By Donna M. Recktenwalt
A number of different aquarists who have kept Rivulus species have noticed that many of these fish seem to actually like being out of the water – even when there is no apparent reason for them to be.
Some Rivulus species (and some tetras, as well, particularly Copeina arnoldi) are known to spawn out of the water. In the aquarium, this spawning behavior results in placement of the eggs on the aquarium glass or some equivalent surface above the water line.
Collectors have noted that in the wild rivulins usually inhabit the shallow fringes of pools, and have often observed the fish lying in the wet leaves or mud above the water line. Whether this is a predatory or a defensive technique is unknown, but conjecture indicates that it may be a hunting adaptation. [Ed. note: Since many rivulins live in small, low oxygen content pools, could this behavior also provide a source of additional oxygen, with the fish “respirating” through their skin, like amphibians?]
Jaap Vlaming, in “Killie Nieuws”, wrote that in Cameroon he observed A. cameronense jumping out of the water and onto the riverbanks, and A. splendopleure jumping onto floating pieces of wood, both for no apparent reasons. When he entered the water near the A. cameronense, they all jumped back into the water – all together, and with none jumping in the wrong direction. This would seem to indicate that they do this often, and that their eyesight is very good both in and out of the water.
Other aquarists have observed similar behavior in captive fishes. Cal Him says that he has observed a Pterolebias xiphophorous that stuck to the side of an aquarium for a period of up to 10 minutes. When the fish saw him, it jumped back in, and seemed to suffer no ill effects. Another aquarist had some R. milesi in a tank with an exposed rock, where he often found them lying quietly.
Working with this apparent preference, Glen Collier drapes a long spawning mop across the top of a 2″ square of styrofoam, which he then floats in the breeding tank for Rivulus deltaphilus and other rivulins he keeps. Regularly, he finds eggs in the part of the mop above the water line, and sometimes one or both of the spawning fish lying there as well.
Little direct study has been done on this phenomenon of spawning behavior, but it can be assumed that by placing their eggs in the moist moss or algae at the water’s edge, as do some of the rain forest frogs, fish that spawn in this manner assure their eggs of greater oxygen supplies, adequate moisture, and lower levels of predation during development.
— G.C.K.A. Newsletter, March 1997