Abstract
Chronic aeromoniasis in turbot (Scophthalmus maximus), caused by Aeromonas salmonicida, provokes cutaneous lesions that occasionally expose dermal tubercles, which may act as unconventional ecological niches. A storm in February 2024 caused severe damage at a turbot farm in Galicia (Northwestern Spain), leaving various tanks uncovered and subsequently exposing fish with chronic aeromoniasis to several days of intense solar irradiation. Afterward, farm staff observed green tufts over the animals. Two affected turbots were submitted for necropsy. Morphological features identified the epibiont as the alga Ulva spp., covered by a biofilm containing A. salmonicida DNA (detected by real-time polymerase chain reaction). The algae did not trigger additional host responses beyond those induced by aeromoniasis, indicating true epibiosis rather than infection. However, algal colonization physically obstructs re-epithelialization and may prolong lesion chronicity, while the associated biofilm could serve as an environmental reservoir for the pathogen. This case illustrates how environmental disruptions can generate unexpected host-pathogen-epibiont interactions. To our knowledge, this is the first report of Ulva epibiosis on fish.
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