Caligus minimus Otto, 1821 (Copepoda: Caligidae): A commercially important but poorly described parasite of cultured European Sea Bass, Dicentrarchus labrax (Linnaeus, 1758)

Caligus minimus Otto, 1821 has been known for over two centuries and it is the second oldest of the approximately 275 species of Caligus O. F. Müller, 1985. Despite the numerous records of this species from European waters, it has never been fully described to modern standards. The lack of a comprehensive modern description has resulted in numerous misidentifications, even in recently published reports, and this is especially problematic for a species that is known to have a significant economic impact in aquaculture. This study presents a detailed description of both sexes and documents newly observed features of C. minimus collected from the buccal cavity of farmed European Sea Bass (ESB), Dicentrarchus labrax (Linnaeus, 1758). The morphology of C. minimus was examined using light microscope (LM), scanning electron microscope (SEM), and confocal laser scanning microscope (CLSM), and new details are revealed regarding the structure and ornamentation of the marginal membrane of the cephalothorax, maxilliped, antenna, sternal furca, abdomen, and legs 1, 3, 4, and 6. The ornamentation of the marginal membrane of the cephalothorax is unique and its impact on the functioning of the cephalothoracic sucker requires further investigation. Additionally, partial COI gene region sequences were obtained from four individuals of C. minimus and provided for future references. A phylogenetic analysis was conducted in conjunction with Caligus sequences available in the NCBI GenBank database. [Display omitted] •Caligus minimus (Caligidae) causes high mortalities on cultured European sea bass.•Since its discovery, C. minimus has never been fully described to modern standards.•Previous poor descriptions are likely a key cause of recent misidentifications.•.Re-examination of C. minimus using CLSM and SEM revealed new morphological details.•New COI gene region sequences provided for future reference studies.