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Four unarmored heterotrophic dinoflagellates were isolated from the coastal waters of southern Korea. The rDNA sequences of four clonal cultures were determined, and the morphology of one of the four strains was examined using light and scanning and transmission electron microscopy. The large subunit (LSU) and small subunit (SSU) rDNA sequences of each of the strains differed by 0-0.9% from those of the other strains, and the SSU rDNA sequence of the strain differed by 1.8-4.4% from those of other Gyrodinium species, whereas the LSU (D1-D2) rDNA sequence differed by 12.4-22.2%. Furthermore, phylogenetic trees showed that Gyrodinium jinhaense n. sp. formed a distinctive clade among the other Gyrodinium species. Meanwhile, microscopy revealed an elliptical bisected apical structure complex and a cingulum that was displaced by approximately one-quarter of the cell length, which confirmed that the dinoflagellate belonged to the genus Gyrodinium. However, the cell surface was ornamented with 16 longitudinal striations, both on the episome and hyposome, unlike other Gyrodinium species. Furthermore, the cells were observed to have pusule systems and trichocysts but lacked mucocysts. Based on morphology and molecular data, we consider this strain to be a new species in the genus Gyrodinium and thus, propose that it be assigned to the name G. jinhaense n. sp.

Polykrikos hartmannii is a phototrophic dinoflagellate with chloroplasts, and it swims as a single zooid or forms a two-zooid pseudo-colony. To investigate the feeding ability of P. hartmannii, its feeding occurrence, prey species, and feeding mechanism were explored. Furthermore, the growth and ingestion rates of P. hartmannii on the mixotrophic dinoflagellate Cochlodinium polykrikoides, the optimal prey, as a function of prey concentration were measured. This study reported for the first time that P. hartmannii is a mixotrophic dinoflagellate. When diverse algal species were provided as potential prey, P. hartmannii could feed only on chain-forming toxic mixotrophic dinoflagellates C. polykrikoides and Gymnodinium catenatum. P. hartmannii ingested cells of these prey by engulfment after anchoring a prey cell using a nematocyst–taeniocyst complex. With increasing mean prey concentration, the ingestion rate of P. hartmannii on C. polykrikoides increased, but reached saturation at a prey concentration of 945 ng C ml−1 (1350 cells ml−1). The maximum ingestion rate of P. hartmannii on C. polykrikoides was 1.9 ng C predator−1 d−1 (2.7 cells predator−1 d−1). However, the maximum mixotrophic growth rate of P. hartmannii with added C. polykrikoides cells (0.030 d−1), at 20 °C under a 14:10 h light-dark cycle of 100 μE m−2 s−1, was slightly greater than of those without added prey (0.023 d−1). The low maximum ingestion rate of P. hartmannii may be responsible for this small difference between the maximum mixotrophic and autotrophic growth rates. The calculated grazing coefficients for P. hartmannii on co-occurring C. polykrikoides were up to 0.324 d−1 (28% of the population of C. polykrikoides was removed by P. hartmannii populations in 1 d). The results of the present study suggest that P. hartmannii can have a considerable grazing impact on C. polykrikoides populations.

A heterotrophic dinoflagellate isolated from the waters of Masan Bay, Korea was, at first glance, morphologically similar to Katodinium glandulum, yet had unreported plate patterns and DNA sequences. This dinoflagellate had thin plates arranged in a Kofoidian series Po, X, 4′, 2a, 6″, 6c, PC, 3 + s, 5‴, 0p, and 2⁗, similar to species in the family Pfiesteriaceae. However, it did not have a closing plate (cp) and only small sulcal plates (3 + s). Furthermore, this dinoflagellate had a conspicuous apical hook (finger-like projection) on the epitheca, like K. glandulum and Katodinium asymmetricum, but unlike the other Katodinium species and the other species in the family Pfiesteriaceae. The epitheca had a conical shape and was markedly larger than the hemispherical hypotheca. Cell lengths and widths of live biflagellate cells satiated with the phototrophic dinoflagellate Heterocapsa triquetra were 21.2–29.2 μm and 18.1–24.9 μm, respectively, while those of biflagellate cells starved for 4 d were 10.2–17.3 μm and 7.7–14.6 μm, respectively. The ratio of the width of the epitheca relative to the hypotheca (0.9) is similar to that of K. glandulum, but larger than that of K. asymmetricum (0.5–0.7). The dinoflagellate had a peduncle but lacked an eyespot. The DNA sequences differed from those of K. glaucum, the only Katodinium species with reported rDNA sequences. Compared to the other known genera in the family Pfiesteriaceae, the rDNA sequences of the small subunit (SSU), ITS regions (ITS1, 5.8S, ITS2), and D1–D3 large subunit (LSU) differed by 1.9–6.8%, ≥4.4%, and ≥5.6%, respectively. In a phylogenetic tree based on the SSU and LSU rDNA, a clade containing this species belonged to a larger clade of the family Pfiesteriaceae, but was divergent from a clade containing K. glaucum. Due to its morphological similarity to K. glandulum and genetic differences from K. glaucum and the other species in the family Pfiesteriaceae, it is proposed that this species Aduncodinium glandula be assigned to be a new genus and a new combination, A. glandula.

Ostreopsis is an epiphytic and potentially toxic dinoflagellate. Here, we have provided the first report about the occurrence of Ostreopsis cf. ovata in Korean waters. In general, the morphology of the Korean strain of O. cf. ovata isolated from the waters off Jeju Island, Korea, was similar to the original description of O. ovata isolated from Japanese water. However, the cell length of the Korean strain was 20–30% smaller than that of the Japanese strain. In addition, the cingulum of the Korean strain was displaced and undulated in the ventral view, but that of the Japanese strain was not displaced. We have reported, for the first time, the presence of a line of small knobs on the Po plate. The plates of the O. cf. ovata Korean strain were arranged in a Kofoidian series of Po, 3′, 7″, 6c, 6s, V p , R p , 5‴, 1p, and 2⁗. When properly aligned, the small subunit (SSU) rDNA sequence of the O. cf. ovata Korean strain was 5.8% different to the O. cf. ovata Malaysian strain, which is the closest strain. In addition, compared to the south-east Asian, Pacific, Atlantic, and Mediterranean strains of Ostreopsis sp., O. ovata , and O. cf. ovata , the sequences of the internal transcribed spacer region (ITS1 and ITS2) plus 5.8S, D1–D3, and D8–D10 large subunit (LSU) rDNA of the O. cf. ovata Korean strain showed a difference of 3.6–16%, 15.8–19.7%, and 0–3%. Furthermore, in the phylogenetic trees based on D1–D3 LSU rDNA regions, the O. cf. ovata Korean strain was located in the basal position of the other O. ovata and O. cf. ovata clades. Therefore, we suggest that the O. cf. ovata Korean strain is genetically distinct among the O. ovata and O. cf. ovata strains reported to date.

The marine phototrophic dinoflagellate Gymnodinium smaydae n. sp. is described from cells prepared for light, scanning, and transmission electron microscopy. Also, sequences of the small (SSU) and large subunits (LSU) and the internal transcribed spacer region (ITS1-5.8S-ITS2) of ribosomal DNA were analyzed. This newly isolated dinoflagellate possessed nuclear chambers, nuclear fibrous connective, an apical groove running in a counterclockwise direction around the apex, and a major accessory pigment peridinin, which are four key features for the genus Gymnodinium. The epicone was conical with a round apex, while the hypocone was ellipsoid. Cells growing photosynthetically were 6.3-10.9 μm long and 5.1-10.0 μm wide, and therefore smaller than any other Gymnodinium species so far reported except Gymnodinium nanum. Cells were covered with polygonal amphiesmal vesicles arranged in 11 horizontal rows, and the vesicles were smaller than those of the other Gymnodinium species. This dinoflagellate had a sharp and elongated ventral ridge reaching half way down the hypocone, unlike other Gymnodinium species. Moreover, displacement of the cingulum was 0.4-0.6 × cell length while in other known Gymnodinium species it is less than 0.3 × cell length. In addition, the new species possessed a peduncle, permanent chloroplasts, pyrenoids, trichocysts, pusule systems, and small knobs along the apical furrow, but it lacked an eyespot, nematocysts, and body scales. The sequence of the SSU, ITS1-5.8S-ITS2, and LSU rDNA region differed by 1.5-3.8%, 6.0-17.4%, and 9.1-17.5%, respectively, from those of the most closely related species. The phylogenetic trees demonstrated that the new species belonged to the Gymnodinium clade at the base of a clade consisting of Gymnodinium acidotum, Gymnodinium dorsalisulcum, Gymnodinium eucyaneum, etc. Based on morphological and molecular data, we suggest that the taxon represents a new species, Gymnodinium smaydae n. sp.

To investigate heterotrophic protists grazing on Symbiodinium sp., we tested whether the common heterotrophic dinoflagellates Gyrodinium dominans, Gyrodinium moestrupii, Gyrodinium spirale, Oblea rotundata, Oxyrrhis marina, and Polykrikos kofoidii and the ciliates Balanion sp. and Parastrombidinopsis sp. preyed on the free-living dinoflagellate Symbiodinium sp. (clade E). We measured the growth and ingestion rates of O. marina and G. dominans on Symbiodinium sp. as a function of prey concentration. Furthermore, we compared the results to those obtained for other algal prey species. In addition, we measured the growth and ingestion rates of other predators at single prey concentrations at which these rates of O. marina and G. dominans were saturated. All predators tested in the present study, except Balanion sp., preyed on Symbiodinium sp. The specific growth rates of O. marina and G. dominans on Symbiodinium sp. increased rapidly with increasing mean prey concentration < ca. 740–815 ng C/ml (7,400–8,150 cells/ml), but became saturated at higher concentrations. The maximum growth rates of O. marina and G. dominans on Symbiodinium sp. (0.87 and 0.61/d) were much higher than those of G. moestrupii and P. kofoidii (0.11 and 0.04/d). Symbiodinium sp. did not support positive growth of G. spirale, O. rotundata, and Parastrombidinopsis sp. However, the maximum ingestion rates of P. kofoidii and Parastrombidinopsis sp. (6.7–10.0 ng C/predator/d) were much higher than those of O. marina and G. dominans on Symbiodinium sp. (1.9–2.1 ng C/predator/d). The results of the present study suggest that Symbiodinium sp. may increase or maintain the populations of some predators.