Sea fans (Order Gorgonacea)
… are actually colonies of tiny invertebrates (soft corals) working together as a single organism. These polyps secrete minerals that form a semi-flexible skeleton to which they all attach. They are primarily nocturnal, withdrawn into their holes in the skeleton during the day. At night, they extend their tentacles to capture micro-organisms that drift by in the water current.
The polyps of some species of sea fan also contain algae, existing in a symbiotic relationship - the algae photosynthesizes and provides sugars to the polyp, while the polyp provides nutrients through the prey items it captures. The polyps share nutrients with each other through a layer of living tissue. Sea fans are most commonly found in the shallow waters of Florida and the Carribean basin, though species occur worldwide.
A few organisms, such as a couple species of pygmy seahorse, are obligate residents of sea fans (that is, never found anywhere else). Sea fans are very similar to soft corals, but differ in their anchor point: soft corals anchor on hard substrates, while sea fans are anchored in sand or mud.
photo by Greg Grimes (Thespis377) on Flickr
(via: Peterson Field Guides)