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An attempt was made to infer the phylogeny of Pseudo-nitzschia species by using the mitochondrial-encoded gene, cytochrome c oxidase subunit I (cox1), and comparing it with the nuclear-encoded large subunit ribosomal DNA (LSU rDNA). A pair of primers targeting Pseudo-nitzschia cox1 was designed in silico and used to infer the molecular phylogeny of Pseudo-nitzschia. The primer pair was tested using genomic DNAs isolated from six species of Pseudo-nitzschia from Malaysia. The phylogenetic inference of cox1 was then compared to the LSU rDNA phylogeny. Phylogenetic reconstructions of both data sets revealed monophyly of Pseudo-nitzschia species complexes. The range of genetic divergences among Pseudo-nitzschia species were higher in the cox1 data set (3.5–20.4 %) compared to the LSU rDNA data set (0.1–8.8 %). The present study suggests that high genetic divergence in cox1 of Pseudo-nitzschia species could be a useful genetic marker for DNA bar coding.

Paradilepis scolecina (Rudolphi, 1819) Hsü, 1935 is a tapeworm widely distributed across the globe that uses piscivorous birds as definitive hosts. Although various piscivorous birds inhabit the Republic of Korea (Korea), this species has not previously been documented in the country. In this study, we report its occurrence in resident great cormorants, Phalacrocorax carbo sinensis (Staunton, 1796), based on integrated morphological, molecular, and ultrastructural analyses. The recovered specimens exhibited 20 rostellar hooks of parvoid type arranged in a double row consistent with the scolecina pattern, allowing distinction from other congeners. Partial LSU sequences of the specimens in this study showed the highest identity to P. scolecina isolates from South Africa and Lithuania (GenBank accession no. MH062156 & PQ570010, 100% identity), while SSU sequences were closest to Paradilepis sp. references (MH699816, 98.1%). Combined BLAST and phylogenetic analyses supported their identification as P. scolecina. Notably, scanning electron microscopy revealed a previously undescribed cirrus surface featuring rose-thorn–shaped spines interspersed with capilliform filitriches—representing the first such ultrastructural description within the family Gryporhynchidae. The shift of Ph. c. sinensis increasing tendency toward year-round residency in Korea may enhance parasite transmission by increasing contact with local freshwater hosts, a hypothesis that warrants further ecological investigation. Additionally, the presence of P. scolecina may have implications for wild fish populations and nearby aquaculture facilities, particularly if intermediate or paratenic hosts include economically important species. These findings contribute to the growing knowledge of the parasite fauna of cormorants in Korea and underscore the need for further research into the transmission dynamics and host range of trophically transmitted cestodes in East Asian freshwater ecosystems.

Highlights •First study investigating ciguatera-causing organisms in Seychelles, southwestern Indian Ocean.•Gambierdiscus belizeanus identified using combined morphological and molecular approaches.•Isolation of a G. belizeanus strain exhibiting low ciguatoxin-like activity.•Evidence of intraspecific variability in toxin production among G. belizeanus strains from the same locality.

Although the taxonomy of oligotrich ciliates has been widely investigated, yet the species diversity remains poorly known. We newly designed a pair of oligotrich-specific LSU rDNA primers covering the 600-bp D1/D2 region, and it was effective for detecting oligotrich species. Using the primers, we constructed the cloning libraries to investigate the species diversity of oligotrichs in the northern coastal waters of the South China Sea. In total, 165 oligotrich sequences were obtained from five widely separated sampling sites. Sixty operational taxonomic units (OTUs) were obtained at 99% similarity threshold, and low-abundance OTUs with no more than two sequences contributed most of these (about 78%). Our findings are consistent with previous morphological studies, Strombidium was found the most abundant and widely distributed genus in this area. In addition, the BLAST search in the NCBI database resulted in 95% OTUs matching with named oligotrich species in similarity below 99%. Therefore, oligotrich morphospecies diversity has been underestimated as low-abundance species, and the LSU rDNA oligotrich sequence database needs to be better promoted.

Large subunit ribosomal DNA (LSU rDNA) sequences have been increasingly used to infer the phylogeny and species identity of organisms, a few previous studies, however, have observed high intraspecific and even intraindividual variability in LSU rDNA in some dinoflagellate species due to, assumably, large copy numbers of rDNA in dinoflagellates. Since the copy number of LSU rDNA varies tremendously among dinoflagellate species, the intraspecific and intraindividual diversity for a species of particular interest thus needs to be investigated individually. As a toxic and HABs-forming dinoflagellate, Margalefidinium (= Cochlodinium) fulvescens has been observed to approach blooming density in Jiaozhou Bay, China since 2015 after numerous blooms having been reported from other countries. In trying to identify the source of this newly observed HABs-forming species in China by sequencing the LSU rDNA for both field samples and clonal cultures, we noticed and thus further investigated high intrapopulational and intraindividual genetic diversities of the dinoflagellate. The D1–D6 region of the LSU rDNA (1,435 bases) was amplified from 7 field samples (pooled cells) and 11 clonal cultures, cloned, sequenced, and analyzed phylogenetically for 2,341 sequences obtained. All the numbers of sequences obtained from each clonal culture were far less than the estimated rDNA copy number in M. fulvescens. In the clone library, only one unique sequence was contained in all samples as the most dominant sequence. We found high intrapopulational and intraindividual genetic diversity in M. fulvescens as reflected in the number of polymorphic sites and unique sequences in the clone library for different field samples and clonal cultures in comparison to other species. The mean number of nucleotide differences of each sequence from different field samples and clonal cultures were 6.43 and 4.42 bases, respectively, with the highest being 132 bases, nearly 10%. The sequences with highest variability may be easily annotated as different species if they were obtained from environmental genomic studies because sequence-based species identification in meta-barcoding studies often use "97% identity" threshold. Based on that the mean and overall intrapopulational genetic diversity calculated for 7 field samples was equivalent to the mean and overall intraindividual variability for 11 clonal cultures in indices of genetic diversity, together with the result of AMOVA analysis, we infer that the variability within individual cells (i.e. variability among LSU rDNA polymorphic copies) caused both the intraindividual and intrapopulational genetic diversities observed in the M. fulvescens population, and a higher interpopulational diversity may exist among different geographic populations. The results provide an insightful basis for such a comprehensive interpopulational comparison and important implications for identifying species and establishing new taxa based on the similarity comparison to reference sequences deposited in databases.