Bull. Eur. Ass. Fish Pathol., 30(3) 2010, 114 * Corresponding author’s email: jmallg@unileon.es NOTE Mixed germ cell-stromal testicular neoplasia in a crucian carp ( Carassius carassius L.): a case report J. M. Fregeneda-Grandes, M. J. García-Iglesias, C. Pérez-Martínez and J. M. Aller-Gancedo * Departamento de Sanidad Animal, Facultad de Veterinaria, Universidad de León, 24071 León, Spain Abstract This report documents a case of spontaneously occurring gonadal neoplasm in a crucian carp (Carassius carassius L.) caught in the wild. On the basis of the histopathological findings, the neoplasm was diagnosed as a mixed germ cell-stromal testicular neoplasia (gonadoblastoma). Teleosts generally resemble other vertebrates in their basic susceptibility to developing neoplastic lesions. A large variety of neoplasms of various tissues have been recognized and described in a broad range of freshwater and marine species of fish (Hayes and Ferguson, 1989). The aetiology of tumours is generally complex and many of the factors that contribute to the initiation and growth of tumours remain unknown. Factors contributing to the formation of tumours in fish include viruses, chemical or biological compounds, physical agents, hormones, and the age, sex, genetic predisposition and immunological competence as predisposing factors of the host (Roberts, 1989). Tumours can arise in virtually any tissue of the body but they are more frequent in tissues such as skin, gills, liver and gut, where cell proliferation is normally active and may be exposed to noxious substances (Roberts, 1989). In contrast, neoplasms of reproductive tissues, especially germ-cell neoplasms (seminoma in males and dysgerminoma in females), are generally considered to be rare in fish (Masahito and Ishikawa, 1997). Cases of these neoplasms have been described in several fish species, such as African lungfish Protopterus annectens (Nigrelli and Jakowska, 1953), P. aethiopicus (Masahito et al., 1984) and P. dolloi (Hubbard and Fletcher, 1985), bagrid catfish Hemibagrus macropterus (Majedd and Wang, 1994), zebrafish Danio rerio (Smolowitz et al., 2002), barbel Barbus barbus (Palikova et al., 2007) and Japanese medaka Oryzias latipes (Hawkins et al., 1996). Non-germ gonadal cell neoplasms have also been described in fish including leiomyoma in yellow perch Perca flavescens (Budd et al., 1975) and large-mouth bass Micropterus salmoides (Herman and Landolt, 1975), testicular mesothelioma in blue shark Prionace glauca (Borucinska et al., 2003),