Vol. 18: 181-185.1994 DISEASES OF AQUATIC ORGANISMS Dis. aquat. Org. 1 Published April 14 X-cell pseudotumors in a hardhead catfish Arius felis (Ariidae) from Lake Pontchartrain. Louisiana. USA Ariel Diamant*, John W. Fournie *, Lee A. Courtney U.S. Environmental Protection Agency. Center for Marine and Estuarine Disease Research. 1 Sabine Island Drive, Gulf Breeze, Florida 32561-5299. USA ABSTRACT: X-cell epidermal lesions are described from a single specimen of the hardhead catfish Anus felis (Ariidae). The lesions exhibited an unusual growth pattern but did not involve any visceral organs. Histologically, the lesions resembled those previously described for coldwater f~shes. This is the first report of X-cell lesions from Lake Pontchartrain, Louisiana, USA, and the hardhead catfish repre- sents the first warmwater species affected with the d~sease. KEY WORDS: X-cell . Arius felis . Hardhead catfish INTRODUCTION The nature and origin of X-cells are unknown. These ovoid, pale-staining cells with a large, round nucleus and prominent nucleolus are known to occur only in certain lesions, termed X-cell pseudotumors, reported only from marine teleosts. X-cells may produce large aggregations that superficially resemble true neo- plasms (Brooks et al. 1969, Harshbarger 1984), or less commonly, they may also occur in small, isolated clus- ters of individual cells (Diamant & McVicar 1987). They usually occur in such organs as slun, pseudo- branch, and gills (e.g. Peters et al. 1978, Morrison et al. 1982, Diamant & McVicar 1990), but have also been observed in renal, splenic and gonadal tissue (Diamant & McVicar 1987), and from an internal pseudotumor (Kent et al. 1988).X-cells have been studied with con- ventional histology, electron rnicroscopy, rnicrofluoro- metric DNA analysis, varous DNA-binding dyes and isozyrne analyses. It has been suggested that there is a correlation ' Present address: Nat~onal Center for Mariculture, Israel Oceanographic & Limnological Research Institute, PO Box 1212, Eilat 88112, Israel ' ' Addressee for correspondence between X-cell incidence and environmental pollution (Stich et al. 1976) or that X-cells are virally transformed fish cells (Peters et al. 1978, 1981). Alternatively, X- cells may be invasive unicellular parasites embedded in the fish host's tissue (Alpers et al. 1977, Dawe 1981). Although an increasing body of evidence supports the latter hypothesis, conclusive proof of the parasitic on- gin of X-cells has yet to be provided. In this paper, we report on the occurrence of an extensive epidermal X-cell lesion in hardhead catfish Arius felis from Lake Pontchartrain, Louisiana, USA. Thls record extends the condition to include a new tel- eost taxon (Order Siluriformes, Family Ariidae) as well as a new zoogeographic region. MATERIALS AND METHODS Fish were collected in eastern Lake Pontchartrain, using a custom-designed high rise 16 ft (ca 4.9 m) otter trawl with a chain sweep. Two trawls were made with tow durations of 10 min and a towing speed of 2 to 3 knots against the prevailing current. Following cap- ture, fish were counted and examined for evidence of gross pathological abnormalities. A total of 21 hard- head catfish were fixed in Dietrich's solution for histo- O Inter-Research 1994