Phylogenetic and Autecology Characteristics of Five Potentially Harmful Dinoflagellate Alexandrium Species (Dinophyceae, Gonyaulacales, Pyrocystaceae) in Tropical Waters: A. affine, A. fraterculus, A. leei, A. pseudogonyaulax, and A. tamiyavanichii

Five species of <i>Alexandrium</i> (<i>A. affine</i>, <i>A</i>. <i>fraterculus</i>, <i>A</i>. <i>leei</i>, <i>A</i>. <i>pseudogonyaulax,</i> and <i>A</i>. <i>tamiyavanichii</i>) are commonly found in Vietnamese waters. They were distinguished based on their apical pore complex (A.P.C), precingular first plate (1'), ventral pore (Vp), and sulcal platelets. A genetic analysis was conducted using nuclear rDNA sequences of ITS and LSU (D1-D3, D8-D10). The growth rates of <i>A</i>. <i>fraterculus</i>, <i>A</i>. <i>leei</i>, <i>A</i>. <i>tamiyavanichii</i>, and <i>A</i>. <i>pseudogonyaulax</i> were quite similar. Specifically, these four species had the highest growth rates at two temperature levels of 24 °C and 27 °C, at salinities ranging from 25 psu to 35 psu. Furthermore, these species were able to adapt to a low salinity of 20 psu at temperatures from 18 °C to 27 °C. No Paralytic Shellfish Toxins (PSTs) were found in the two <i>Alexandrium affine</i> strains, VINVN01-1 and VINVN01-2. The detection limit for PSTs ranged from 0.45 to 15.5 fg cell^-1, depending on the molecular response and available biomass.