Fish with a high level of familiarity are more aggressive towards each other
(PHYSorg.com Biology)

Aggressiveness among animals may increase the longer individuals live together in stable groups. This is the finding of a recent study carried out by researchers from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB). The study, published in the journal Animal Behaviour used the Amazon molly, a naturally clonal fish species that produces genetically identical individuals to isolate the effects of familiarity on behaviour. The study provides new insight into how groups function, and why it is sometimes better for fish to leave their group and join a new one.