Sturgeon species are well-suited for aquaculture because of their favourable characteristics, including robustness, suitability for farming in facilities unsuitable for other fish species, and adaptability to diverse farming conditions. The Russian sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeburg 1833) is one of the most prominent farmed species
however, like other aquaculture species, it is susceptible to significant losses from bacterial and viral diseases. Beyond infectious causes, there are few reports documenting conditions that produce cutaneous masses in Russian sturgeons. This study presents a multidisciplinary investigation of six farmed Russian sturgeons exhibiting discrete, multiple cutaneous masses. Bacteriological analysis of tissue samples revealed the presence of Morganella morganii and Aeromonas veronii biovar sobria, identified as opportunistic bacteria. Virological assays targeting the principal viruses affecting sturgeon, Acipenser iridovirus and Acipenser herpesvirus, yielded negative results. Ultrastructural analysis with direct negative staining revealed no evidence of biological agents. Histologically, the dermal masses were well-demarcated, expansile, and moderately cellular, consisting of spindle-to-stellate neoplastic cells that were multifocally periodic acid-Schiff-positive and embedded in abundant alcianophilic ground substance. Immunohistochemistry with the S-100 antibody confirmed cytoplasmic staining of the neoplastic cells. A final diagnosis of cutaneous nerve sheath tumour with myxoid differentiation was made, replicating findings from a similar tumour in rainbow trout. To the best of our knowledge, this represents the first description of multiple cutaneous nerve sheath tumours in sturgeon species. The potential factors contributing to the development of this neoplastic condition are discussed.