Young tomato clownfish adjust their white stripes depending on whether adult individuals live nearby.
A study by the Okinawa Institute of Science and Technology has shown that if a juvenile grows up in the presence of adults, the second white stripe disappears 24 days earlier than in isolated peers. The reason lies in the social hierarchy: adults react more aggressively to fish with one stripe, while two stripes make juveniles "less noticeable" in conflicts. When the fish is accepted into the group, the stripe disappears — this is a signal of entry into the hierarchy.
At the cellular level, the process is controlled by programmed cell death of iridophores (cells that form the white coloration) under the control of caspase 3 and thyroid hormones activated by social signals. The stripe does not move — the cells die, and their place is taken by orange pigment cells.
From an evolutionary perspective, the ability to lose stripes has arisen independently in different species living in small groups, where direct aggression is particularly dangerous. For clownfish, the stripe is not just decoration, but a social tool: it helps to fit into the school, avoid fights, and find its place in the reef community.
