https://phys.org/news/2018-11-zebrafish-virtual-shoal-partners-based.html

Shape, colour, smell, sound and a whole range of other factors may play a role. For a long time, biologists had assumed that conspecific animals recognise each other, in a holistic fashion, based on a combination of characteristics. Experiments performed with zebrafish in virtual reality have now shown that a fish-like motion pattern alone can convince a fish that it is in the presence of a member of its own species.

Zebrafish begin to form shoals with conspecifics when they are between 10 and 20 days old. The bigger the group, the stronger the attraction. Johannes Larsch from the Max Planck Institute of Neurobiology built a virtual environment for free-swimming zebrafish to find out which visual cues are critical for fish to enter a shoal. This experiment revealed which digitized patterns the fish were interested in and which they did not care about. Surprisingly, a simple black dot was sufficient to attract a young zebrafish within just a few seconds – provided that the dot was animated in a specific way.