Azaspiracids are polyether marine algal toxins produced by several species of dinoflagellates from the family Amphidomataceae. Within the genus Amphidoma, Am. languida is the only species known so far to produce azaspiracids, while all the other toxin producers belong to the genus Azadinium. Strains of Am. languida collected in the Northeastern Atlantic have been found to produce AZA-38 and -39 as major metabolites. Cultures of Am. languida (24,000–30,000 cells mL⁻¹) fed to mussels (Mytilus edulis), confirmed the bioaccumulation of AZA-38 and -39 in shellfish tissues. AZA-38 and -39 were found to reach a combined 75.2 μg kg⁻¹ (AZA-1 equivalents) in mussel tissue. The tentative identification of new derivatives resulting from the biotransformation of AZA-38 and -39 in the shellfish tissue was performed by LC-HRMS/MS. Although toxin concentrations in the tissue never reached AZA-1 regulatory limits, the study demonstrates that toxins from Am. languida can readily bioaccumulate and biotransform in shellfish and the toxicity of AZA-38 and -39 and their products of biotransformation should now be assessed. Importantly, a snapshot of biotoxin data from the Irish monitoring program in 2020 also identified AZA-38 and -39 in some shellfish species, albeit at low levels, from locations around the Southwest coast of Ireland.