A mood disorder in humans could be modelled by a zebrafish that stops swimming when left alone.
In 2005, when Herwig Baier of the University of California, was screening thousands of zebrafish for vision problems, he found one that seemed a bit 'off'.
If alone, especially after repeated periods of isolation, the fish would 'freeze': just sit at the bottom of the tank, reports Nature.
If fish that swum normally were put in the tank, the relatively inactive fish became normal and swam around too.
Baier looked at the genetic mutations in the 'frozen' fish and found one in the glucocorticoid receptor, a protein that is found in almost every cell and that senses cortisol - a hormone involved in the stress response.
Baier found that levels of all three hormones - CRH, ACTH and cortisol - were higher in the frozen fish than normal.
He guessed that the animals were unable to respond properly to chronic stress - a problem that is known to trigger anxiety or depression in humans.
"The fact that the key elements of stress and stress response are conserved in zebrafish is exciting because of the many experimental advantages of that model organism," says chemical geneticist Randall Peterson of the Massachusetts General Hospital, Harvard Medical School, Charlestown.
The findings were discussed at the Society for Neuroscience meeting in San Diego, California.