Care
The Spotted Onesided Livebearer is known from the clear, sub-tropical lowland tributaries of the Rio Uruguai. This unusual, viviparous fish is a member of the Family Anablepidae, which also includes the Four-eyed fishes (Anableps spp.) and is only seldomly seen in the trade. Jenynsia species are what's known as "onesided" livebearers: many experts asserting that they only mate on one side, yet recently, other sources have disputed this. It is believed that the male is restricted by only being able to direct his gonopodium to one particular side, and the female can only ever be fertilised from one side due to a modification of the genital opening. So it was thought that, for example, that "left handed" males could only mate with "right handed" females; however, it appears that left handed males can go "head-to-tail" with a left handed female and still fertilise her (and vice versa for right handed fish). The aquarium should be mature and well-filtered, with plenty of shady areas provided amongst hardy aquatic vegetation (both rooted and floating) and decor such as spindly driftwood pieces and rocky caves. The fish may nibble on soft-leaved plants, although will not usually cause too much damage if a diet with a vegetable component is offered. This is a fairly peaceful species that is best kept in a species tank environment, with 2 or more females to every male; this will reduce the amount of attention that any one female receives from the often extremely amorous males and will also further your chances of raising more fry. A shoal of at least 8-10 fish is preferable in a good sized tank, and although there may be a bit of minor chasing between rival males, they are rarely aggressive towards one another and generally do not cause damage. Tankmates are possible, but do ensure these are short-finned, fast-swimming, and are happy in the same cool conditions - although be aware that in such a community setting, the J. onca fry may be at risk from predation. Be sure to keep the temperature cool as these fish do not fare well above 24 °C (75 deg F); a means of chilling the water may be necessary during the warmer summer months. Filtration should be efficient and the water well-oxygenated, with partial water changes carried out on a very regular basis in order to keep nitrogenous wastes to a minimum. All members of the genus Jenynsia (of which there are currently 15 recognised species) are diagnosable by the possession of an unscaled tubular gonopodium formed chiefly by the third, sixth, and seventh anal-fin rays, and by the possession of tricuspid teeth in the outer mandibular series in adults. There is also a lack of segmentation on both the proximal and distal quarters of the sixth anal-fin ray of adult males, and vertically inclined proximal radials associated with the first six anal-fin rays of adult males. J. onca is distinguished from its congeners by the following autapomorphies: (1) presence of a large convex expansion in the distal segments of the right half of sixth anal fin-ray of adult males; and (2) distinct ovoid to circular dark spots confined to ventral half of flank posterior to pelvic fin, which gradually become more circular towards the ventral profile. Interestingly, the specific name "onca" is in allusion to the colour pattern, which resembles that of the jaguar (Pantera onca).
Feeding
Flake, green flake, micropellets, small meaty frozen foods e.g. bloodworm, white mosquito larvae, daphnia, vitamin-enriched brineshrimp, plus some vegetable matter such as blanched spinach, lettuce, kale etc. These fish have a fast metabolism and should be offered small amounts of food 2 or 3 times per day; however, be sure to employ efficient filtration and perform partial water changes very regularly to keep nitrate as low as possible.
Breeding
J. onca are livebearers, which means that the females give birth to live young. Well conditioned fish should reproduce quite readily under favourable conditions. As detailed above, these fish are either "right handed" or "left handed", the female only able to accept the male's gonopodium from one side, and the male only able to direct his gonopodium left or right. Males tend to be very amorous a lot of the time, so try to ensure you have 2 or more females to every male. The smaller male in the above photo continually tried to mate with the larger female whilst in the photo aquarium - he being left handed, approaching this right handed female continually from her right hand side. After fertilisation occurs, the female gestates for 6 weeks, after which time she will give birth to between 10 and 20 free-swimming fry. The adults (and other fish species in the aquarium) may predate on the fry if not separated. However, a good number will normally manage to survive onto adulthood when kept in a well planted aquarium with lots of natural hiding places.
