Twospot astyanax
(Astyanax bimaculatus)
Image source: Jo's Animal Database
General data
The twospot astyanax, also called the two-spot tetra, is a small species of freshwater fish native to South America.
Astyanax bimaculatus is omnivorous, eating plants, zooplankton, and various forms of detritus.
Astyanax bimaculatus has an enormous distribution. The rivers and basins it can be found in include, but are not limited to: the Amazonas estuaries and the main channel, Apure, Araguaia, Atrato, Beni-Madre de Dios, Branco, Cauca-Magdalena-Sinu, Coppename, Suriname, Saramacca, Corentyne-Demerara, Essequibo, Guapore, Itapicuru-Mearim, lapura, middle-lower Madeira, Mamore, Maroni-Approuague, Oiapoque, Orinoco (upper and lower), Parnaiba, Purus, Putumayo, upper Tocantins, Ucayali, and upper Xingu.
Astyanax bimaculatus reaches a maximum total length of 17.5 cm (6.9 in), and a maximum weight of 90 g (3.23 oz).
The scales are large, and lack distinct radii originating from a single point in the center of each; the presence of such lines was a trait once ascribed to A. bimaculatus, but has since been recognized as a trait of congener A. abramis instead.
There are 33 lateral-line scales, 26–27 dorsal-fin rays, and 26–30 anal-fin rays.
Before taxonomic revisions, the reported variation in the numbers of fin-rays and lateral-line scales was much greater, due to separate species all being grouped under a single name.
The back is dark-green, the sides a silver-yellow, and there is a clear silver lateral stripe. The base of the dorsal fin and the upper lobe of the caudal fin may be strongly red, pink, or yellow, with reduced coloration in the rest of the fin. There is a strong black humeral spot, and another strong blotch of pigment at the base of the caudal fin that extends onto the middle caudal-fin rays; these two spots are shared amongst all members of the A. bimaculatus species complex, which is often considered one of the defining characteristics therein, even if a small handful of species do not conform, including A. elachylepis and A. varzeae. Markings also characteristic of this complex include two brown vertical bars in the humeral region.
