Catatropis johnstoni is a fluke from the United States. It was first described in 1956 by Martin, who had found cercariae (a larval stage of a fluke) released by the snail Cerithidea californica in southwestern California. When the cercaria were fed into chickens (Gallus gallus domesticus), they developed into mature worms; Martin speculated that the natural host was a waterbird. In 1970, a study of helminths of the marsh rice rat (Oryzomys palustris) in a saltmarsh at Cedar Key, Florida, found flukes similar to C. johnstoni. The fluke was present in 30% of 110 examined rice rats, with the number of worms per rat varying from 1 to 500 (average 91). Some Cerithidea scalariformis snails from this marsh also released cercariae similar to C. johnstoni from California. When introduced into chic