Sea pen

Sea pens are a type of marine cnidarian classified under the superfamily Pennatuloidea. These soft-bodied, colonial animals anchor themselves to the seafloor, with small polyps set within a fleshy matrix. The oldest confirmed fossils of sea pens come from the Cambrian-era Burgess Shale, specifically the genus *Thaumaptilon*. Older fossils from the Ediacaran period might represent the earliest sea pens, but scientists have not reached a consensus on what those ancient specimens actually are.

Taxonomically, sea pens belong to the order Scleralcyonacea and are colony-forming, benthic filter feeders. The superfamily Pennatulacea includes 16 families and 44 living genera, encompassing roughly 235 recognized species. The families are Anthoptilidae, Balticinidae, Chunellidae, Echinoptilidae, Funiculinidae, Gyrophyllidae, Kophobelemnidae, Pennatulidae, Protoptilidae, Pseudumbellulidae, Renillidae, Scleroptilidae, Stachyptilidae, Umbellulidae, Veretillidae, and Virgulariidae.

Because sea pens are found worldwide and have a long evolutionary history, there is considerable genetic diversity among species. Most sea pens have retained the original order of their mitochondrial genes, but one particular group shows unique rearrangements, as revealed by ancestral state reconstruction. Many sea pen populations live in Indian waters. Genetic changes mainly affect their polyps, which have spread across different waters and islands, and these polyps—with their tentacles—are used for defense, both for the sea pen itself and for other species.

As octocorals, sea pens are colonial animals made up of multiple polyps, each with eight tentacles, resembling tiny sea anemones. Unlike other octocorals, however, a sea pen’s polyps are specialized for different jobs. One polyp becomes a rigid, upright stalk called the rachis, losing its tentacles and forming a bulbous base or peduncle. The other polyps branch from this central stalk, creating structures for taking in water (siphonozooids), feeding (autozooids, which have stinging cells called nematocysts), and reproduction. The whole colony is reinforced with calcium carbonate spicules and a central axial rod.

Using their root-like peduncles to hold fast in sandy or muddy bottoms, the exposed part of a sea pen can reach up to 2 meters (6.6 feet) tall in some species, such as the tall sea pen (*Funiculina quadrangularis*). Some sea pens are brightly colored; the orange sea pen (*Ptilosarcus gurneyi*) is a well-known example. They are rarely found in water shallower than 10 meters (33 feet), preferring deeper areas where turbulence is less likely to dislodge them. Certain species live at depths of 2,000 meters (6,600 feet) or more.

Though generally stationary, sea pens can move and re-anchor if needed. They position themselves to take advantage of currents, ensuring a steady supply of plankton, their main food. Their primary predators are nudibranchs and sea stars, some of which eat only sea pens. Sea pens tend to cluster together in unpredictable patterns, which makes it harder for sea stars to prey on them. When touched, some sea pens emit a bright greenish light—a form of bioluminescence. They can also force water out of their bodies as a defense, rapidly deflating and pulling back into their peduncle.

Like other anthozoans, sea pens reproduce by releasing sperm and eggs into the water, either seasonally or year-round. Fertilized eggs develop into free-swimming larvae called planulae, which drift for about a week before settling on the seabed. Adult sea pens provide shelter for other animals, such as juvenile fish. Growth rings on the rachis suggest sea pens may live for 100 years or more, assuming those rings are indeed annual.

Some sea pens display glide reflection symmetry, a pattern rarely seen in living animals.

Sea pens are occasionally sold for aquariums, but they are difficult to care for because they require a very deep substrate and have specific dietary needs.