Alternative name: Rivulus(Oditichthys) cajariensis.

Rivulus cajariensis was originally described as Rivulus cajariensis and was collected at single locality near the village of Cajari, Amapá, northern Brazil. Specimens of the type series were found in pools and shallow channels, about 5-30 cm deep, of a flooded area, about 30 m wide, adjacent to the right bank of a clear water stream, which was about 1-4 m wide. The stream is part of the upper course of the Cajari River, which directly empties in the main course of the lower Amazonas River. The whole aquatic habitat was within dense rain forest.

The name cajariensis referring to the
occurrence of the new species in the Cajari River drainage.

The species is close related with Rivulus igneus known from French Guiana and neighbouring Surinam.

Side of body purplish gray, with dark reddish brown dots. Side of head purplish gray, to golden on lower part of opercle and infraorbital region; dark greenish blue blotch on upper portion of opercle. Lower jaw orange. Dorsum brown. Venter yellow. Iris brown. Dorsal
fin pale yellow, with small dark brown spots on whole fin except distal region. Anal fin orangish yellow, with small dark reddish brown spots on basal portion; black stripe on distal margin. Caudal fin pale yellow with reddish brown dots; lower marginal region reddish orange, with narrow black margin. Pectoral fin reddish orange, basal portion yellow. Pelvic fin reddish orange. Females. Side of body light brown, with dark reddish brown dots. Side of head purplish gray, to pale greenish golden on lower part of opercle and infraorbital region; dark green blotch on upper portion of opercle. Lower jaw orange. Dorsum brown. Venter yellow. Iris brown. Dorsal fin pale yellow, with small dark brown spots on whole fin except distal region. Anal fin pale yellow, with small dark brown spots on basal portion. Caudal fin pale yellow to orangish yellow on borders; brown dots on middle of fin; round black blotch on upper portion of caudal-fin base. Pectoral fin yellowish hyaline. Pelvic fin pale yellow.

Keeping and breeding is easy but a tight fitting cover on the aquarium is necessary because their ability to jump, even through the smallest opening, is unbelievable.

This species can, if there is enough space for them, be housed in a tank with more than one male and several females. If separated before and brought together again however, they can be very aggressive towards each other and fight to become the dominant fish. The best way to produce numbers of fry is to put a pair or a trio in a small tank of about 10 litres with a small filter in it or with some aeration. On the bottom of the tank place some peat moss or dark gravel and a floating "mop" to give space for laying their eggs. The fish will, if they are fed a rich variety of live food, spawn during their whole adult life and produce daily between 5 to 15 eggs.

The eggs should be collected by hand and stored for about 14 days in a small container. Eggs are 1.6 mm and amber coloured. It is wise to add an anti fungus to the water you store the eggs in, to prevent fungal attacks.

After hatching the young fry can eat fresh artemia nauplii without any problem. It takes 4 to 5 months to raise them to maturity. Often the males far out number females. For some species this can become a major problem for the breeder, as he has to raise many fry to enable distribution of a few pairs to other hobbyists. Recent personal tests found that lower pH levels during the time of hatching gave a higher percentage of females. To achieve this I used some peat moss that I added to the container with the eggs just before they were ready to hatch. This peat moss also is a good way to prevent eggs from being attacked by fungus.

Their lifespan can be up to 3 years in captivity.

Remarks :

This species is not in the hobby yet. All data above in this chapter are general for Rivulus.

Max. size 5.7 cm.
Dorsal 10-11,
Anal16-17,
D/A 0.0,
LL scale count (average) 39
Pre- dorsal length to % SL –78.4 %
Depth to % SL – 19.4 %