After long exposure to sunlight, the patterns on its tail fins shine vividly when darkness arrives.
The way its two tail fins flutter while it swims has earned it the nickname “BEAUTIFLY of the Sea.”
The line running down its side can store sunlight. It shines vividly at night.
Swimming and fluttering its two tail fins, it looks like a BEAUTIFLY. At night, the patterns on its tail fins softly shine.
It absorbs sunlight with the pink areas of its skin, which then shine. This appears to be a form of camouflage.
— Pokédex entries (via PokeAPI)