One of the oldest genera in South American Cyprinodont fishes. In the first description from F. Poey he described the species Rivulus cylindraceus from Cuba. Till the redefinition of the whole group of species that was assigned to Rivulus at the end of 2011 by Costa Rivulus members can be found from Florida in North America to far south on the S.A. continent in Argentina. Till now, only in Chile they seemed not present. At that time the genus Rivulus was grown to a total of about 145 species.

In their shape, their body size and their behaviour as well they show huge differences and by several occasions scientists did try to reorganize this group of species that contains over 145 accepted names in the mean time.( Oct. 2011)

Parenti for example saw all smaller species as one genus and proposed to separate the larger species into an own group. This failed because she had studied only the extremes within the genus Rivulus. There are however many species that could not easely be assigned to the larger- or smaller ones just because they are neither large or small.

Huber found another way by erecting a number of sub- genera. These genera names are containing species with overall shared colouration and other meristic characters. In fact this is the same as giving a clade a subgenus status. In former times we spoke about "superspecies" or "species- groups". The fact is that there was a need to classify this huge divers group of fishes. This was implemented by Costa from Brazil in October 2011 in a paper in which he upgraded some species groups to the genus status and added genera as well. These were named Anablepsoides, Atlantirivulus, Cynodonichthys, Laimosemion, and Melanorivulus. Costa only left Rivulus cylindraceus and R. insulaepinorum, both from Cuba, in the genus Rivulus and was not able to confirm if R. roloffi, from the caribbean island Hispanola, was also one of the remaining species in the genus Rivulus.

Rivulus is separated from all the new genera by having a hypural plate not divided by a gap (vs divided by a gap) and caudal fin covered for about 50 % by scales (vs only a few scales covering the caudal base)

Behaviour:

Rivulus are non- annuals, eggdevelopment takes place mostly within 2 to 3 weeks. They lay their eggs not in the bottom like annual fishes do but attach them between plants and roots at the edge of their habitat.

Rivulus and the related genera have one thing in common. That is their ability to survive low waterlevels by jumping over land seeking new places to live. They use this ability also to find niches that bring a good source of food and provides protection against predation. They mainly eat insects like ants and mosquito larvae but also like the tadpoles that are placed by frogs in there isolated environments. To avoid competition they find niches, sometimes far from the creeks and swamps in the neighbourhood, even hillside- up, where no other fish can enter to eat them or there food. If still in the main water- source they seek the outer boundaries between leaves and roots to escape from predators. If predation occurs, they jump out of the water and stay there till danger is over. This makes Rivulus to the most effective regulators of malaria outbreaks as the mosquito's lay their eggs in this type of standing waters. Amerindian people use often lianas that contain a poisonous liquid to catch their food easy by poisoning the fishes in the creeks. By doing so, they are killing also a great number of "Rivulus", to small to serve as food, and bring there own life in danger as malaria take over in their communities.