Since COVID-19 is ruling our daily routines, we recognize more than ever how important interpersonal relationships are for well-being, productivity, and mental health. Humans and other social species such as monkeys, rodents, and even fish have an innate need for interactions with others. However, it is still not fully understood how early-life social isolation shapes brain development. 

A team of scientists at the Champalimaud Center for the Unknown in Portugal studied one-week old larval zebrafish raised in isolation and found that early-life social isolation has an impact on the strength of social avoidance reactions and that the lateral line organ — a system of organs in aquatic vertebrates that detect movement in surrounding water — plays a crucial role in shaping them.

Larval zebrafish don’t swim continuously but make discrete movements called swim bouts. These swim bouts can be combined into complex behaviors, such as hunting, escaping, and social interactions. Scientists use high-speed video tracking to automatically detect those swim bouts and can classify them into different swim types. This allows detailed dissection of behavioral phenotypes for comparison between individuals or experimental groups.

Analyzing the differences in swimming behavior between group- or isolation-raised larvae, the scientists found that isolation-raised larvae display enhanced social avoidance. This behavior is independent of the visual system; blind fish behave similar to seeing fish. Mimicking swimming neighbors with local water vibrations is sufficient to induce this enhanced reaction and suggest a role of the mechanosensory system. The scientists used pharmacology to damage the lateral line mechanosensory organ and discovered that this reduced the social avoidance behavior of isolation-raised larvae.

One-week old larval zebrafish are not considered social yet, as they do not display social attraction behavior. The observation that one-week old isolation-raised larvae display strong avoidance reactions to water vibrations and swimming neighbors around them suggests that early-life social isolation has a strong impact on the development of the sensory system and marks the fishes behavior much earlier than previously thought.