An international team of ichthyologists has described a remarkable new genus and species of miniature fish from the Rio Negro, one of the best explored parts of the Amazon.

The fish is only 7 mm longer than the world’s smallest fish, and seems to only appear at night, but its bright blue belly caught the eye of the ichthyologists who spotted it was a new species and genus.

The species has been named as Cyanogaster noctivaga. ‘Cyanogaster’ means blue belly and ‘noctivaga’ means night wanderer.

“It is a strange little animal, completely transparent with an otherwise unique color pattern,” said Dr Ralf Britz from the Natural History Museum, London, who co-authored a paper describing the new fish species and genus in the journal Ichthyological Exploration of Freshwaters.

C. noctivaga has large eyes, and unusual-looking snout, mouth and teeth.

The fish was discovered in the Rio Negro, the largest tributary of the Amazon River. This area of the Amazon basin is probably one of the best explored, so finding not only a new species but a new genus too, was quite a surprise.

The ichthyologists could only find the blue-bellied fish in one place on the Rio Negro, and it could only be found at night.

“The fish appeared as a fast swimming blue streak in the net,” Dr Britz said. “The largest Cyanogaster individual we collected was 17.4 mm long, which is about 7 mm longer than the largest Paedocypris progenetica.”

The two little fish seem to prefer similar habitats too. “The Rio Negro in Brazil, like the Asian peat swamp forests, has acidic blackwaters and like the peat swamps, it harbors a large number of miniature species,” Dr Britz explained. “Small size seems to be favored in mineral-poor water and Cyanogaster is another example of this rule.”

The number and shape of teeth, or dentition, is very useful for naming and classifying fish and especially those of the order Characiformes, the group C. noctivaga belongs to.

The fish has 2 rows of teeth in the upper jaw, an inner and outer. There are only 4 teeth with several cusps in the inner row and 1 conical tooth in the outer row.

“All other members of the subfamily Stevardiinae and actually most members of the family Characidae have a different number and arrangement of teeth. So this helps to demonstrate that our little ‘bluebelly’ is something quite different, a new genus,” Dr Britz concluded.

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Bibliographic information: George M. T. Mattox et al. Cyanogaster noctivaga, a remarkable new genus and species of miniature fish from the Rio Negro, Amazon basin (Ostariophysi: Characidae). Ichthyol. Explor. Freshwaters, vol. 23, no. 4, pp. 297