Sea pens are so named for their resemblance to an old-fashioned quill pen. Each sea pen is made up of a colony of polyps that work together for the survival of the whole. The primary polyp loses its tentacles and becomes the stalk of the sea pen, while the secondary polyps form the sea pen’s branches. Some sea pens feed on plankton by using nematocysts, specialised cells containing stinging thread which can be projected towards prey.