Taxonomy & naming
Danionella cerebrum was formally described in 2021 in a paper published in Scientific Reports (Nature Publishing Group) by Ralf Britz, Kevin Conway and Lukas Rüber, working from material collected on the southern and eastern slopes of the Bago Yoma range in Myanmar. The Catalog of Fishes (Eschmeyer, CAS) records the valid name as Danionella cerebrum Britz, Conway & Rüber, 2021 with no prior combinations — this is an entirely new species, not a renamed or transferred taxon.
The genus Danionella belongs to the tribe Danionini within the family Cyprinidae. It is a small, tightly circumscribed genus of Myanmar endemics characterised by their extreme developmental truncation (paedomorphism): the adults retain larval skeletal features and the bones of the skull remain incompletely formed, leaving the brain exposed and visible through the skin. This paedomorphic condition is what makes the genus so attractive to neuroscience, since the brain and neural circuit activity of living, behaving fish can be imaged through the body wall without surgery or clearing. The specific epithet cerebrum is Latin for 'brain,' a direct reference to the exposed, visible brain that is the species' most striking character.
Morphology
Danionella cerebrum is one of the tiniest adult vertebrates on record, with a maximum standard length of about 0.5 in according to FishBase. Like other members of the genus it is entirely or nearly transparent: internal organs, the vertebral column and the brain are all visible through the body wall in living specimens. The incompletely ossified skull roof — a paedomorphic retention of a larval condition — means the dorsal surface of the brain lies exposed beneath translucent skin rather than being shielded by fused cranial bones.
The body is slender and elongate, the mouth terminal and small, and the fins are colourless and delicate. Externally the species is superficially similar to its congener Danionella translucida, from which it was distinguished by subtle differences in skeletal and meristic characters documented by Britz and colleagues in the original description. The species offers no bright coloration and few of the features by which most aquarium fish are identified at a glance; what makes it remarkable is entirely the architecture it lacks — the skull, the pigment — rather than what it displays.
Habitat
The type localities for Danionella cerebrum lie on the southern and eastern slopes of the Bago Yoma mountains in Myanmar: the Tandabin Chaung and Bala Chaung in Yangon Division and an unnamed stream northwest of Daikme Chaung in Bago Division, as documented in the Natural History Museum of Bern's announcement of the species. These are low-altitude, slow-flowing streams, shallow and soft-watered, with FishBase recording a conductivity of roughly 20–100 µS and temperatures of 77–86 °F.
The streams have a neutral to very slightly alkaline pH in the range of 7.4–7.5 per FishBase, which is softer and more neutral than the acidic blackwater streams typical of Sundaland but cooler and gentler than the turbulent hill torrents of the Western Ghats or the alkaline plateau waters of Inle. Characteristically these are forested Burmese lowland streams with slow or intermittent flow, likely carrying organic leaf material and fine silt. The species' extreme small size, transparent body and tolerance for soft, warm water are consistent with life in sheltered, shaded stream reaches rather than exposed or fast-flowing reaches.
Feeding
No detailed dietary study specific to Danionella cerebrum has been published. By analogy with other members of the genus and with Danionini generally, it is presumed to be a micropredator feeding on tiny invertebrates — copepods, ostracods, midge larvae and similar microcrustaceans and insect microlarva — taken from the water column and near the substrate. Its minute gape limits prey size to the very smallest aquatic organisms.
In research aquaria the species is typically maintained on live or frozen copepods, Artemia nauplii, paramecia and similar microfoods. The demanding feeding requirements of small Danionella species are a practical limitation on their wider keeping in the hobby, and keepers should have a live-food culture running alongside any attempt to maintain them.
Mating
Danionella cerebrum is an egg-scattering, non-guarding spawner, consistent with the reproductive mode of other danionins. There is no nest, no bubble-nest, no mouthbrooding and no lasting pair bond. In courtship a ripe male pursues a gravid female through the water column and among any available cover, and eggs and milt are released together in brief contact spawning embraces.
The species is notable for its acoustic behaviour during courtship. The 2021 original description in Scientific Reports documented sound production exceeding 140 dB re 1 µPa at 3 ft in males, generated by a specialised rib-striking mechanism involving the fifth ceratobranchial — a morphological novelty that is possible because of the species' paedomorphic skeletal configuration. The function of this unusually loud call in so small a fish is likely mate attraction and male-male competition, though the full behavioural context remains under active study in laboratory settings.
Breeding
Danionella cerebrum scatters adhesive or weakly adhesive eggs over fine substrate and plant material, with no parental care after spawning; adults will consume both eggs and newly hatched larvae given the opportunity. The reproductive biology broadly follows the pattern of other tiny cyprinids and has been observed in research aquaria more than in dedicated hobbyist breeding trials.
Because the species is maintained in neuroscience laboratories specifically for its transparent, small body and externally visible brain, basic life-history parameters — clutch size, egg development rate, larval stage duration — have been investigated in that context. Larvae are minute at hatching and require paramecia or other suitable microfoods in their first days. Juveniles grow slowly relative to larger danionins, reflecting the species' paedomorphic biology. At this stage the species remains specialist territory for aquarists with experience in very small, delicate fish and reliable access to live microfoods.
In the aquarium
Danionella cerebrum is not a fish of the mainstream hobby. Its extreme small size, demanding microfeeding requirements and status as a recently described research species mean that it circulates primarily in laboratory settings and among specialist nano-fish keepers rather than through ordinary retail channels. The conditions it needs — warm (77–86 °F), soft, mildly neutral water (pH ~7.4–7.5) with slow, gentle flow and a well-planted or moss-covered tank — are achievable but must be carefully maintained.
Given its size and delicacy, Danionella cerebrum should be housed in a dedicated species-only nano aquarium or with similarly minute, peaceful companions. Any flow must be negligible; standard internal filters are too powerful and should be replaced with sponge filters or similar low-flow equivalents. Water quality must be high and changes slow and small. The species' appeal is primarily scientific and aesthetic — watching an animal this small with its brain and internal organs visible through the body wall is genuinely extraordinary — but it should not be treated as a casual purchase.
Conservation
Danionella cerebrum has not yet been formally assessed by the IUCN Red List. It was described new to science in 2021 and its evaluation has not caught up with its description, so it carries the designation Not Evaluated (NE). This is a common situation for newly described species, particularly those from narrow ranges in understudied drainages.
The species is endemic to a small cluster of streams on the Bago Yoma slopes in Myanmar — Yangon and Bago Divisions — which represents a restricted range. Myanmar's lowland forests and stream systems face well-documented pressures: deforestation for agriculture, sedimentation from land clearance, and expanding agricultural and urban footprints in the lowlands around Yangon. A species restricted to a handful of known localities in a rapidly changing landscape warrants precautionary attention, even if a formal threat category has not yet been assigned. Until an IUCN assessment is completed, the species' conservation status remains formally unknown, though its narrow endemism and the habitat trends in its range suggest vigilance is warranted.