Rheocles  (Rheocles vatosoa)

Classified as Vulnerable (VU) on the IUCN Red List (1).

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).

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).

A freshwater species, Rheocles vatosoa occupies clear waters of both rapidly flowing rivers and small brooks, over gravel or coarse sand bottoms (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).

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).

Although there are no known specific conservation measures currently in place for Rheocles vatosoa, the catchment of streams in the Lokoho basin is now protected by the Marojejy National Park (1).

View information on this species at the UNEP World Conservation Monitoring Centre.

 To find out about wildlife conservation in Madagascar see:

• Conservation International:
  http://www.conservation.org/explore/africa_madagascar/madagascar
• WWF:
  http://wwf.panda.org/who_we_are/wwf_offices/madagascar
• Wildlife Conservation Society:
  http://www.wcs.org/where-we-work/africa/madagascar.aspx