Astyanax boliviensis

Astyanax boliviensis can reach roughly 9 cm (3.5 in) standard length (SL). The lateral line has 39–42 scales. There are 10–14 rays in the pectoral fin, 6–7 in the pelvic fin, and 23–30 in the anal fin. The dorsal lobe of the caudal fin has 12–14 rays, and the ventral lobe has 10, making for 22–24 rays total in the caudal fin. The body is somewhat compressed, and is at its greatest depth just past the start of the dorsal fin. No sexual dimorphism is known. The base body color is a middle silver, as with most Astyanax, and includes a lighter belly, as well as a reflective silver lateral stripe. The upper half of the body turns a chestnut-brown upon preservation in alcohol, while the lower half turns yellowish; the silver stripe remains prominent. The fins are largely hyaline (clear), though may be slightly grayish. Unlike several other members of the genus Astyanax, A. boliviensis has no dark lateral stripe; it does, however, have a series of prominent chevron markings that extend down either side. These chevrons are extended at either end to connect to the next marking, creating a continuous pattern that overlays a silver lateral stripe rather than a dark one. There is also a dark spot on the caudal peduncle; the only member of the Astyanax orthodus species group not to have a caudal-peduncle marking is A. ghandiae. …

Astyanax boliviensis was originally described from a small stream inside the Iturralde Province of Bolivia. Today, it is also known to also be found in the Madidi and Mamoré rivers, which are both tributaries of the Madeira river. Specific biotope preferences, as well as diet and sympatry, are unknown. The Maderia basin hosts a large portion of the Llanos de Moxos floodplain, which is a patchwork-esque spread of savanna and wetland biomes. There are also occasional patches of forest. The Mamoré river is found in the same lowlands as a large portion of these floodplains. To the southwest, the Madidi river is surrounded by savannas as well, and further by lowland rainforest.

Because A. boliviensis is endemic to Bolivia, its specific name originates therein; the suffix "-ensis" denotes presence in or affiliation with a location. The genus Astyanax has a name which is an allusion to the Iliad, in which a Trojan warrior named Astyanax appeared. The reason for this allusion was not made clear by Spencer Fullerton Baird and Charles Frédéric Girard when they named the genus, but it may originate in the scales of type species, Astyanax argentatus, which are large and silver, like armor or a shield.