Astyanax argyrimarginatus

Astyanax argyrimarginatus reaches a maximum of 4.5 cm (1.8 in) standard length (SL). There are 42–46 scales in the lateral line, 27–31 rays in the anal fin, and a single tooth in the maxillary bone; the premaxilla sports multiple multicuspid teeth. The head averages a length of 26.8% of the body, and the eye is large, taking up 34–38% of the head's length. The scales are silver. The fins are largely red, with more intense coloration on the caudal fin and on the front rays of the anal fin. There is a dark humeral spot in the shape of a horizontal oval, and two dark-brown vertical bars in the same region of the body. A dark blotch of color on the caudal peduncle extends to the tips of the middle caudal-fin rays. Perhaps the most distinctive feature is the dark lateral stripe, which has a bright silver border.

Sexual Dimorphism. On the anal fins of males larger than 7.5 cm (3.0 in), bony hooks are present. This is a feature not uncommon in the entire family Characidae. Otherwise, few morphometric or coloration differences can be discerned upon examination. Some congeners demonstrate more dramatic instances of sexual dimorphism, such as A. aramburui; mature males therein bear the same bony hooks, but they also grow small fleshy protrusions on the head and scales.

Astyanax argyrimarginatus was originally described from the Araguaia River basin of Brazil. Its range also includes the Tocantins and Xingu river basins, in the Brazilian states of Goiás and Mato Grosso. The Xingu is a more recent addition, from a study of the species' geographical range in 2012. Generally, it can be found in river headwaters and in various small streams. The Araguaia and Tocantins rivers are a part of a single river system, despite differing ecotypes. The Tocantins is relatively clear, with rapids and waterfalls; the Araguaia is muddy and winding, part of the local floodplains. The Xingu is another clearwater river, and it runs through the stony areas of the Brazilian geoscape. Little is known regarding the physiology and feeding habits of A. argyrimarginatus. Other members of the genus are generally adaptable omnivores, and are not shy of allochthonous material; this may remain the case for A. argyrimarginatus, but specifics are sparse.

The specific epithet originates in Greek. The word "argyros" means "silver", and "marginatus" means "bordered"; this is in reference to the silver lining around the black lateral stripe. The genus name, Astyanax, is an allusion to the Iliad, in which a Trojan warrior named Astyanax appeared. The reason for this allusion was not made clear in the nominal text, but it may originate in the scales of type species, Astyanax argentatus, which are large and silver, like armor or a shield. In terms of a common name, A. argyrimarginatus is one of many species of small fish sometimes referred to as "lambari" in South America.