Cardinal tetra

The cardinal tetra was described in 1956 by the American ichthyologist Leonard P. Schultz in the aquarium magazine Tropical Fish Hobbyist, as Cheirodon axelrodi. George Myers and Stanley Weitzman were about to publish the same species as Hyphessobrycon cardinalis: the rule of priority kept Schultz's name, dedicated to Herbert R. Axelrod, the magazine's founder.

It differs from the neon tetra by its red band covering the whole underside, from head to tail. A revision of tetra classification published in 2024 now places it in the family Acestrorhamphidae, separate from the Characidae.

It frequents small forest streams and slow side channels. In the Rio Negro basin these are igapós and igarapés with black water: very acidic, almost without hardness, stained brown by humic substances from decaying leaves, under dense bank vegetation. In the Orinoco it lives rather in morichales, with clear water over sand.

The rhythm of the floods shapes its life: in the dry season the fish gather in large shoals in the channels, then disperse into the flooded forest as the water rises. Genetic studies have shown that the upper Rio Negro populations are ancient, distinct lineages, considered priorities for conservation.