Gorgonian fan

A sea fan, or gorgonian, is a form of sessile colonial cnidarian, similar to a sea pen or a soft coral, found in tropical and subtropical seawater. Individual tiny polyps form a colony that is erect, flattened, branching, and reminiscent of a fan. A colony can be several feet high and across but only a few inches thick. They may be brightly coloured, often purple, red, or yellow.

The main structural skeleton of a sea fan colony is formed from a flexible, horny substance called gorgonin, with living polyps covering the surface. Unlike corals, sea fans do not attach themselves to a hard substrate: instead, they anchor themselves in mud or sand. Each sea fan polyp has eight tentacles which catch plankton that is consumed. The "fan" is often oriented across the prevailing current to maximise water throughput and hence food supply. They may also contain algae which photosynthesise.

Sea fans are classified in the phylum Cnidaria, class Anthozoa, alongside the orders Alcyonacea (soft corals) and Pennatulacea (sea pens).

The Pygmy seahorse makes the sea fan its home.