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.