This is a wonderful creature, which unfortunately is very difficult to keep alive in captivity. Here it is just after I purchased it. It is about 8 inches long and perhaps 3/4 inches in diameter.
After a day or so, it buried itself in the sand, where it normally lives. There is a fox coral on the left, a button coral skirt on the right, and a yellow watchman goby in the background.
This what the sea pen looks like at night, when it comes out of the sand. It is perhaps12 inches long, and covered with feeding polyps. The fox coral is in the background. When the sea pen is in the sand, only about 1/2 inch is visible above the sand. This peculiar way of protecting itself during the day must require a lot of energy. Since these only come out at night, some divers may have crossed places which they thought were quite boring during the day: had they returned at night, they might have been very surprised. Imagine a forest of these things…
A close-up of the feeding polyps which cover the sea pen.
Another close-up of the feeding polyps.
And closer still to the feeding polyps.
A final close-up of the feeding polyps. A truly beautiful creature.