The Acanthurus coeruleus (yellow doctor) fish changes color during growth. There are several reasons why this juvenile coloration occurs; for the juvenile yellow doctor this coloration signifies a variation in growth where the younger ones are more of yellow and the juvenile ones are less of it.
As juveniles, the edges on their dorsal and anal fins and the rings around their eyes are purple-blue, blue or blue-green. Their colors change during growth from a yellow juvenile, yellow tailed blue sub-adult and the blue adult phase.
Coloration also protects juvenile fish from aggression from adults in their same species. If a fish can recognize that a juvenile is their species, they may react differently than they would to an adult.
This gradual change in color is normal and it signifies successful transition of the fish.
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