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Although named after their feather-like appearance redolent of antique fountain pens, only Sea Pen species belonging to the suborder Subselliflorae live up to the comparison. Those belonging to the much larger suborder Sessiliflorae lack feathery structures and grow in club-like or radiating forms. The latter suborder includes what are commonly known as Sea Pansies.

As octocorals, Sea Pens are composed of many tiny individual animals called polyps (somewhat like miniature sea anemones), each with eight tentacles. Unlike other octocorals, however, a Sea Pen's polyps are specialized to specific functions: a single polyp develops into a rigid, erect stalk (the rachis) and loses its tentacles, forming a bulbous "root" or peduncle at its base. Other polyps branch out from this central stalk, forming water intake structures (siphonozooids), feeding structures (autozooids) with nematocysts, and reproductive structures. The entire colony is fortified by calcium carbonate in the form of spicules and a central axial rod.

Using their root-like peduncles to anchor themselves in sandy or muddy substrate, the exposed portion of sea pens may rise up to 2 metres in some species, such as the Tall Sea Pen (Funiculina quadrangularis). Sea Pens are sometimes brightly coloured; the Orange Sea Pen (Ptilosarcus gurneyi) is a notable example. Rarely found above depths of 10 metres, Sea Pens prefer deeper waters where turbulence is less likely to uproot them. Some species may inhabit depths of 2,000 metres or more.

While generally sessile animals, Sea Pens are able to relocate and re-anchor themselves if need be. They position themselves favourably in the path of currents, ensuring a steady flow of plankton, the Sea Pens' chief source of food. Their primary predators are nudibranchs and starfishs, some of which feed exclusively on Sea Pens. When touched, Sea Pens emit a bright greenish light; this is known as bioluminescence. They may also force water out of themselves as a defensive act, deflating and retreating into their peduncle.

Like other anthozoans, Sea Pens reproduce by co-ordinating a release of sperm and eggs into the water column; this may occur seasonally or throughout the year. Fertilized eggs develop into larvae called planulae which drift freely for about a week before settling on the substrate. Mature Sea Pens provide shelter for other animals, such as juvenile fish. Analysis of rachis growth rings indicates Sea Pens may live for 100 years or more, if the rings are indeed annual in nature.

The Sea Pen fossil recordEver since recorded history began, and probably before, people have found pieces of rock and other hard material with indentations from the remains of dead organisms. These are called fossils, and the totality of these artifacts and their placement in roc is patchy and disputed by some; while the earliest accepted fossils are known from the CambrianThe Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon and ended about 490 million years BP with the beginning of the Ordovician period. It is the first peri-aged Burgess Shale, similar fossils from the late ProterozoicIn geology, the Proterozoic is an eon prior to the first abundant complex life on earth. Classically, the boundary between the Proterozoic and the Paleozoic was set at the base of the Cambrian period when the first fossils of animals known as trilobites a ( Vendian) may represent the dawn of Sea Pens. Precisely what the Proterozoic fossils are, however, is not decided.