Astyanax abramis is a small freshwater fish from the inland rivers of South America. It is plentiful in almost the entire continent, with a habitat including the La Plata, upper Amazon, and Meta River basins. The body shape is subrhomboidal (somewhat rhomboidal) and laterally compressed, and the scales are largely silver, with a blue or green tint on the dorsal side. As a cryptic species, A. abramis could possibly constitute multiple species that are currently undescribed. A. abramis is also a part of a species complex that includes itself and various congeners, including Astyanax bimaculatus, the namesake of the complex, and Astyanax lacustris, which is very closely related to A. abramis and looks very similar. Astyanax is a particularly speciose genus wherein cryptics and complexes are not uncommon. Description Abramis astyanax is a small fish that has a maximum recorded length of 14.0 cm (5.5 in) in standard length (SL) and a maximum recorded weight of 29.70 g. The body is compressed laterally and is subrhomboidal. It is a silvery fish with a green or blue tint on the back, and there is a dark spot, starting on the caudal peduncle, that reaches the posterior margin of the caudal fin. Its fins, while largely transparent, can also have a slight red, orange, or yellow tint. There is a dark humeral spot on each side behind the top of the head ("humeral" means "around or related to the shoulder", as in "humerus"). This spot manifests as a horizontal oval, and the humeral region also sports two vertical brown bars. There is a black lateral stripe, just above the lateral line, that starts at the humeral spot or just behind it. Morphometric characteristics include 1 dorsal-fin spine, 10 dorsal-fin rays, and 30 to 34 anal-fin rays. A. abramis also has 42 to 48 lateral-line scales; this can be used to differentiate it from closely-related congener A. lacustris, which has 36 to 39. The scales are generally small, with concentric grooves near the posterior margin. A. abramis lacks maxillary teeth, which can be used to tell it apart from various other similar congeners; however, the premaxilla still sports teeth, and A. abramis is believed to demonstrate unilateral replacement of missing teeth. Sexual Dimorphism While there are few morphometric differences between the sexes, male A. abramis can be distinguished from female by the presence of small hooks or barbs on the anal-fin rays of