Danionella cerebrum

Danionella cerebrum is a cyprinid fish species reported in 2021 from turbid low altitude streams on the southern and eastern slopes of the Bago Yoma mountain range in Myanmar as well as from an irrigation canal southwest of the town of Hmawbi in Yangon Division. It was previously erroneously identified as Danionella translucida due to the close resemblance and similar geographical distribution of the two species. Adult fish of the species measure only 10–13.5 mm in size and have a brain volume of just 0.6 mm3 which is thus far the smallest known adult vertebrate brain. Danionella cerebrum larvae have been shown to exhibit similarities but also differences in their locomotor activity compared to the zebrafish (Danio rerio) to which they are evolutionary closely related.

Due to its miniature size, rich behavioural repertoire, and optical translucency that persists into adulthood, Danionella cerebrum holds great promise for non-invasive whole-brain in vivo imaging analyses with single cell resolution in an adult vertebrate and is beginning to emerge as a novel important model system in current neuroscience research.

The species makes the loudest sound for its size of any fish, exceeding 140 decibels, using muscles to tension a cartilage; this is released to strike the swim bladder. The sound appears to be used for intraspecific communication, as the loudest few males effectively suppress the sound production of other males. Despite its size, and the physics of sound making interaural cues very small, it has directional hearing.