1) Killifish 
Rivulus is a genus of freshwater fish in the Rivulidae family of Cyprinodontiformes. Formerly the largest genus in its family, work, mostly by Wilson J. E. Costa, has split this genus into several new genera (including Anablepsoides, Atlantirivulus, Cynodonichthys, and Laimosemion) leaving only a few species left in Rivulus. Some of these species ...
[n] - found in small streams of tropical America
noun found in small streams of tropical America; often kept in aquariums; usually hermaphroditic