These glowing, quill-shaped colonies are actually sophisticated, multi-polyp organisms anchored in the deep sea.

Nature · Published 3 October 2026 Swipe more like this →

In short: Named for their resemblance to antique writing instruments, sea pens are not single animals but complex colonies of specialized polyps. These cnidarians thrive in soft mud, using a unique hydraulic system to inflate or deflate their bodies, and can even emit a brilliant greenish light when disturbed by predators.

Sea pens belong to the order Pennatulacea, a group of colonial marine invertebrates within the class Anthozoa. While they appear as singular, feather-like structures, each colony is a sophisticated assembly of specialized polyps. A single primary polyp forms the central stalk, which includes a lower peduncle for anchoring into sandy or muddy substrates and an upper rachis that supports the rest of the colony. Other polyps branch off to perform distinct roles: some act as water intake valves, others capture plankton for food, and some manage reproduction.

The physical structure of these organisms is remarkably varied. Some species, such as Funiculina quadrangularis, can reach heights of up to two meters, while others remain small and leaf-like. They are found globally, from polar regions to the tropics, and inhabit depths ranging from shallow coastal waters to over 6,000 meters. Because they rely on a central axial rod and calcium carbonate spicules for support, they prefer stable, deeper environments where water turbulence is minimal, though they are capable of relocating if necessary to find better current flow.

Survival for a sea pen involves a mix of passive defense and active biology. When threatened by predators like sea stars or nudibranchs, they can rapidly expel water to deflate and retreat into the sediment. Many species also exhibit bioluminescence, glowing when touched. Reproduction occurs when colonies release gametes into the water column, resulting in free-swimming larvae that eventually settle to the seafloor. Some research suggests that growth rings found in their structure could indicate lifespans exceeding 100 years, though this remains a subject of ongoing study.

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