Sea Pen (Pennatulacea) ✨
As their name suggests, sea pens can look like writing pens, or they can resemble feathers, whips or worms. But these diverse and delicate underwater animals, which are a type of coral, are also hard to pin down: A single sea pen is both an individual and a colony.
Sea pens are a type of octocoral, named for the eight stinging tentacles they use to capture plankton (tiny floating plants and animals) to feed themselves. The basic unit of a sea pen, like other corals, is a polyp, which consists of a sac-like body cavity enclosed by a mouth and surrounded by a ring of tentacles. Once it has found the perfect substrate — sand, mud, rubble or, in a few cases, solid rock — a larva settles down and becomes the primary polyp. It buds off into daughter polyps, and the sea pen grows. All are supported on a stemlike structure.
Some sea pens use a bulb inflated with water to anchor them to the sea floor. All have hard, internal skeletons, and at least some of them can glow in the dark.
Photo by @ocean_miki