Strains of epiphytic and toxic dinoflagellates were collected from the seawaters surrounding Ulleungdo and Dokdo islands in Korea, and their morphology, molecular phylogeny, and toxicity were analyzed. Each dinoflagellate strain was examined under a microscope for genus-level identification, while species-level confirmation was achieved for Amphidinium operculatum, Ostreopsis sp. (type 1), Protoceratium reticulatum, Coolia canariensis, and C. malayensis through molecular phylogenetic analysis. The genera Gambierdiscus, Heterocapsa, and Prorocentrum were identified based on morphological traits, with Gambierdiscus characterized by a round-flattened shape with distinct thecal plates, Heterocapsa exhibiting a fusiform to oval shape, and Prorocentrum showing an oblong-to-ovate shape. Toxicity assessments for five species involved exposing Artemia salina nauplii to concentration-dependent extracts of the dinoflagellates. At a concentration of 100 ppm, P. reticulatum, A. operculatum, C. canariensis, and Ostreopsis sp. resulted in significant mortality among the nauplii, with survival rates dropping to as low as 0% over a 24-hour period. These findings underscore the potential ecological and toxicological impacts of these species and highlight the necessity for further research to evaluate their behavior under varying environmental conditions. This study marks the first documentation of subtropical epiphytic dinoflagellates in the waters around Ulleungdo and Dokdo islands, encompassing twelve strains from eight epiphytic species. Additionally, we investigated the toxicity of five species, including a toxic planktonic dinoflagellate. The results suggest a potential northeastward shift in their distribution, likely originating from Jeju Island, and being transported by the Tsushima Warm Current through the East Sea. This shift has enabled the successful establishment of populations in these areas under favorable conditions influenced by changes in the oceanic climate.

Abstract
1. INTRODUCTION
2. MATERIALS AND METHODS
    2.1. Sampling and strain setup
    2.2. Microscopy
    2.3. DNA extraction, PCR, and sequencing
    2.4. Phylogenetic analysis
    2.5. Preparation of extracts of dinoflagellatesand toxicity assessment
3. RESULTS
    3.1. Isolated epiphytic and toxic dinoflagellate
    3.2. Phylogeny
    3.3. Toxicity
4. DISCUSSION
5. CONCLUSION
CRediT authorship contribution statement
Declaration of Competing Interest
ACKNOWLEDGEMENTS
REFERENCES

• Seung Joo Moon(Environment and Resource Convergence Center, Advanced Institute of Convergence Technology, Suwon 16229, Republic of Korea)
• Hangy Lee(Environment and Resource Convergence Center, Advanced Institute of Convergence Technology, Suwon 16229, Republic of Korea)
• Jun-Ho Hyung(Environment and Resource Convergence Center, Advanced Institute of Convergence Technology, Suwon 16229, Republic of Korea) Corresponding author
• Jaeyeon Park(Environment and Resource Convergence Center, Advanced Institute of Convergence Technology, Suwon 16229, Republic of Korea) Corresponding author