Rivulus atratus is a member of the subgenus Anablepsoides.

This species was initially described as Rivulus atratus, later placed in the newly erected subgenus Anablepsoides by Huber in 1992, and designated as the type species of the initially monotypic subgenus Anablepsoides. Costa described some species that would also fit this Anablepsoides subspecies, which are: Rivulus amanan, R. atratus, R. gamae, R. henschelae, R. ornatus (former obscurus), and R. ottonii.

In 2011 Costa upgraded the subgenus Anablepsoides and gave it the status of a genus. He assigned a 42  taxa, preliminary in Rivulus, to this newly erected genus. This move is not followed herein because of unstable diagnoses.

Rivulus atratus is rarely seen in the hobby but commercially exported from Peru, so now and then in small numbers to Europe and the USA. Rivulus atratus was discovered by W. James et al. in September 1865 in the River Jutahy area, Amazonas, Brazil.

Atypical to all other species, even in its group, this fish cannot maintain buoyancy and is always found at the surface of the waters they live in. They always float at the water surface. These waters are very shallow, but still, it is a strange way of behavior for a Rivulid fish species.

The species belongs to the smallest species with a maximum length of 4 cm, and this species does not show bright coloration. Nevertheless, even if they do not show colors other than silver and black, they are very popular with Killi hobbyists as it is a challenge to breed them successfully. Although they have been imported several times, it seems complicated to maintain the strains for a more extended period.

Males have a mixed pattern of dark bars and blotches in the caudal fin that are not visible in females. Fin- and body coloration shows little differences between both sexes.

Juveniles have in the earliest stage a black band just behind the eye that covers the front half of the body

Keeping and breeding are not easy, and an excellent fitting cover to the aquarium is necessary because their ability to jump, even though the smallest opening, is unbelievable.  

If there is enough space for them, this species can be housed in a tank with more than one male and several females. If separated before and brought together again, they can be very aggressive towards each other and fight to become the dominant male. The best way to breed good numbers of offspring is to put a pair or a trio in a small tank from 10 liters with a small filter in it or with some aeration. Place some peat moss or dark gravel on the bottom and a floating "mop" to give space for laying their eggs. If they are fed a wide variety of life food, the fish will spawn during their whole adult life and produce between 3 to 5 eggs daily.  

The breeder can choose to keep the fry in the tank with parents when providing much space to hide at the surface. The eggs can also be collected by hand and stored until they hatch (about 17 days) in a small container. Eggs are 1.6 mm and amber-colored. It is wise to add some anti-fungus medication to the water. This medication (Tripavlavine or Acriflavine) prevents unfertilized eggs from spreading fungus, quickly infecting other eggs in the container.

After hatching, the fry can eat freshly born Artemia nauplii but need other smaller food such as infusorians, micro eel, or other small foods. It takes 5 to 6 months to raise them to maturity. It takes the right eye to see the difference between the sexes of not fully grown juveniles. Often the number of males is far above the number of females. This species can become a significant problem for the breeder as he has to raise many young fishes to enable him to distribute a few pairs to other hobbyists. Recent personal tests learned that lower pH levels during hatching gave a higher percentage of females. To achieve this, I used some peat moss to add to the container with the eggs before they were ready to hatch. This peat moss also is an excellent way to prevent eggs from being attacked by fungus. Their lifespan can be up to 2 years in captivity.

Remarks :

Rivulus (Anablepsoides) atratus is for experienced breeders only, and even they are not always successful. Eggs are produced in low numbers and only by younger fishes. The older they become, the lower the egg production. Fry should have much infusoria for food, and the best thing to do is breed them with a lot of floating plants in the tank.

Max. size 4.0 cm.
Dorsal 9.0,
Anal 9.0,
D/A 12,
LL scale count (average)31.0
Pre- dorsal length to % SL – 80.1 %
Depth to % SL – 15.0 %

Garman, S.W. 1895. The Cyprinodonts. Mem. Mus. Comp. Zool. Harvard, 19 (1): 140.

Huber, J.H. 1992. Review of Rivulus. Ecobiogeography - Relationships. Cybium Suppl., Société Française d'Ichtyologie Publ.: 43.