Morphology, phylogeny and seasonal dynamics of Prorocentrum species ( P. koreanum, P. gracile, P. micans and P. concavum ), in the coastal waters of Djibouti, western Gulf of Aden

To date, approximately 84 species of the genus Prorocentrum have been identified. Most are planktonic, but nearly 30 are associated with benthic habitats. Some of these species produce toxins, including okadaic acid (OA), dinophysistoxins (DTXs) and their derivatives, which can be responsible for diarrhoetic shellfish poisoning by molluscs. Some are known to cause harmful algal blooms leading to significant economic impacts on marine aquaculture, fisheries and tourism, as well as profound environmental and human health consequences. This study provides a detailed analysis of four Prorocentrum species collected from coastal waters of Djibouti in the Gulf of Aden (western Indian Ocean), combining morphological approaches using both light microscopy (LM) and scanning electron microscopy (SEM), with molecular analyses based on the sequences of the large subunit ribosomal DNA (LSU rDNA) and the internal transcribed spacer region (ITS1–5.8S–ITS2). The species studied comprise three planktonic species (P. koreanum, P. gracile and P. micans) and one benthic species (P. concavum). The Liquid Chromatography–Tandem Mass Spectrometry (LC-MS/MS) analysis, performed only on P. koreanum and P. gracile, shows no production of okadaic acid (OA) or dinophysistoxins (DTXs). An analysis of the seasonal dynamics of the three planktonic Prorocentrum species in the Gulf of Aden indicates their near-permanent presence, with minimal densities during winter and a summer peak reaching its maximum in June. This research is not only the first report of the genus Prorocentrum in the Gulf of Aden but also constitutes the first record of Prorocentrum koreanum in the region spanning the Red Sea, the Arabian Gulf and the Arabian Sea.