Spontaneous branchioblastoma in a koi carp

Gill neoplasia in fish is rare but has been reported in multiple elasmobranch and teleost species. Although more commonly a site of metastatic disease, primary neoplasms of the gill may occur, and both spontaneous and chemically induced tumors have been reported. Here we describe a spontaneous branchioblastoma in a koi carp (<i>Cyprinus rubrofuscus</i>) with no known history of chemical exposure. A soft-tissue mass on the inner surface of the dorsal opercular chamber appeared to originate from either a gill arch or a pseudobranch. Histologically, the mass was comprised of 3 well-differentiated cell types: blastemal cells, epithelial cells arranged in a lamellar fashion, and islands of cartilage resembling those present in gill filaments. To our knowledge, this is only the fourth case of spontaneous branchioblastoma recorded in koi.