Djibouti Sea Pen

A slender feather standing upright from bare sand at night, its polyps arranged along both sides of a central stalk. The lower part of that stalk is buried and swollen into an anchor, and the whole colony can pull itself down into the sediment - a lit or touched animal deflates and disappears, leaving no sign of where it stood.

A sea pen is not one animal but a colony with a division of labour: one founding polyp grows into the stalk, and everything else buds from it, some polyps feeding, others doing nothing but pumping water to keep the structure inflated. Many species glow when disturbed, a wave of green light running along the colony. Gravier described this one in 1906 from 🇩🇯 Djibouti, at the southern entrance to this sea, and records anywhere are very few - sea pens live on the sort of featureless night-time sand that divers cross rather than search.