Rivulus is a genus of small freshwater fish in the CyprinodontiformesfamilyRivulidae. It was traditionally considered to be the largest genus in its family; however, the genus's size is currently in dispute. Wilson J. E. Costa split this genus into several new genera in 2004 and 2011, leaving only a few Greater Antillean species in Rivulus itself. Despite being moved to other genera, some of the species retain the common name "rivulus", like the well-knownmangrove rivulus. Shortly after the review by Costa, another review authored by J.H. Huber refuted the split, moving the proposed genera back in Rivulus and again making the genus the largest in the family Aplocheilidae. If the split is recognized, the genus Rivulus only includes three non-annual killifish species from western Cuba and Hispaniola where they inhabit streams, rivers and lagoons from the highlands to the coast. If the split into multiple genera is not recognized, Rivuluscontains more than 150 non-annual killifish species found in fresh and brackish waters of the tropical and subtropicalAmericas, ranging from Argentina in south to Mexico in north, including the Caribbean region.
A fourth species, R. insulaepinorum, has been recognized, but it is a synonym of R. cylindraceus. The two Cuban species are close relatives, but the position of the HispaniolanR. roloffi has not been confirmed and its placement in this genus is preliminary. Because of the disputed validity of the split into several genera, there is a level of confusion about the species included in Rivulus, despite it being clearly defined in 2011. For example, "R." azureus, "R." pivijay, "R." ribesrubrum, "R." tomasi and "R." xi were described in 2013 by authors that did not recognize the split of Rivulus, but noted that they were part of Cynodonichthys and Laimosemion, by them only considered subgenera of Rivulus. This has resulted in the unusual situation where FishBase, despite recognizing the split into several genera, places the newly described species that belong in other genera in Rivulus. This is repeated by Catalog of Fishes, which also recognizes the split into several genera, but includes the newly described "R." azureus, "R." pivijay, "R." ribesrubrum and "R." xi, despite them being part of Cynodonichthys according to their description.
