Written by Dan Frase
The Genus Rivulus contains some 65 species living in South and Central America, on various Caribbean Islands, and in brackish coastal waters in Florida. Included in the group is an annual species, R. stellifer, and two auto reproductive hermaphrodites, R. Marmora us and R. ocellatus, that are capable of fertilizing their own eggs. The remainder of the group are probably all non-annual killies whose eggs are best incubated in water.
The members of the genus Rivulus may not be as brightly colored as some of their African relatives, but they are still an interesting group, with many species that exhibit strange and unusual patterns of aerial behavior. Among these are the habit of traveling overland by flipping, the ability of R. stagnates to climb waterfalls by flipping and sticking, the habit of basking out of the water when given the opportunity, the habit of egg stranding (laying eggs above the surface of the water on damp vegetation or algae), and the habit of aestivating (going through a period of dormancy during droughts) out of the water (Fromm).
Rivulus travel overland for several reasons. The adaptation of terrestrial flipping allows them to forage for insects, to move down watercourses that are drying up by flipping from pool to pool, or, in the case of R. stagnatus at least, to move up a watercourse (Fromm, 4). There is no evidence, however; that they travel between drainages overland.
Another testament to their jumping abilities is that the type specimen of R. heyei collected itself by jumping into a pail of water that had been left at streamside overnight (Fromm, 19).
The sight of a Rivulus basking out of water can be slightly unnerving the first time you observe it, but it is normal behavior. They have been known to bask on the leaves of floating plants or on top of floating corks (Fromm, 19). I’ve seen R. agilae D 5/18 stick to the bottom of the glass cover on their tank for long periods, and watched R. bondi basking on top or their mop. There is evidence that R. maramoratus is actually capable of “breathing” air through capillaries close to their dorsal skin surface (Specht, 177). Perhaps other Rivulus species can also breathe air in this way and basking is simply a way for them to breathe.
Many Rivulus species prefer to spawn as close to the water surface as possible. Occasionally they spawn above the water on damp vegetation or on mats of floating algae in nature, or on their mops in an aquarium. John Chapek said that he once found Rivulus eggs on the bottom of a glass cover. This behavior is call egg stranding and may serve as protection from predation for the eggs.
There are four Rivulus species that are known to aestivate out of the water during periods or drought. Rivulus chucunaque and R. hartii have been found aestivating under logs and in leaf litter (Fromm, 19). Large numbers of R. maramoratus have been found aestivating together in crab holes along the western coast of Florida which is unusual because R. maramoratus is usually carnivorous toward other members of its species.
Apart from Rivulus’ unusual aeiral adaptations, the genus contains two species that are unique among vertebrates, as well as being the only euryhaline (capable of living in both fresh and salt water) members of the genus. Rivulus maramoratus is from southern Florida, the tip of the Yucatan Peninsula, many of the Caribbean Islands, the northern Venezuela, and Rivulus ocellatus is from the east coast of South America (Spect, 179). These killies are the only auto reproductive hermaphrodites among vertebrates and can breed in fresh or salt water. Each female is a breeding population since they are capable of fertilizing their own eggs to produce perfect clones of themselves. Males of both species do exist and can be produced by incubating the eggs below 68 degrees Fahrenheit (Fromm, 28), but the males are not able to reproduce with the females.
When keeping Rivulus in home aquaria water conditions are not particularly important except when keeping R. xiphidius which prefers soft, acid (below 6.0 pH) water to breed in.
Rivulus species will eat almost anything offered to them, although fry need to be started on newly hatched brine shrimp. Rivulus bondi has even been known to eat paste food made from ground goat guts and oatmeal (Thomason & Taphorn, 108).
Although Rivulus can be kept and bed in small containers, tanks with a two and a half capacity or larger should be used to house and breed smaller species, and larger species such as R. harti, which reaches a total length of six inches (Fromm, 31), should be kept in five gallon tanks or larger.
Since Rivulus are skilled jumpers and can exit through the smallest of openings, their tanks should be well covered with small openings taped up or otherwise covered to prevent their escape.
Most Rivulus are shy fish and need hiding places in aquaria. Usually a large top mop is sufficient for them to hide in.
Breeding of most of the available Rivulus species is not difficult. Most will try to lay their eggs as close to the water surface as possible in top mops, but some have been known to breed in bottom mops or even in gravel. I had a pair of R. milesi that refused to use top and bottom mops, having a preference for breeding in the gravel substrate.
The eggs of all Rivulus are large and, subsequently, so are the fry. All Rivulus fry can take newly hatched brine shrimp from the first day. The fry grow moderately fast, usually maturing between four and eight months of age.
Not all Rivulus species are easy to breed, however. In fact, some species such as R. rectocxaudatus, R. glaucus, R. zygonectes, and R. brunneus have probably never been bred. With other species such as R. uroflammeus and R. xiphidius egg production of one to two eggs a day is considered good (Fromm, 33).
- Huber, “A Short Note on a Recent Collection of Rivulus Species (Cyprinodontidae; Pisces) from the Brazilian Coastal Plains”. Journal of the American Killifish Association, Inc. 19:1, 1986.
- Fromm, Dan. “Rivulus as Live Animals”. Journal of the American Killifish Association, Inc. 19:1, 1986.
- Spect, Harry O. “Rivulus maramoratus”. Journal of the American Killifish Association, Inc. 22:5, 1989.
- Thomerson, Jim & Taphorn, Don. “The Caracas Killifish: Rivulus bondi”. Freshwater and Marine Aquarium Magazine, 10 Sept. 1987.