| Summary: | Trabajo presentado en la International Conference & Exposition Aquaculture Europe, celebrada en Funchal, Maderia (Portugal) del 04 al 07 de octubre de 2021. [Introduction]: Turbellarian flatworms are controverted organisms with changing taxonomical adscription. Most of these platyhelminths are terrestrial and aquatic free-living organisms, but also include symbiotic species, and few cases of parasitic ones associated to fish, crustaceans and molluscs. Here, we report an epizootic due to a rhabdocoelan infection in cultured red drum (Sciaenops ocellatum) in a sea-cage farm in a tropical area. We describe the morphological, histological and molecular approaches for its identification. [Methods]: Two subsequent outbreaks in 2018-2019 affected red drum in the first year after entering the farm (weight ranging from 12g to 180 g). Water temperature ranged from 23 to 29.5°C and salinity was 36 ‰. Different types of samples of fish presenting acute mortalities and clinical signs were taken. Fresh smears of gills were observed at light microscope on site. Necropsied gills were fixed in 10% neutral buffered formalin (NBF), processed routinely, embedded in Technovit7100-resin, sectioned at 1 µm and stained with Giemsa, PAS, and alcian blue-PAS. Some NBF-fixed specimens were dehydrated, cleared with dimethyl phthalate and stained with iron acetic carmine. Another sample set was stored in 70% ethanol for molecular identification. The ribosomal genes 18S and 28S were PCR-amplified (Giribet et al., 1996; Jovelin & Justine, 2001) and used concatenated to infer a phylogeny by Bayesian inference. Other representatives of Rhabdocoela present in GenBank were included in the analyses. Bacterial samples were taken for basic bacteriological analysis. [Results]: Prevalence of infection was 100 % in some stocks, and mortality ranged from 5% to 60%. Clinical signs included anaemia, weight loss, pale and necrotic gills with mucous masses, desquamation and erosion of the skin, and asphyxiation. Bacteriological results were ...
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