Tiny Transparent Fish Could Revolutionize Brain Science
One of the world’s leading brain research centers is betting big on a fish smaller than a grain of rice. The Howard Hughes Medical Institute’s Janelia Research Campus has announced an ambitious decade-long initiative to decode how a vertebrate brain generates behavior — and at the center of the effort is Danionella cerebrum, a tiny, transparent fish that lets scientists watch brain activity in real time.
“It’s a big, risky bet,” says Gerry Rubin, Janelia’s founding executive director and head of biology. “But that’s what makes it interesting.”
The Brain-Behavior Mystery
One of the most fundamental questions in biology remains stubbornly unanswered: How do physical processes in the brain — the firing of neurons — give rise to memory, decision-making, social interaction, and consciousness? This is known as the brain-behavior question, and solving it could transform our understanding of neurological and psychiatric disorders like Alzheimer’s disease, autism, and depression.
Traditional approaches have studied brain circuits in isolation, but Janelia’s new initiative aims to observe an entire vertebrate brain at work simultaneously. The challenge is that in most species, the brain is hidden behind a skull and skin. Danionella cerebrum is different: it lacks the top part of its skull and has see-through skin, remaining transparent throughout its entire life.
“Having an animal that has a clear head and a clear body [is] extremely useful for neuroscience,” says Matt Lovett-Barron, a neurobiologist at UC San Diego who studies the fish. His lab places Danionella into virtual reality environments — “like little video games with virtual social partners” — to observe how their brains manage social behaviors.
A New Model Organism
Danionella cerebrum was only officially identified as a separate species in 2021 by ichthyologists Ralf Britz, Kevin W. Conway, and Lukas Rüber, as documented in Scientific Reports. Its name comes from the Latin for “brain,” reflecting its unique suitability for neuroscience.
Adult Danionella measure just 10 to 13.5 millimeters and have approximately 650,000 neurons — about three times that of a fruit fly but still a tiny fraction of the 86 billion neurons in the human brain. Yet their transparency offers something no other vertebrate model can: the ability to watch brain activity across the entire organ while the animal performs complex behaviors like mating, social interaction, and decision-making.
Most model organisms have significant limitations for whole-brain imaging. Rodents have brains hidden by skull and skin. Fruit flies have an opaque exoskeleton. Zebrafish, the current standard, are only transparent as larvae, with limited behavioral repertoires. Danionella remains clear into adulthood, allowing researchers to study the full range of vertebrate behaviors.
From Fruit Flies to Fish
Janelia is famous for its work on fruit flies, including a landmark 2024 project that mapped all 54.5 million connections in the insect’s brain. Now the center is taking on an even greater challenge.
Janelia plans to triple the space dedicated to fish to 6,000 square feet, adding thousands of new tanks. The number of scientists working on Danionella is expected to rise from about 10 to 100 or more. The center will also create a complete map of every connection in the fish brain, much like the connectome it built for the fruit fly.
“We all evolved from fish, and our brains share many features of the brains of fish,” says Nelson Spruston, Janelia’s executive director.
AI as a Scientific Partner
A key innovation of the initiative is what researchers call “AI-in-the-Loop” methodology. Rather than using artificial intelligence merely as a data analysis tool after experiments are complete, AI systems will actively participate in the scientific process — interpreting data in real time, generating hypotheses, designing experiments, and directing experimental execution alongside human scientists.
“This is going to produce so much data that we’re going to need something like artificial intelligence to analyze it,” Rubin says.
The discovery loop compresses from months to days, with AI and laboratory experiments working together as one continuous system. AI can simulate proposed approaches before a single experiment runs, detect unexpected patterns, flag candidate circuits, and update predictions as fresh data arrive.
This represents a paradigm shift in how science is conducted. The effort works in close partnership with AI@HHMI, the institute-wide initiative bringing AI methods to biomedical research across a wide range of organisms.
The Engineering Challenge
Many of the necessary tools do not yet exist. Scientists still do not know whether AI systems can reliably help generate mechanistic biological understanding at the level the project requires. And some of the engineering challenges are formidable.
Currently, scientists often immobilize Danionella fish to study their brains. The ultimate goal is different. “The ultimate goal is to do these experiments in freely swimming animals,” Spruston says. “That’s going to require that we tackle some serious engineering challenges.”
HHMI President Erin O’Shea acknowledges the uncertainty but embraces it. “The test is simple: Does this research require Janelia to succeed? We are concentrating Janelia’s distinctive capacity on big bets that no one else can make.”
What Success Looks Like
Even with the best tools and AI partnership, answering the brain-behavior question is a long-term goal. O’Shea says she “would be ecstatic if in 10 years we [understand] just one complex behavior in the fish, like schooling.”
Researchers at UC San Diego have already made progress in this direction. A 2024 study published in Current Biology found that Danionella depend on their sense of vision to coordinate social swimming, and that the ability to school develops as individuals mature — much like how human social skills develop with age.
Implications for Human Health
If successful, this research could provide the first complete mechanistic account of how a vertebrate brain produces behavior, potentially revolutionizing our understanding of what goes wrong in neurological and psychiatric disorders. The answers that emerge could unravel the logic of brain function and lead to new therapeutic approaches.
“Twenty years ago, Janelia committed to determining the wiring diagram of the fruit fly brain,” Rubin says. “Many thought such efforts were infeasible, but we succeeded. The same approach can be applied to Danionella. But having a transparent animal where brain-wide activity patterns can be measured and interpreted using AI provides possibilities we lacked in the fly.”
What to Watch For
In the coming years, watch for Janelia to release open-source tools, genetic methods, and datasets that make Danionella accessible to the broader scientific community. The initiative’s success will depend on whether AI systems can truly partner with humans in biological discovery — and whether the lessons learned from a 650,000-neuron fish brain can illuminate the workings of our own.
As O’Shea puts it: “We are tackling a problem whose solution will benefit not only science but human health, and in the process, we are reenvisioning how science is done.”
This article was based on reporting by NPR and additional sources including MedPath Trial, Wikipedia, and UC San Diego Today.