Danionella is a genus of miniature freshwater fish from Myanmar and neighboring parts of South Asia, in the same broad group as danios and rasboras. At roughly 0.5 in as adults, its species are among the smallest known vertebrates, and that extreme miniaturization comes with an unusual developmental story: Danionella retain a number of larval, cartilaginous features into adulthood rather than fully ossifying, a pattern researchers describe as developmental truncation. Several species, including the effectively see-through Danionella translucida, are also notably transparent, which together with their small size has made the genus a favored model organism in neuroscience labs studying brain activity in an intact, living vertebrate.
In the aquarium hobby, Danionella has become a niche darling of the nano-tank scene, prized for schooling in large numbers within a small footprint; Danionella dracula, the "dracula fish," is a particular favorite for the small fang-like projections males use when sparring with rivals. Care is generally straightforward for an experienced nano-fish keeper — soft, stable water and very small live or micro foods — though their size makes them easy to lose to larger tankmates or strong filter outflows. Conservation data are thin: three of the five species in the atlas are Data Deficient and one has not been evaluated, so the wild status of most Danionella is genuinely unclear rather than reassuring.