Sea pens are octocorals in the order Pennatulacea. Development begins with one polyp, the oozooid, which elongates into the peduncle anchoring the colony and the rachis carrying everything above. All other polyps arise as buds from that founder and remain connected through a shared internal canal system.
The polyps divide their labour. Autozooids bear eight pinnate tentacles and do the feeding, straining plankton from the current. Siphonozooids have reduced tentacles and pump water through the colony, inflating it hydraulically so the rachis stands up into the flow and deflating it when the colony withdraws into the sediment.
Many sea pens are bioluminescent, producing waves of greenish light across the colony when touched. Ptilosarcus and Renilla have been important in luminescence research; green fluorescent protein was first characterised in the hydrozoan Aequorea, but related Renilla proteins became standard laboratory reagents. The usual misconception is that a sea pen is a plant or a single soft-bodied animal, when it is a colony that inflates itself with seawater and can vanish into the seabed.