Molecular phylogeny of selected dorid nudibranchs based on complete mitochondrial genome.

Dorid nudibranchs are a large group of mollusks with approximately 2,000 recorded species. Although agreement exists on the monophyletic nature of the dorid nudibranch group, the interfamily relationships of the suborder are subject to debate. Despite efforts to elucidate this issue using short molecular markers, the conclusiveness of the findings has been hindered by branching polytomy. Mitogenomes are known to be effective markers for use in phylogenetic investigations. In this study, eight mitogenomes of dorid nudibranchs were decoded and analyzed. Gene content and structure showed little change among species, reflecting the conserved mitogenomes of dorid nudibranchs. For most genes, the direction was typical for nudibranchs; nevertheless, tRNACys had an inverse direction in Cadlinidae species. Phylogenetic trees based on nucleotide and amino acid datasets revealed a relatively consistent pattern of interfamily relationships with little difference for positions of Phyllidiidae and Cadlinidae. Species of Cadlinidae were clustered together and did not form a clade with Chromododidae. Additionally, Goniodorididae was sister to Aegiridae, whereas Discodoridae was sister to Dorididae. This finding was supported by tree topology test based on mitogenome data. The results of the present study indicate that complete mitogenomes are promising markers for investigating interfamily relationships among dorid nudibranchs.

Characterization of the complete mitochondrial genome of Okenia hiroi (Baba, 1938) (Nudibranchia, Goniodorididae).

Okenia is a speciose genus of the family Goniodorididae with more than 50 valid species. The phylogenetic relationships within the genus are little known. The mitogenome is a good marker to understand the phylogenetic relationships of relative species. This study was performed to sequence the mitogenome of O. hiroi. The mitogenome of O. hiroi was 14,583 bp in size and was composed of 37 genes, including 13 protein-coding genes, two ribosomal RNA genes, and 22 tRNA genes. The nucleotide composition was 30.5% A, 13.6% C, 16.5% G, and 39.4% T. The phylogenetic analysis showed that O. hiroi is sister to Notodoris gardineri (Aegiridae). This study recorded the first mitochondrial genome sequence of the family Goniodorididae.

A report of a new species and new record of Cadlina (Nudibranchia, Cadlinidae) from South Korea.

Of the four species in the genus Cadlina present in the northwestern Pacific region, C. japonica has been the only species recorded from South Korea. For the purpose of investigating Cadlina in Korean waters, specimens were collected from the Korean East Sea (Sea of Japan) by scuba diving. The radula and morphology of these specimens were examined by stereoscopic and scanning electron microscopy. Based on morphology, three species were identified in Korean waters, including the new species, Cadlina koreana sp. nov., C. umiushi (first record in South Korea), and C. japonica. Cadlina koreana sp. nov. somewhat resembles C. umiushi but differs in both its morphology as well as the structure of its radula. The background color of Cadlina koreana sp. nov. is translucent white, tubercles on the dorsum are opaque white and the yellow marginal band is absent. The radular formula of Cadlina koreana sp. nov. is 57 × 23.1.23 with a rectangular rachidian tooth. In addition, mitochondrial cytochrome c subunit 1 (COI), 16S ribosomal RNA (16S rRNA), and nuclear 28S ribosomal RNA (28S rRNA) gene sequences were generated and used for analysis of Automatic Barcode Gap Discovery (ABGD) and reconstruction of the phylogenetic tree. Morphological distinction and genetic analyses confirm that three Cadlina species are present in Korean waters of which Cadlina koreana is a new species.

An integrative approach to the systematics of the Berthella californica species complex (Heterobranchia: Pleurobranchidae).

Berthella californica (W. H. Dall, 1900) is a widespread species of heterobranch sea slug distributed across the North Pacific Ocean, from Korea and Japan to the Galapagos Islands. Two distinct morphotypes are observed in B. californica, which differ in external coloration, egg-mass morphology and geographic distribution (with the exception of a small range overlap in Southern California). Molecular and morphological data obtained in this study reveals that these two morphotypes constitute distinct species. The name B. californica (type locality: San Pedro, California) is retained for the southern morphotype, whereas the name Berthella chacei (J. Q. Burch, 1944) (type locality: Crescent City, California) is resurrected for the northern morphotype. Moreover, molecular phylogenetic analyses recovered B. californica as sister to Berthellina, in a well-supported clade separate from Berthella, suggesting that the classification of B. californica may need additional revision.

The complete mitochondrial genome of Aplysia kurodai (Anaspidea: Aplysiidae).

Complete mitochondrial genome is sequenced from an opisthobranch gastropod Aplysia kurodai. Mitochondrial genome size of the species is 14,113 bp. The mitochondrial genome of A. kurodai contains 13 protein coding genes, two ribosomal RNA genes, and 22 tRNA genes like mitochondrial genomes of congeneric species. The gene order of protein coding genes is identical to that of other Aplysia species. A+T content (65.9%) of the mitochondrial genome is included in the range for A+T content of congeneric species. This genome data provides evolutionary and systematic implications for the related species.

Phylogeography of the Rock Shell Thais clavigera (Mollusca): Evidence for Long-Distance Dispersal in the Northwestern Pacific.

The present-day genetic structure of a species reflects both historical demography and patterns of contemporary gene flow among populations. To precisely understand how these factors shape current population structure of the northwestern (NW) Pacific marine gastropod, Thais clavigera, we determined the partial nucleotide sequences of the mitochondrial COI gene for 602 individuals sampled from 29 localities spanning almost the whole distribution of T. clavigera in the NW Pacific Ocean (~3,700 km). Results from population genetic and demographic analyses (AMOVA, ΦST-statistics, haplotype networks, Tajima's D, Fu's FS, mismatch distribution, and Bayesian skyline plots) revealed a lack of genealogical branches or geographical clusters, and a high level of genetic (haplotype) diversity within each of studied population. Nevertheless, low but significant genetic structuring was detected among some geographical populations separated by the Changjiang River, suggesting the presence of geographical barriers to larval dispersal around this region. Several lines of evidence including significant negative Tajima's D and Fu's FS statistics values, the unimodally shaped mismatch distribution, and Bayesian skyline plots suggest a population expansion at marine isotope stage 11 (MIS 11; 400 ka), the longest and warmest interglacial interval during the Pleistocene epoch. The lack of genetic structure among the great majority of the NW Pacific T. clavigera populations may be attributable to high gene flow by current-driven long-distance dispersal of prolonged planktonic larval phase of this species.