Interesting Creatures... The Pencil fish (Nannostomus)

Interesting Creatures… The Pencil fish (Nannostomus)

Nov 16, 2014 Features / Columnists, Interesting Creatures in Guyana

Nannostomus (from the Greek: nanno = small, stomus = mouth) is a genus of fish belonging to the Lebiasinidae family of characins. All of the species in this genus are known as pencil fish or pencil fish, a popular name that was first only applied to two species in the 1920s, Nannostomus unifasciatus and Nannostomus eques. However, but by the late 1950s they would come to be applied to all members of the genus. Several of the species have become popular aquarium fish due to their attractive coloration, unique shape, and interesting demeanour.

Most species are slender, pencil-shaped fish ranging in size from under one to about two in length. N. marginatus, N. rubrocaudatus, and N. mortenthaleri possess shortened, blockier outlines reminiscent of pencil stubs. All but one species, Nannostomu espei, possess one to five horizontal black or brown stripes with gold or silver iridescence appearing dorsal to the primary stripe. Most also display red, orange, or maroon highlights in their fins, and many have flashes of these colours on their flanks, as well.

The recently described N. Mortenthaleri and N. rubrocaudatus are especially vividly coloured. For N. espei alone, horizontal stripes are only weakly present and are supplanted by five dark comma-shaped blotches. This pattern is assumed by other species at night, but only N. espei displays the pattern permanently and in daylight. The adipose fin is present in some species and absent in others, while in certain species, N. eques for example, it may be present or absent within the species.

All swim in a horizontal attitude except N. unifasciatus and N. eques, which assume an oblique, ‘snout-up’ posture. A range of sexual dimorphism occurs in the genus, with males being more brilliantly coloured in some species, especially with regard to colour present in the fins, and far less evident in other species. However, a reliable indicator of gender for most of the species rests in the anal fin of adult males which is enlarged and elongated (as in N. espei, N. eques, etc.) and/or the anal fin of males is more colourful (as in N. harrisoni, N. marginatus etc.). The popular aquarium species, N. trifasciatus, is an exception in this regard.

The genus has a vast distribution in South America, from Colombia, Venezuela, and the Guianas in the north, to the southern Amazon basin and Bolivia in the south, to Peru in the west and Belém, Brazil, in the east.

Several of the individual species have a distribution nearly as vast. As a result, many of the species are polymorphic and manifest marked colour variations depending on the population. Over the years, some of these colour variants have been erroneously described as separate species. Such names as ‘Nannostomus ocellatus’, ‘N. anomalus’ and ‘N. auratus’, among many others, are now known to be junior synonyms to the various species.

To date, only two species, N. beckfordi and N. harrisoni, have been commercially raised for the aquarium trade in fisheries, mostly in Asia. All of the remaining species that find their way to home aquaria are wild-caught from South American waters.

Nannostomus species thrive in home aquaria when provided with soft, moderately acidic water, low nitrate levels, and temperatures in the range of 72 to 82 °F. The addition of aquatic plants, including floating varieties, is recommended. The latter will reduce the likelihood of the fish jumping, which is a common occurrence for some of the species, especially N. espei and N. unifasciatus. They should be kept in schools of at least six.

If kept in a community aquarium, the best tankmates are other species of Nannostomus, small peaceful characins, and corydoras. Aquaria with strong water current, large tankmates, or swift-moving species are contraindicated. If kept in a thickly planted single-species aquarium with the above water parameters, most species will spawn, eggs will not be eaten, and the fry will be found among the floating plants.

Baby brine shrimp, live or frozen, and other small-sized foods are required for both fry and adults. They are also avid biofilm grazers and, for most of the species, algae are under-reported staples of their diet. In most species, the males will establish small territories and defend them. Their defensive actions are usually harmless, but in two species, N. mortenthaleri and N. trifasciatus, antagonistic behaviour directed at conspecifics can have deleterious results if sufficient space and plant cover are not provided. Once acclimated to the aquarium and provided with suitable conditions, they are hardy, often living for five or more years.

(Source: Wikipedia – The Free Online Encyclopedia)