From Bill Rudman (Click here for the fact sheet):
Sea pens are cnidarians or coelenterates, related to sea anemones, corals and jellyfish. Each ‘animal’ is a colony, with a fleshy anchor which sticks down into the mud or sand, and an upper ‘feather-like part with layers of flaps which can expand out into the water. At the edge of the flaps are a row of polyps, which look like small sea anemones. They live offshore in sheltered bays, feeding on small organisms in the water.
Leanne and David Atkinson provided the photos below and explain:
They are something we see in the Port Stephens-Great Lakes Marine Park regularly over all of the dive sites. Here are our second series of photos. The first photo shows a Sea Pen stalk with Polyps out. It was taken at Fly Point, Port Stephens-Great Lakes Marine Park, Port Stephens, NSW Australia in 8 metres of water. The next two show Sea Pens from The Pipeline with a close up of the Polyps. Both of these were taken at The Pipeline, Nelson Bay Port Stephens.
If you’re interested in reading the full article, look here
What a relief!