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Type locality is ‘Maduni Creek, Guyana’, and this species is widely-distributed throughout Guyana and Suriname, plus the middle-to-lower Amazon basin east of the Andes mountains in Peru, Colombia, and Brazil.
For more information see the description section below.
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Nannostomus marginatus, (from the Greek: nanos = small, and the Latin stomus = relating to the mouth; from the Latin: marginatus = marginated), commonly known as the dwarf pencilfish, is a freshwater species of fish.
Type locality is ‘Maduni Creek, Guyana’, and this species is widely-distributed throughout Guyana and Suriname, plus the middle-to-lower Amazon basin east of the Andes mountains in Peru, Colombia, and Brazil.
Records thus exist from various major river systems including the Rupununi, Takutu, Essequibo and Demerara in Guyana, the Nickerie and Courantyne/Corantijn in Suriname, Inírida and Caquetá in Colombia, the Ucayali, Javari, Ampiyacu, and Putumayo in Peru, the Napo in Ecuador and Peru, Juruá in Brazil, and throughout most major left-bank tributaries of the Amazon in Brazil, including the Japurá, Negro, Branco, and Trombetas.
Records from the upper Río Orinoco in Venezuela also exist but may require confirmation.
There also exist a number of minor differences in colour pattern between populations.
For example some Peruvian forms have yellow pigmentation on the body and fins, and those from Guyana and Suriname possess a short red stripe on vertical scale rows 6-9, above the central dark stripe on the body.
Rio Negro forms have a similar red stripe but overlapping the central dark stripe and extending along most of the flank, while a form from Colombia has an elongate red stripe which is broken in two beneath the dorsal-fin.
One form from Peru was described as a subspecies, N. m. picturatus, by Hoedeman (1954) but is currently considered synonymous with N. marginatus.
It has a distinctive red blotch on the caudal-fin base, between the primary and secondary dark lateral stripes.
This variability has yet to be studied in depth.
