Nannostomus anduzei (common name: Anduzi’s pencilfish) is a freshwater species of fish belonging to the genus Nannostomus in the characin family Lebiasinidae. It is native to Venezuela and northern Brazil, particularly the upper Orinoco and Rio Ererê, a tributary of the Rio Negro. The adults grow to a maximum length of only 16 mm, making them one of the smallest pencilfish. Other common names include miniature pencilfish, miniature nannostomus, and Anduzi’s nannostomus.
Nannostomus anduzei is restricted to the Amazonian region of the South American continent and has only been formally reported from two localities in the north of that area. The first, and site of discovery, was 20 km north of the small Venezuelan town of Puerto Ayacucho in a small lagoon, in the upper regions of the Rio Orinoco, at 5° 50′ N, 67° 30′ W. The second site was some 700 km to the southeast, in the Lago Caatinga, off the Rio Ererê, a tributary of the Rio Negro at 0° 14′ S; 63° 53′ W. These limited data are reflected in the range map above, where it is not possible to say where else the species might be found. However, because these fish occasionally do appear in the aquarium trade (see also below), their true range likely is somewhat wider than indicated. While the two known sites are geographically widely separated in two river systems, the Orinoco and the Negro are linked by the Casiquiare canal, so migration between the two sites is possible. N. anduzei does not appear in a systematic list of fishes of Columbia, though the northern Orinoco locality is only a few kilometres from the Colombian border. This could represent undersampling in what is a remote part of the forest.
This is a very small fish: even fully grown adults are only a maximum of about 16 mm and it can become sexually mature at only 11 mm. As such, it is the smallest member of its genus currently known, the next smallest being N. minimus, which is not sexually mature until about 14 mm in length. Though small in size, the species is strikingly colored. The fish is mainly golden brown with a metallic gold stripe running the length of the body along the lateral line. The most notable feature is a pair of brilliant, rich red patches on the anal and caudal fins. Some anatomical features discriminate N. anduzei from related fish, notably reduced fin ray counts on the pelvic and caudal fins and the possession of only four premaxillary teeth. These three features are perhaps related to the reduced size. N. anduzei is sexually dimorphic in that consistent differences are seen between males and females. Males show brighter coloration around the red patches on the tail and anal fins, especially when in breeding condition. Males are also slimmer-bodied and have a longer anal fin (3 mm in males; 2 mm in females), with some of the surrounding muscle modified to enable mating, a common feature shared with other members of the genus. Females can have a brighter longitudinal body stripe; the red patches are almost always somewhat paler.