Rivulus is one of the oldest genera of Cyprinodont fishes living in South- and Central America's. The type species is Rivulus cylindraceus Poey, 1860, found on Cuba.

Poey's description of the genus was short. Gunther (1866) reviews Rivulus in more detail. Garman (1895) also reviews Rivulus adding other features. Eigenmann (1909), Regan (1912), Meek & Hildebrand (1916), Myers (1927), Goline (1949) all did add some knowledge to the genus Rivulus. Then Hoedeman (1959) provides a detailed synthetic diagnose with complementary data on sensory organs and frontal scalation. Parenty (1981) proposed a ful review of all Cyprinodont fishes including Rivulus based primarily on osteological characters.

Parenti (1981) saw all smaller species as one genus, and proposed to separate the larger species into another group. But this proposal was based on examination of only the extremes within the genus Rivulus. There are many species that could not be easily assigned, because they are neither large or small. Huber (1992) proposed erecting a number of subgenera. Clade's sharing coloration and meristic characters or biogeographic limits were proposed to become a sub generic status.

Around the nineties of the last century we spoke about "species-groups" or "Super species"' because scientist were searching for absolute and unique features that clearly separate one lineage to the other. The fact is that there was a growing need to classify this more and more expanding and diverse group of fishes for better understanding of of their origin, interspecific relationship and history using key phylogenetic characters. It is believed the genus Rivulus forms a monophyletic assemblage.

Huber (1992) published his book "Review of Rivulus, Ecobiogeogarphy-relationships" and describes in it a new and, at that time, monotypic subgenus Anablepsoides and proposed further grouping of "Super species" based on key systematic characters. Huber describes the need for a new grouping of species into phylogenetic lines.

In the year the book was released it mentioned 110 valid species.

Today (April 2019) we have a total of 189 valid species in the genus Rivulus. Additionally we mention here some synonymized taxa just to inform visitors about their former existence and why their status has been changed. 

The Brazilian scientist Dr. Wilson Costa, very active in reordering/splitting genera in Cyprinodont fishes published a major work (2006) in which he proposes a the use of subgenera, based on the present morphological phylogeny and previous molecular
analyses. Seven subgenera are recognized which are: Rivulus, Cynodonichthys, Anablepsoides, Benirivulus n. subgenus., Laimosemion, Owiyeye n. subgen., and Melanorivulus n. subgen.

He further proposed in October 2011, to upgrade of most subgenera to generic status. These were: Anablepsoides (Huber 1992), Atlantirivulus, Cynodonichthys (Meek 1904), Laimosemion (Huber 1999) and Melanorivulus, Rivulus (Poey 1860). In the same publication he "merges" some known subgenera like Owiyeye, Benirivulus and Oditichthys in the new genera without proper explanation.

Following Costa Rivulus itself was separated from all the other new genera by having a hypural plate not divided by a gap (vs. divided by a gap) and caudal fin about 50% covered by scales (vs. only a few scales covering the caudal base)

After Costa's work a new paper was released by Huber (2012) in which Huber stated that the paper from Costa was not well supported by evidence and in which he proved Rivulus was based on 144 external characters was monophyletic.

Behavior:

Rivulus  in general are non-annuals, egg development takes place within 2-4 weeks. Eggs are attached to plants and roots at the edge of their habitat, not on/in the substrate.

Recent collections of species in the Rivulus tecminae species group from the Amazonas Territories in Southern Colombia, Venezuela and East Peru however provide evidence that some species can show annual behavior as well. Egg development periods of 5-6 months are reported.

Rivulus and Kryptolebias have one thing in common. Their ability to survive low water levels by jumping over land, seeking new places to live their lives. They use this ability also to find niches with a good source of food, and protection against predation. They also use it to get out raging current during rainfall by seeking save haven on the land. They mainly eat insects (such as ants and mosquito larvae) but also consume tadpoles that are placed by frogs in these isolated environments. To avoid competition they find niches, sometimes far from the creeks and swamps, even moving uphill in steep creeks or overland when moist, to places where no other fish can enter to eat them and/or compete for their food. If still in the main water-source, they seek the outer boundaries, between leaves and roots, to hide from predators. If a threat is sensed, they jump out of the water, and stay there until the danger has passed. Rivulus do produce relatively large eggs for the size they have and only few per day. They do not take care for their offspring other than live in places baby's have best chance to survive, even if the numbers of produced offspring are little and produced day after day during their adult life. When compared to fishfamilies that produce many hundreds of eggs in a single spawning the strategy works very well as we know.

This makes Rivulus the most effective regulator of malaria as mosquitoes lay their eggs on the surface of this type of standing water. Amerindian people use often lianas (vines which contain a poisonous liquid) to catch their food, by poisoning the fishes in the creeks. By doing so, they kill a great number of Rivulus (too small to be eaten by people), and endanger their own life, as malaria may take over their communities.

Literature:

The description of R. cylindraceus appeared in a major work of F. Poey, Cuba. Below the correct reference to that work which was brought out in partitions (Tomo's).

Poey, F., 1860 [1858-1861] Memorias sobra la historia natural de la Isla de Cuba, acompañadas de sumarios Latinos y extractos en Francés. Tomo 2. La Habana. Entrega secundo [= second delivery], including Poissons de Cuba. Espèces nouvelles: 115-336, pls. 10-12 and 14.

Costa, W.J.E.M., 2011. Phylogenetic position and taxonomic status of Anablepsoides, Atlantirivulus, Cynodonichthys, Laimosemion and Melanorivulus (Cyprinodontiformes: Rivulidae). *Ichthyological Exploration of Freshwaters* 22 (3): 233-249.

Huber J.H., 2012. Reappraisal of the Phylogeny of Rivulus and its Allied Focused on External Characters.

This moves with numerous namechanges appears then more preliminary (tree-naming driven and splitting-driven) with little added value to the nomenclature which promotes stability and reason.
This move will then not be followed by Killi-Data and this site, until strong and solid diagnoses are proposed which show that the split situation reflects better the philogeny than previously.
In practical terms, the names prevailing in Killi-Data until further evidence are the previous ones, with only 1 genus Rivulus and and several subgenera.

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