Jenynsia onca
Striking are the black elongated markings on the body. Jenynsia onca owes its name to this characteristic. This is derived from the Portuguese “onça”, a local name for the jaguar. The body is elongated and the eyes are relatively large. The ventral side is lighter in color and may have an orange-yellow sheen. Lucinda et al (2002) give a maximum length of 4 cm for the females and indicate that the males remain one cm shorter. However, experience shows that the species gets bigger. The females can grow up to 7 cm. The males remain significantly smaller. The appearance of the males and females is identical. Pregnant females are slightly plumper in build. Males are distinguished from females by their anal fin (gonopodium) that has grown into a sexual organ. Despite the simple drawing, Jenynsia onca is an attractive species.
Distribution area
The species is found in the Río Ibicuí river basin in Brazil and the Río Negro river basin in Uruguay.
Care and breeding
There are not many experiences with keeping and caring for this species because it has not been kept in the hobby for very long. In 2006 G. Rickling from Germany gave me some fish of this species and our experiences form the basis of this article. The species can be cared for broadly as a goodeid. The temperature does not have to be kept constant all year round. A cooler period of around 16ºC seems advisable. During the warmer period, the temperature can rise to 23ºC. A higher temperature is not good for these fish. Regularly changing a large part of the water is necessary. All the food offered is eaten, at a higher temperature these fish are hearty eaters.
Breeding
Breeding Jenynsia onca is not that easy. Occasionally a few young are born that measure 2 cm at birth. I managed to get the fry out of the aquarium with the parent fish in time to raise them separately for the first few months. Then they can be put back with their larger counterparts. There may be more young left if the females are separated. The maximum number I could get from the plants now was 4. It is difficult to judge whether the females are about to give birth, they hardly gain weight during the gestation period. Also very rarely young are born. Possibly reproduction is similar to Jenynsia multidentata (Mai et al, 2007).
Research into a population from the Patos lagoon in Brazil shows that large females give birth twice between September and May. The females born during the first litter also reproduce within this period. The average number of young per litter in this species is 30. As expected, larger females give birth to more young than smaller ones. I have never encountered so many youngsters at J. onca. Mai et al also indicate that in nature J. multilineata has more females than males. They have no explanation for this.
Maintaining J. onca in the aquarium has not been very easy so far. Besides the fact that few young are born, the fish do not seem to get very old. Occasionally specimens get a kind of sores and then languish. Hopefully in the future it will be possible to multiply this species considerably. It is an attractive fish that deserves a larger audience.
Source: leo-aquarium.blogspot.nl
Author
Kees de Jong
Copyright images
Leo van der Meer
