Rivulus, The Ugly Ducking
by Norm Ruebsamen, WAKO

My first stint at keeping South American killifish was in the mid-seventies. At that time, the killies classified as Rivulus were affectionately known as brown worms by most hobbyists. When I returned to the hobby some twenty years later, the ugly duckling Rivulus had been transformed into a very diverse and colorful genus of killifish. With some 92 known species more are being discovered every year. Today there are more species of Rivulus than Aphyosemion.

Most species of Rivulus, as their name implies, inhabit rivulets and other shallow marginal habitats such as springs, swamps, roadside ditches, and have even been found occupying water-filled hoof prints left by cows that had come to the river to drink. A few species prefer brackish water and some can even tolerate full-strength seawater.

Rivs are seldom seen in nature unless one is looking among dense vegetation. Wet leaves are a common habitat. They feed strictly on live food (face it, they have teeth) and eat larvae and insects that fall into water. As hunters, they dart and sit awhile watching for prey, then move to a new spot and again sit very still. Rivulus will actually come out of the water onto the bank to get ants and other ground insects. That is why you will often find your Rivulus out of the water, laying on top of the plants or the cork of the mop. Although they are very sedentary by nature, they are capable of moving considerable distances over land as swamps and pools are reduced or disappear during the dry season. In the aquarium, they jump and stick to the sides of a glass tank or the underside of the lid. Rivs will easily flip their way up and out of a tank by sticking to the sides and then escaping through the smallest crack in the cover, making a tight fitting lid is a must.

Rivulus offer the killie hobby something for everyone; from the very easy to the very challenging. Good beginner fish that have color would be cryptocallus, cylandraceus, tenuis, derhami, magdalenae, and luelingi. Intermediate level Rivs would include: agilae, amphoreus, birkhahni, xiphidius, bahianus, and brasiliensis. The most challenging Rivulus that I have dealt with are: mahdia, paryagi, obscurus, and rectocaudatus. These fish will only breed in a small window of parameters of special water conditions and food. Most tend to be very male heavy.

Rivulus are plant spawners, but their eggs often take much longer to hatch than other plant spawners. Most species lay a few eggs a day over a period of several days, are fairly easy to reproduce, and need little space. Their eggs are large, and easy to find. But some eggs will depause even when water incubated. One species, Riv. marmoratus, is unique because it is a hermaphrodite. It is the only killifish that can internally produce and self fertilize its eggs. Most Rivulus are very peaceful and can be raise with other killies. Exceptions would be Rivs like gransabanae or breviceps which are very aggressive.

The next time you attend a show, check out the Rivulus. You will find that they have come a long way from the brown worms of the past.