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• Ciguatoxin-like compounds are immunolocalized in Gambierdiscus australes cells. • 8H4 anti-CTX antibody confirms the presence of ciguatoxin-like compounds in G. australes. • Immunostaining localizes ciguatoxin-like compounds on thecal plates surface and inside cytoplasm. • Ciguatoxin-like compounds are present in small dots and reticulate tubule-like structures within cytoplasm. Dinoflagellates of the genera Gambierdiscus and Fukuyoa are able to produce potent neurotoxins like ciguatoxins (CTXs), which, after biooxidation in fish, are responsible for ciguatera intoxication. An isolate of G. australes from the Canary Islands, that revealed the presence of CTX-like compounds by immunosensing tools, was studied by immunocytochemistry to localize intracellular CTX-like compounds, using 8H4 monoclonal antibody that specifically recognizes the right wing of CTX1B and CTX3C analogues. Confocal microscopy observations of immunostained whole cells revealed a strong positive reaction on cell surface and all along the cell outline, while no reaction was detected inside the cells, probably because the antibody was not able to pass through thecal plates. Cell sections showed a positive antibody staining not only on thecal plates, but also inside cytoplasm, with numerous small dots and larger tubule-like reticulate structures. Small fluorescent dots were detected also on the nuclear surface. These observations indicate that CTX-like compounds are present in G. australes cytoplasm, and then are, at least in part, released to cover the cell surface. [Display omitted] [ABSTRACT FROM AUTHOR]