Belonging to the genus known commonly as the Madagascan silversides, Rheocles vatosoa is one of several small, attractively coloured fish inhabiting Madagascar’s waterways (1)(2)(3). It is unique among Rheocles species in that only a single bone (a pterygiophore) supports the dorsal fin, as opposed to the usual three (2). The scientific name of this species, vatosoa, is Malagasy for ‘crystal’ or ‘gemstone’ and refers to its jewel-like colouration (2)(3); both sexes share the same body coloration of pale brown on the back, shading to a creamy yellow on the sides, and silvery yellow to white on the underside (2). The male’s second dorsal fin, anal and tail fin are creamy yellow at the base, fading into burgundy with a black edge, whereas all the female’s fins are clear yellow with dusky edges (2). The male Rheocles vatosoa grows to a larger size than the female, and typically has a deeper body with longer dorsal and anal fins(2).
Very little information is available on the biology of Rheocles vatosoa. Its diet appears to be somewhat opportunistic, consisting of both terrestrial insects, likely to have fallen into the water from overhanging trees, and aquatic insect larvae(2).
Rheocles vatosoa begins to spawn in late October and early November and is believed to have an extended breeding season (2). Based on the reproductive behaviour of a closely-related species, Rheocles alaotrensis, along with an absence of aquatic plants in its habitat, it is thought that Rheocles vatosoa deposits its eggs in patches of coarse gravel (2).
Rheocles vatosoa is currently known only from the Lokoho River in north-eastern Madagascar, where it occurs in the upper and middle reaches of the main river channel in the general vicinity of the town of Andapa (1)(2)(3)(4). It typically occurs between 400 and 940 metres above sea level (2)(5).
According to local residents in the Andapa region, Rheocles vatosoa was formerly abundant throughout the Lokoho river basin; however, numbers have declined substantially over the past half century as native vegetation has been replaced with rice paddies, and this species is now restricted to streams that drain the slopes of the surrounding mountains, which still retain some forest cover (2). Continuing loss of habitat from deforestation and the presence of exotic predators and competitors, such as the eastern mosquitofish (Gambusia holbrooki), the green swordtail (Xiphpphorus helleri) and the spotted snakehead (Channa maculate), pose the principal threats to the long-term survival of Rheocles vatosoa(2)(3)(5)(6).
The fin on the under surface of a fish, behind the anus.
The area surrounding a stream or river from which it receives water, either from rainwater run-off or from tributaries.
The fin found on the back of the body of fish.
A category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
Stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Produce and deposit large quantities of eggs in water.
Stiassny, M.L.J., Rodriguez, D.M. and Loiselle, P.V. (2002) Rheocles vatosoa, a new species of freshwater rainbowfish (Atherinomorpha: Bedotiidae) from the Lokoho River basin in northeastern Madagascar. Cybium, 261: 71-77.
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