The sail gillfin has replaced it's ancestor, the gillfin. The sail gillfin has changed little from it's ancestor but is far more successful because of a change in it's reproduction method. Sail gillfin eggs no longer detach from their parent's tail. They don't detach because a thin membrane is constantly coating the tail of the sail gillfin. This membrane is used allow it's tail to move easier through the water, and has the added benefit of 'sealing' the eggs onto the tail. Also, female sail gillfins will lay their eggs either onto another members tail (female or male) or will lay their eggs directly into the water and then 'catch' them with their tail. This means that either sex can care for the eggs until they hatch. Upon hatching, baby sail gillfins won't break-free from the parent's tail right away. Baby sail gillfins will instead grow for a short time beneath the safety of the membrane. After growing strong enough to swim on their own, the young will 'eat' their way through the parents tail-membrane to freedom. Sail gillfins live in large schools.