained isolated since the formation of the lakes after the Last Glacial Maximum. We analyzed mussels from marine lakes located on different islands and conducted morphological, phylogenetic and population genetic characterization to clarify their evolutionary history. The mussels were morphologically classified into three differentiated morphs: NS, ON, and MC. Their common characteristics were consistent with the Brachidontes-Hormomya complex of the Mytilidae family. Phylogenetic analysis based on the nuclear 18S ribosomal RNA gene supported the taxonomic position of the mussels among the Mytilidae. In the mitochondrial cytochrome c oxidase subunit I (COI) gene lineage, NS-and MC-morphs were highly diverged from each other; their estimated time of divergence dates back to the mid-Pleistocene. ON-morph was more closely related to MC-morph, although the shell morphologies of ON- and MC-morphs were easily distinguishable. Population genetic analysis revealed the coexistence of highly diverged haplotypes within a population of ON-morph, indicating introgression of mtDNA among the morphs. Our data suggest that morphological differentiation of marine lake mussels can occur in a relatively short period under different environmental conditions. Thus, the marine lakes provide a unique site for the study of diversification in mussels.

Mitochondrial DNA (mtDNA) at Co-1 gene region was sequenced for 7 scorpion fish species (in total, 16 sequences of at least 552 bp) from the Far East of Russia and compared with 15 other sequences of Scorpaeniformes comprising altogether 29 scorpion-like fish sequences and two outgroup sequences (Cypriniformes). The analysis of the protein-coding Co-1 gene revealed statistically substantiated bias in the (T + C) : (A + G) content, proving basic findings. The average scores of p-distances for different scales of the evolutionary history at Co-1 gene revealed a clear pattern of increased nucleotide diversity at four different levels: (1) intraspecies, (2) intragenus, (3) intrafamily, and (4) intraorder. The scores of average p-distances for the compared fish groups were: (1) 1.00 ± 0.20%, (2) 3.80 ± 0.20%, (3) 12.40 ± 1.20%, and (4) 18.00 ± 0.38%, respectively (mean ± SE). These data support the concept that speciation in the order Scorpaeniformes, in most cases, follows a geographic mode through accumulation of numerous small genetic changes over a long period of time. However, intraspecies diversity was surprisingly high among scorpionfish. Phylogenetic trees for 29 sequences of scorpionfish and 2 other fishes belonging to ray-finned fishes (Actinopterigii) were developed using Co-1 gene and four different analytical approaches: Bayesian (BA), maximum likelihood (ML), neighbour-joining (NJ), and maximum parsimony (MP). The analysis revealed a monophyletic origin for the representatives of Cottidae, which is the principal scorpionfish family investigated (100, 96, 98% support level in our BA, MP, and NJ analyses). Similarly, the monophyletic origin of up to the three compared scorpion-like fish genera was supported by molecular phylogenetic data. Species identification on individual basis (barcoding tagging) was high. A few taxonomic complications arose during the analysis and they are discussed here in.

Although evidence for the evolution of terrestrial species on islands continues to rapidly accumulate, little is known about the evolution of marine species in geographically isolated environments such as islands as ocean currents often facilitate gene flow among populations. In this study, we focused on marine lakes of the Palau Islands, which are considered to be true analogues of terrestrial islands for marine species. To examine evolutionary processes in marine lakes, we conducted population genetic analyses on marine lake and lagoon populations of the striped silverside, Atherinomorus endrachtensis, using two mitochondrial DNA markers differing in evolutionary rate, the cytochrome b gene and the control region. The analyses revealed that the amount of genetic diversity of marine lake populations is much lower than that of lagoon populations and high levels of genetic differentiation occur among marine lake and lagoon populations. The present study has shown that marine lake populations have been completely isolated and have differentiated from lagoon populations, and each marine lake population is experiencing different evolutionary processes. These findings clearly demonstrate that marine lakes are excellent environments for the evolutionary study of marine species.

Mitochondrial DNA of the Co-1 gene region was sequenced for 8 fish species (in total 17 sequences of at least 532 bp) from the far east of Russia and compared to 3 other perciform sequences, altogether comprising 20 perch-like fish sequences and 2 outgroup sequences (Cypriniformes). The analysis of the protein-coding Co-1 gene revealed a statistically substantiated bias in the (T+C):(A+G) content, confirming the basic findings on the pyrimidines-prunes ratio. The average scores of p-distances for different scales of the evolutionary history of the Co-1 gene revealed a pattern of increased nucleotide diversity at 4 different levels: (1) intraspecies, (2) intragenus, (3) intrafamily and (4) intraorder. The scores of the average p-distances of the 4 categories of comparison were: (1) 0.11 ± 0.04%, (2) 1.87 ± 0.68%, (3) 12.67 ± 0.28%, and (4) 16.52 ± 0.10%, respectively (mean ± SE). These data support the concept that speciation in the order Perciformes in most cases follows a geographic mode through accumulation of numerous small genetic changes over a long time. The phylogenetic trees for 18 and 21 sequences of perch-like fishes together with 2 other fishes belonging to the ray-finned fishes (Actinopterygii) were developed using the Co-1 gene and 4 different analytical approaches: neighbour-joining (NJ), maximum likelihood (ML), Bayesian (BA) and maximum parsimony (MP). The analysis revealed a monophyletic origin for the representatives of the Stichaeidae, which was the principal percoid family investigated (86, 96 and 100% support in our NJ, ML and BA analyses, respectively). Species identification on a per individual basis or Co-1-based DNA barcoding was high.

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