Danionella

Four out of five described species of Danionella are found in Myanmar. D. translucida is described from the Ayeyarwady River basin, and D. mirifica was described from the Kamaing area in upper Myanmar.^[1] Danionella priapus is endemic to India.^[2]

When first described, Danionella translucida was the smallest ostariophysan and the smallest adult vertebrate to inhabit fresh water.^[3] Its adult size ranges from 10–12 millimetres (.43–.47 in) SL.^[3] D. mirifica gets slightly larger, at about 14 mm (.55 in) SL, but is still one of the smallest freshwater fishes.^[1][4]

Danionella species lack scales and barbels,^[3] but possess a lateral line.^[5]

D. mirifica has a single row of melanophores between the pelvic fins and the tips of the cleithra, and there is a lack of melanophores on the underside of the abdomen.^[1]

D. dracula reaches 17 mm in length. It is neotonous, lacking 44 bones that develop late in the related zebrafish Danio rerio. They have teeth made of bone, rather than the true teeth of other fishes, and the males have a pair of boney fangs which may be used during male-male competitions over nesting sites. Britz et al. believe the lineage lost true teeth about 50 Ma.^[6]

Danionella is used in scientific experiments as it is small, optically transparent throughout its lifespan, genetically amenable and has a short generation time.^[7][8]

A particular scientific advantage of an optically transparent organism is that it allows neural activity recordings (typically calcium imaging) to be performed in the same animal that will later be used for neural circuit reconstruction. This allows researchers to bypass many of the animal-to-animal variability problems caused when trying to correlate the behavior observed in one animal with the reconstructed circuitry of a different animal.