Determining whether a population is introduced or native to a region can be challenging due to inadequate taxonomy, the presence of cryptic lineages, and poor historical documentation. For taxa with resting stages that bloom episodically, determining origin can be especially challenging as an environmentally-triggered abrupt appearance of the taxa may be confused with an anthropogenic introduction. Here, we assess diversity in mitochondrial cytochrome oxidase I sequences obtained from multiple Atlantic and Pacific locations, and discuss the implications of our findings for understanding the origin of clinging jellyfish Gonionemus in the Northwest Atlantic. Clinging jellyfish are known for clinging to seagrasses and seaweeds, and have complex life cycles that include resting stages. They are especially notorious as some, although not all, populations are associated with severe sting reactions. The worldwide distribution of Gonionemus has been aptly called a “zoogeographic puzzle” and our results refine rather than resolve the puzzle. We find a relatively deep divergence that may indicate cryptic speciation between Gonionemus from the Northeast Pacific and Northwest Pacific/Northwest Atlantic. Within the Northwest Pacific/Northwest Atlantic clade, we find haplotypes unique to each region. We also find one haplotype that is shared between highly toxic Vladivostok-area populations and some Northwest Atlantic populations. Our results are consistent with multiple scenarios that involve both native and anthropogenic processes. We evaluate each scenario and discuss critical directions for future research, including improving the resolution of population genetic structure, identifying possible lineage admixture, and better characterizing and quantifying the toxicity phenotype.

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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.

Chironomids of the subgenus Chaetocladius s. str. from the Amur River basin are revised using both morphological characters and molecular data. Three new species, C. egorych sp. nov., C. lopatinskiy sp. nov. and C. yavorskayae sp. nov., are described and figured. The pupa of C. fedotkin is described for the first time. Adult males of C. ligni and C. piger, little-known in the Far East, are redescribed and annotated, and key to males of the Chaetocladius s. str. from the Amur River basin is provided. A reference 658 bp barcode sequence from a fragment of the mitochondrial gene cytochrome oxidase I (COI) was used as a tool for species delimitation. Comparisons with corresponding regions of COI between 5 species in the subgenus produced K2P genetic distances of 8.3-12.6%, values well associated with interspecific variation. Molecular data were also used for the reconstruction of the phylogenetic relationships within the subgenus Chaetocladius s. str.

We obtained new data on the complete mitochondrial DNA (mtDNA) and the ribosomal operon of the trematode Carassotrema koreanum (Digenea: Haploporata: Haploporidae), an intestinal parasite of Carassius auratus, using next-generation sequencing. The mtDNA of C. koreanum contained 13,965 bp, including 12 protein-coding genes, two ribosomal genes, 22 transport RNA (tRNA) genes and a non-coding region. The ribosomal operon of C. koreanum was 10,644 bp in length, including ETS1 (1449 bp), 18S ribosomal RNA (rRNA) gene (1988 bp), ITS1 ribosomal DNA (rDNA) (558 bp), 5.8S rRNA gene (157 bp), ITS2 rDNA (274 bp), 28S rRNA gene (4152 bp) and ETS2 (2066 bp). Phylogenetic analysis based on mtDNA protein-coding regions showed that C. koreanum was closely related to Parasaccocoelium mugili, a species from the same suborder Haploporata. Bayesian phylogenetic tree topology was the most reliable and confirmed the validity of the Haploporata. The results of sequence cluster analysis based on codon usage bias demonstrated some agreement with the results of the phylogenetic analysis. In particular, Schistosoma spp. were differentiated from the other members of Digenea and the members of Pronocephalata were localized within the same cluster. Carassotrema koreanum and P. mugili fell within different clusters. The grouping of C. koreanum and P. mugili within the same cluster was obtained on the basis of frequencies of 13 specified codons, of which three codon pairs were degenerate. A similarity was found between two haploporid species and two Dicrocoelium spp. in the presence of TTG start codon of the mitochondrial nad5 gene. Our results confirmed the taxonomical status of the Haploporata identified in the previous studies and revealed some characteristic features of the codon usage in its representatives.