The purple sea fan in this photograph is a type of gorgonian, a colony of simple animals similar to coral. The filament-like structure is created by the calcified remains of the colony, which supports the colony for new life and growth.

This is an unusual close-up of a Sea Fan, created by a strong back light, with only a touch of light on the front -- a tricky shot due to the inherent scatter in any back-lit underwater photo.