Re‐isolation of <i>Gonyaulax hyalina</i> from the type locality in the Gulf of Aden: Integrated analysis of morphological, molecular, and toxicological characteristics

This study provides a comprehensive investigation of Gonyaulax hyalina, integrating morphological, phylogenetic, and toxicological approaches. Strains were re-isolated from its type locality in the Gulf of Aden (western Indian Ocean) 124 years after its original description by Ostenfeld and Schmidt (1901, p. 141), along with additional Pacific strains collected from Korea and Viet Nam. The primary objective was to clarify the taxonomy of G. hyalina, which has frequently been confused with the morphologically similar G. fragilis. Morphological and molecular data confirmed that the Indo-Pacific strains examined belong to G. hyalina. Diagnostic morphological features supporting this identification included the presence of a characteristic surface ornamentation, a distinct anterior intercalary plate 1a, and comparatively smaller cell dimensions. Phylogenetic analyses based on LSU and SSU rRNA gene sequences revealed that these strains formed a distinct clade, separate from existing G. hyalina sequences in GenBank. This phylogenetic analyses indicated the presence of two ribotypes (A and B) within the species, although morphological characters do not reflect this, providing evidence of cryptic speciation in G. hyalina. Ribotype B corresponded to strains previously associated with gelatinous mucilage aggregates, known from the Mediterranean and New Zealand coastal waters. Toxicological assays conducted on strain DJ_I3 from the Gulf of Aden showed no detectable toxin production. Lastly, analysis of seasonal dynamics in the Gulf of Aden demonstrated that G. hyalina is present throughout the year, with abundance peaking in summer and declining markedly during winter months.