Nannostomus marginatus, the Striped Pencilfish

Nannostomus marginatus (Striped Pencilfish). I don't know why I haven't explored the Nannostomus more. There are over twenty species of Nannostomus, members of the Lebiasinidae, which makes them close relatives of Characins. All of them have long cylindrical bodies and the "tiny mouths" their name specifies. Adults reach almost 2 inches, and, in a flattering reversal, they grow larger for us in aquaria than they are found in nature. Probably they live longer in aquaria, too, under the right conditions. All the Nannostomus have a reputation for being delicate, when they don't have the clean, soft, acidic waters they require.
 
Variable Nannostomus marginatus populations are found in the mid- to lower Amazon, Rio Negro and Orinoco river systems, in the upper Amazon basin of Peru, and in the Guianas too. There is a separate population of N. trifasciatus with a distinct ocellated spot in the tailfin in the Rio Apeu, near Boa Vista, Para, Brazil. N. marginatus have been collected in shoreline reaches of the Amazon itself, but they more characteristically range up into small shaded tributary streams, the kind of forest environment that is most under attack from short-term speculative forestry and cattle-ranching. "Most of the species with wide distributions are geographically somewhat variable regarding life color patterns, scale counts and sometimes vertebral counts," wrote characin guru Stanley Weitzman in a Tropical Fish Hobbyist article, May 2001, "Nannostomus marginatus and a related coral red form." This is the most useful print article on this species and a public introduction to the newly found spectacularly beautiful coral red form, which has been given the name Nannostomus mortenthaleri.
 
Perhaps Nannostomus marginatus are the most beautiful Nannostomus. Mine seem to be from the Rio Negro population, a silvery gold with clean black stripes, with small flecks of nail-polish red. The clear parts of the fins reflect a skim-milk bluish sheen in reflected light. Alpha males in competition flush rosy red. At night the fishes take on a different, protective coloration, drab gray with three vertical dark blotches. In an aquarium that combines small open spaces with dense surface tangles they lose their shyness, but they wouldn't thrive with really boisterous tankmates, even suitably small ones. They always keep that dash-and-freeze behavior that is so characteristic of fry and of many small fishes. The floating tangles in my aquarium are important, for that is where Nannostomus prefer to deposit their spawn, I hear. They hold a station below a leaf or in the pendant roots of Water Sprite, from which males rush out to challenge their conspecifics with fin extensions and broadside shimmying. If that doesn't work, vigorous body-slamming is called for. Though no harm is done, no permanent pecking-order seems to get established either. These overtones of pre-nuptial jockeying behavior are a part of intra-sexual selection, watched at a slight distance by females. A female and a selected male will dive into dense cover, sometimes among the  beech leaf litter I maintain, but I haven't caught mine in the act. 
 
Quite often, schooling behavior is reported: in the Baensch Atlas, for example, or in a capsule bio by U. of Michigan Biology student Mara Zimmerman that's unfortunately been deleted at http://www.umich.edu/bio440/ —but I'm convinced this is shoaling, not schooling, a fear reaction that is a temporary artefact of laboratory conditions. The "schooling" behavior I mean. All that remains of Mara Zimmerman's good brief report, with a bibliography, is the best photo portrait of N. trifasciatus on the web. Currently I have six rival males and three plump females in a densely-planted 10-gallon tank. Males attempt to keep the females loosely herded in a harem. Other males challenge. The females don't seem to have any strong loyalties. With the thought that perhaps the spawnings were being eaten by Melania snails before they hatched, I've given the new snail-free  quarters. 
 
My fishes have the red mid-body splotch   taken as a mark of N. marginatus. Stanley Weitzman and Richard Vari say, "Complex distribution patterns, active speciation, and variable chromosome counts make this a challenging genus for taxonomists, and species limits are ill-defined." (in Paxton and Eschmeyer's Encyclopedia of Fishes, 2nd ed., 1998, p102). No doubt some inadvertent further blurring of Nannostomus populations has taken place in the aquarium world.
 
The TFH article I mentioned was occasioned by the first importation, in late November 2000, by Julio Melgar of Acuario Nanay, of some Nannostomus in which courting males flushed a strong red. The new find comes from small streams in the mid reaches of Rio Nanay, near the village of Albarenga, and from tributaries of Rio Tigre, near Santa Helena, in the province of Loreto, Peru, west of Iquitos, whence these wonderful fishes are being shipped. The males display even to their mirror reflections, and the coral red color fades when they aren't in displaying mood. In courting, the male approaches the female from behind with his anal fin spread wide so as to cup it round the female's vent, concentrating his sperm round the shed egg, which improves chances of fertilization.
 
Nannostomus species at FishBase. Nannostomus species at Wikipedia.