Answer:
Its back fin is dormant.
Discussion:
Sunfish, or mola, develop their truncated, bullet like shape because the back fin which they are born with simply never grows. Instead, it folds into itself as the enormous creature matures, creating a rounded rudder called a clavus. Mola in Latin means “millstone” and describes the ocean sunfish’s somewhat circular shape. They are a silvery colour and have a rough skin texture.