First observation of a <i>Pyrodinium bahamense</i> bloom in the Gulf of Aden, Djibouti: morpho-molecular characterisation and spatio-temporal abundance

At the end of the summer of 2023, in September, an intense water discolouration (red tide) accompanied by fish mortality was observed in the Gulf of Tadjoura. This discolouration of the seawater was caused by a potentially toxic dinoflagellate, <i>Pyrodinium bahamense</i>. The proliferation, located near the coast between the Moucha/Maskali islands and the city of Tadjoura, was significant enough to be detected by satellite imagery. <i>P. bahamense</i> had formed an almost monospecific bloom (90%), with a density of 1.8 × 10⁶ cells l^‒1 (22 September 2023). Other dinoflagellates such as <i>Dinophysis miles, Dinophysis caudata, Tripos furca, Lingulaulax</i> sp. (formerly <i>Lingulodinium</i>), <i>Balechina</i> sp. and <i>Protoperidinium</i> sp. were detected at very low concentrations. This study characterises <i>P. bahamense</i> through an integrated approach, combining detailed morphological analysis with molecular phylogenetic analyses based on the internal transcribed spacer (ITS) region and the D1–D3 regions of large subunit ribosomal (LSU rDNA). For the first time, we have molecularly identified the toxic ribotype from the Indo-Pacific in the Gulf of Aden region. This discovery highlights an emerging risk of this toxic ribotype in the eastern Mediterranean basin. This region, already facing various environmental challenges, may see its vulnerability heightened with the emergence of new risks. A red tide was formed in September 2023 by <i>Pyrodinium bahamense</i>.First morpho-molecular identification of <i>P. bahamense</i> in the Gulf of Aden.The genotype corresponded to the toxic ribotype and the Indo-Pacific ribotype. A red tide was formed in September 2023 by <i>Pyrodinium bahamense</i>. First morpho-molecular identification of <i>P. bahamense</i> in the Gulf of Aden. The genotype corresponded to the toxic ribotype and the Indo-Pacific ribotype.