Snout chondroma in a neon tetra Paracheirodon innesi

An adult neon tetra Paracheirodon innesi with a history of difficulty in feeding caused by a mass overwhelming the snout is presented. Macroscopically the fish presented a unilateral, single, nodular and well-circumscribed, perlaceous, firm to hard mass in the left snout region. Histologically, tumor tissue was formed mainly by abundant, highly cellular elliptic nests of chondrocytes lying in mature hyaline cartilage matrix. Besides this component there were bubbly appearing large cells (physaliphorous-like cells) and a minority of cells having well defined cytoplasmic boundaries, clear eosinophilic pale cytoplasm and a rounded centrally located nucleus (“fried egg–like cells”). Alcian-PAS and Masson’s trichrome stains were applied to demonstrate proteoglycans and collagen of the matrix. Based on topographic localization and histomorphological features a diagnosis of snout chondroma was made. This case can be added to the other rare spontaneous cartilage tumors of fish described in literature.