Redefining the algae Ostreopsis siamensis Schmidt (1901) after more than a century

Ostreopsis siamensis was discovered by Danish scientist Johannes Schmidt in the southern part of Koh Chang, Gulf of Thailand, in 1901. Eighty years later, Fukuyo (1981) described the species found in the Ryukyu Islands (Japan). Many years later, scientists around the world have identified about 11 species in the genus Ostreopsis. The species have almost the same phenotype and exactly the same number of shell plates, so the classification must be based on morphological and genetic features. The problem of species identification becomes difficult, especially since there are many works to identify and describe the species attributed to O. cf. siamensis or Ostreopsis sp. 6 accompanied by genetic analysis and phylogenetic pedigree. The descriptions are often based on a phenotype obtained naturally or in culture with abnormal changes in cell that do not match the description by Schmidt (1901). Several questions are raised: “Has O. siamensis changed its morphology after more than 100 years with environmental changes and global warming; Is this species real or does it not exist in the marine environment?” Because scientists have not found standard specimens in contemporary museums, and this has created a confusing and controversial picture in taxonomy.

Map of specimen collection areas and distribution of Ostreopsis siamensis (red rings). From A to K, pictures of O. siamensis species in Phu Quoc. For a detailed explanation of the image, see: Nguyen-Ngoc et al. 2021;

From 2012 until now, after about 10 years of conducting basic research with funding from the National Foundation for Science and Technology Development (NAFOSTED), Prof. Nguyen Ngoc Lam and colleagues at the Institute of Oceanography (Vietnam Academy of Science and Technology) have made many survey trips to collect coastal samples from the north to the south, from the coastal islands. offshore Vietnam, and during missions in Cambodia, Malaysia, Philippines and Thailand. Hundreds of isolates have been analyzed by leading molecular biologists at home and abroad, and as a result, the species O. siamensis is nowhere to be found. The group has also called for the cooperation of scientists from Malaysia and Germany, but they were all rejected for many reasons ("my group is studying this genus", "studying this genus is too difficult", or "that is a nightmare for us…”).

Figures A, B, E, F and G are the epitypes from Phu Quoc, which are Ostreopsis siamensis from upper shell (A), lower shell (B), longitudinal groove detail with plates small (E), the shell surface has 2 types of large and small holes (F and G) and large holes with substructure (G). Figures C, D, and H show that the cells obtained in Nha Trang Bay have a different shape from the cells in Phu Quoc, but the shell structure, shell surface, and large pores are not different between the two sampling regions.
See detailed explanation in Nguyen-Ngoc et al, 2021;

In the summer of 2018, the research team invited Prof. Jacob Larsen and Dr. Nina Lundholm from the Danish Museum of Natural History to participate in the survey of Con Dao and jointly analyzed DNA by the "single cell" method in the Plankton room. The first rays of light on O. siamensis began to appear, specimens collected from Con Dao, along the coast of My Hoa (Phan Rang, Ninh Thuan), Nha Trang Bay and Ghenh Rang (Quy Nhon) have been described detailed, showing complete agreement with the original description of Schmidt (1901). In parallel, molecular analyzes also identified samples with the same genotype. However, to be accurate and reliable, specimens must be found in the area where the species was first discovered. Through 5 rounds of specimen collection in Phu Quoc (from 2015 to June 2020), the results were not as expected, and no specimens were found likely to be O. siamensis species. However, in the second survey in July 2020, about 10 cells were isolated and kept in culture, some cells were isolated in situ for DNA analysis, the rest were fixed. The results showed that specimens from Phu Quoc waters were identified to share the same morphological and genetic characteristics with those from Con Dao, My Hoa, Nha Trang and Quy Nhon. Although there is a slight variation in cell shape under natural conditions and some morphological variation under culture conditions, all of them are O. siamensis by phylogenetic analysis. With the above results, “Those were anxious moments, waiting for the sequencing results, and then all burst into indescribable joy,” said Prof. Nguyen Ngoc Lam.

Phylogenetic tree of Ostreopsis species based on ITS(A) and LSU(B) gene sequences

GS. Nguyen Ngoc Lam added: "After a long time, the efforts and results of the research team were recorded, specifically the specimen from Phu Quoc was recognized as an "epitype" (sub-standard sample) put into archives with registration number VNPQ218 at the Botanical Museum of the Institute of Tropical Biology, sample E.56950 at the Oceanographic Museum and model C-A-69691 at the Natural History Museum of Denmark. The genetic information of the species has been registered in the world gene bank through gene sequences that have been granted with codes MT953907, MT953908, MT953909, MT953910, MT953911, MT953912, MT953913, MT953914, MT953915, MT953916, MT953917, MT953918 (LSU) and MT968507, MT968508, MT968509, MT968510, MT968511, MT968512, MT968513 (ITS).”

Information on the Vietnamese genus Ostreopsis and the re-identification of O. siamensis has been published in the prestigious ISI journal, specialized in Algaeology (Journal of Phycology), epdf/10.1111/jpy.13157.

Translated by Phuong Ha
Link to Vietnamese version

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