Sea Pen

The Sea Pen is a strange creature. It usually looks like a soft, fat stick, or perhaps a long, thin sock. Soft, fairly smooth, and with virtually no solid structure. It is surprising to learn that the Sea Pen can be mobile, can dig its foot into the sand, and can change its surface shape radically.

Here it is right after I received it. The foot is to the right.

This is what it looks like when it "wakes up" or feeds. Hundreds sof previously invisible polpys sprout up to filter food out of the water. Each polyp is perhaps 5 mm tall. Notice the foot which has become buried in the sand.

Here are the polyps, closer up.

Here you can see some of the details in the polyps. They have a tall, slender body, five (or so) arms, and cillia on the arms. When something they like passes by (even something which you cannot see), the arms close quickly.

And, finally, a closeup of a single polyp. Gotta love that Mavica.

Here is the Sea Pen after it has managed to insert its foot into the sand. I had to built up the sand a bit for it: some Sea Pen require up to 4-6 inches of sand in order to root properly, but I only have about 3. A night feeder, it looked this marvelous first thing in the morning, but rapidly deflated with the light. I have to feed it plankton at night.

The Sea Pen expands and contracts on a daily basis. It "hides" in the sand during the day, and comes out at night to extend its polyps and feed. Initially, I had no idea how dramatic this could be, but after the Sea Pen dug into the substrate/sand (about a week), it became almost astonishing. What follows are two images, each taken at about the same scale (note the position and size of the "hump" in the sand which surrounds the base of the Sea Pen, and the location of the Fox Coral to it's left, as well as the rock structures behind them). One shows the Sea Pen during the day, at which time its external size is about one inch. The next shows the Sea Pen at night, where it is over 12 inches long (difficult to measure, due to its curved nature). An amazing specimen.