Sea penPennatulacea

A sea pen is planted, not attached.

Everything else in this part of the catalogue needs rock. A sea pen lives out on open sand and mud, where there is nothing to hold on to, and gets around it by driving a muscular bulb into the sediment and swelling it with water until the grains lock around it, the way a wall plug expands in a drilled hole. The animal can also let the water back out, pull free and reset itself somewhere better, which makes it one of very few colonial cnidarians that gets to change its mind about where it lives.

The stalk is a single animal. A sea pen begins as one larva that settles, grows out of all proportion and becomes the entire stem; every other polyp on the colony is budded off that founder and stays part of it. So the thing standing in the sand is not a stalk with animals attached to it. It is one animal that turned into a building, with its own offspring arranged in rows along the outside, feeding into a gut they all share.

Many of them make light. Touched in the dark, a sea pen sends waves of green running away from the point of contact along the colony, and the wave keeps traveling after your hand has gone. Nobody has settled what it is for, though a light that spreads outward from whatever bit you is a reasonable way to attract something bigger to deal with it.

This is a sand animal and not a reef one, and it is seldom seen here, partly because divers spend their time over coral and not over the flats. Some species withdraw into the sediment completely in daylight and leave nothing at the surface to find, so a night dive over sand is the way to meet them. The catalogue carries a single species.