''Rivulus'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of small freshwater

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

in the

Cyprinodontiformes Cyprinodontiformes is an order (biology), order of Actinopterygii, ray-finned fish, comprising mostly small, freshwater fish. Many popular aquarium fish, such as killifish and Poeciliidae, live-bearers, are included. They are closely related t ...

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Rivulidae. 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 (including '' Anablepsoides, Atlantirivulus, Cynodonichthys'', '' Kryptolebias'', '' Laimosemion'' and '' Melanorivulus'') 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-known

mangrove rivulus The mangrove rivulus or mangrove killifish, ''Kryptolebias marmoratus'' (syn. ''Rivulus marmoratus''), is a species of killifish in the Family (biology), family Rivulidae. It lives in brackish and marine waters (less frequently in fresh water) a ...

(''Kryptolebias marmoratus''). 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.Huber, J.H., 2012. Reappraisal of the Phylogeny of Rivulus and its Allied Focused on External Characters. Killi-Data Series 2012: 9-25. If the split is recognized, the genus ''Rivulus'' only includes three non-annual killifish species from western

Cuba Cuba, officially the Republic of Cuba, is an island country, comprising the island of Cuba (largest island), Isla de la Juventud, and List of islands of Cuba, 4,195 islands, islets and cays surrounding the main island. It is located where the ...

(including

Isla de la Juventud Isla de la Juventud (; ) is the second-largest Cuban island (after Cuba's mainland) and the seventh-largest island in the West Indies (after mainland Cuba itself, Hispaniola, Jamaica, Puerto Rico, Trinidad, and Andros Island). The island was ...

) and

Hispaniola Hispaniola (, also ) is an island between Geography of Cuba, Cuba and Geography of Puerto Rico, Puerto Rico in the Greater Antilles of the Caribbean. Hispaniola is the most populous island in the West Indies, and the second-largest by List of C ...

where they inhabit streams, rivers and lagoons from the highlands to the coast. If the split into multiple genera is not recognized, ''Rivulus'' contains more than 150 non-annual killifish species found in fresh and

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

waters of the tropical and

subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones immediately to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Ge ...

Americas, ranging from Argentina in south to Mexico in north, including the Caribbean region.

Species

Three species are included in ''Rivulus'': * '' Rivulus berovidesi'' R. R. Silva, 2015 * '' Rivulus cylindraceus'' Poey, 1860 (Green rivulus) * '' Rivulus roloffi'' Roloff, 1938 (Hispaniolan rivulus) A fourth species, ''R. insulaepinorum'', has been recognized, but it is a

synonym A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...

of ''R. cylindraceus''. The two Cuban species are close relatives, but the position of the Hispaniolan ''R. 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." azurescens'', ''"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'' (most species) and '' Laimosemion'' (''"R." tomasi''), by them only considered subgenera of ''Rivulus''. This has resulted in the unusual situation where

FishBase FishBase is a global species database of fish species (specifically finfish). It is the largest and most extensively accessed online database on adult finfish on the web.

, despite recognizing the split into several genera, places the newly described species that belong in other genera in ''Rivulus''.

References

{{Authority control Rivulidae Freshwater fish genera Taxa named by Felipe Poey Fish of the Dominican Republic