What the heck are silversides from Madagascar, you might ask. Well, these small sleek fishes belong to the genus Rheocles and are found, for the most part, in cooler faster flowing waters around Madagascar’s northern territories. However, just to make things difficult, Paul Loiselle, Ph.D., recently discovered a new species of Rheocles in the far South near Ft. Dauphin. There are currently seven species recognized in this genus, which was first described in 1891. Killifish of Madagascar>>
Rheocles species belong to the family Bedotiidae and are closely related to the Madagascar rainbowfishes. Many of the differences that separate the genera Rheocles and Bedotia are skeletal and are difficult for the average hobbyist to detect. Externally, the presence of a Bedotia notch on the premaxillae (upper jaw) of Malagasy rainbowfish helps to readily distinguish Bedotia from Rheocles in all known species. Cichlids of Madagascar>>
Malagasy silversides live in waters ranging from 5.0 to 8.5 pH, low hardness (less than 3 dKH) and cool temperatures usually lower than 73 degrees Fahrenheit. Analysis of gut contents shows that, like Pachypanchax and Bedotia species, Rheocles species are somewhat opportunistic feeders that prey on both aquatic insects and terrestrial insects that fall into the water.
Rheocles species grow to an average total length of 3 inches with larger individuals reaching close to 3¾ inches. Two species — R. vatosoa and R. derhami — show constant sexual dichromatism while the other five species are monochromatic outside the reproductive season.
Basics of Silverside Husbandry
Not much is known about the captive husbandry of Rheocles species, which is both bad news and good news. This is bad news because not only are Rheocles species hard to get hold of, but the little that we know about them shows that they can be finicky when it comes to captive care.
Rheocles alaotrensis, R. vatosoa and R. wrightae have all been kept in aquaria for varying lengths of time. The Lake Alaotra silverside (R. alaotrensis) was bred at the Denver Zoo for four generations before populations stopped breeding and began showing severe signs of physical malformation. During that time, we found that although our R. alaotrensis were collected in soft acidic waters with an average temperature of 66 degrees, they reproduced quite readily in our tap water (pH of 7.2, 8 dGH, 3 dKH) with temperatures ranging from 75 to 82 degrees. We were never able to determine what caused the malformations and die off of our populations; hypotheses included inbreeding depression and mercury poisoning of the founders.
Filtration was easily supplied with undergravel filter plates covered in small-diameter gravel and supplemented with air-driven sponge filters. This filtration was sufficient for 15 specimens in a 50-gallon aquarium. I see no reason to forego canister or hang-on filters if that is your preference, and Rheocles species might appreciate the increased flow rate.
Aquarium décor and lighting should attempt to mimic forested streams, so save the compact fluorescents and bubble curtains for other species. Spawning mops or live plants can provide appropriate cover and subdued lighting is best for these somewhat skittish fishes.
Are you ready for some husbandry challenges? We experienced two problems when dealing with Rheocles species in Denver. The first challenge involves feeding. Rheocles species will grow and reproduce on a diet of quality flake food, bloodworms, blackworms and adult brine shrimp. Once food begins its inevitable descent into the water column, however, Rheocles species lose interest. Heaven forbid any food hit the substrate, for it is certainly beneath these fishes to forage around like common cichlids. When keeping the Malagasy silversides, therefore, feed often and in small amounts to keep uneaten food from fouling the water.
The second challenge with Rheocles species is disease. Like the Malagasy cichlids, Malagasy silversides are highly susceptible to ich (Ichthyophthirius multfiliis) infection and it can be deadly if not caught in time. Treatment with an over-the-counter ich medication (malachite and formalin-based, if possible) coupled with increased temperatures between 78 and 82 degrees does the trick.
More worrisome is the Rheocles susceptibility to a bloatlike condition, at least in wild-caught specimens. The founder population I worked with in Denver developed bloat after one year in captivity. Abdominal swelling with sudden listlessness defied both water parameters adjustment and pharmaceutical use and eventually caused 100-percent mortality in the population. Subsequent generations seem less prone to bloat and no definitive cause was ever identified.
Silverside Captive Reproduction
Rheocles alaotrensis males undergo a dramatic color change when coming into breeding condition, with unpaired fins becoming a deep red with the main body a dark, sooty black. I would hazard a guess that in the four other species that are normally monochromatic (R. wrightae, R. pellegrini, R. sikorae and R. lateralis), a distinct change in coloration is associated with spawning.
Males defend a small territory that is typically centered around a landmark, such as a rock or collection of mops. They display rapid up-down swimming motions when ripe females approach. Rheocles species lay small sticky eggs on the aquarium substrate. Unlike Bedotia species that typically lay their eggs in tight clusters, Rheocles species lay their eggs singly and prefer a firm substrate to spawning mops.
Eggs hatch in six to eight days at temperatures near 80 degrees Fahrenheit. Rheocles fry are small and will have trouble consuming brine shrimp nauplii as their first food so have an infusoria culture or some vinegar eels for starting foods. Keep in mind that Rheocles fry are extremely fragile for the first few months of life, and attempting to move young ones to a rearing aquarium often leaves you with nothing to rear. It is best to move the parents and leave the fry where they hatch.
Want to read the full story? Pick up the November 2009 issue of Freshwater And Marine Aquarium.