Invertebrates
Sea Pen
Pennatulacea · also called sea pens
A sea pen looks less like an animal and more like a feather pushed upright into the sea floor, and in a sense that resemblance is the whole story: it is a colony of hundreds of tiny polyps sharing one skeleton, anchored in soft sand rather than rock. Touch one and, in many species, it answers with a visible blue-green glow.
Overview
Sea pens, order Pennatulacea, are colonial filter feeders anchored in soft sand or mud sea floor worldwide, from shallow coastal water to the deep sea. Numerous tentacled polyps capture plankton, with some polyps within a colony specialised for feeding and others for maintaining the structure’s water flow.
A quill-shaped colony that glows and retreats
Many species are bioluminescent, glowing blue-green when disturbed, and a threatened sea pen can retract its stalk down into the sediment for protection. Unlike reef corals, its soft-sediment anchoring lets it occupy open sea floor most corals cannot use.
Status
Least Concern overall but decreasing in trawled areas, where heavy bottom-contact fishing gear can damage or destroy the delicate, slow-growing colonies.
Sea Pen: common questions
Is a sea pen a single animal or a colony?
A sea pen is a colony made up of hundreds of genetically identical individual polyps sharing a single skeleton and coordinated structure, rather than one single organism in the way most animals are understood, with different polyps within the colony sometimes specialised for different functions such as feeding or maintaining water flow through the structure.
Why does a sea pen glow when you touch it?
Many sea pen species are bioluminescent and produce a visible blue-green glow in response to physical disturbance such as touching, a defensive reaction thought to potentially startle or deter an immediate predator, and in some proposed explanations possibly even attract a larger secondary predator that might then target whatever originally disturbed the sea pen, though the full range of functions of this glow is still studied.
How is a sea pen different from a coral?
While both are colonial cnidarians built from numerous individual polyps, sea pens anchor a bulbous stalk directly into soft sand or mud sediment rather than attaching to hard rock or existing reef structure the way most reef-building corals do, allowing sea pens to occupy open, soft-bottom habitats that are generally unsuitable for typical hard corals.
Why are sea pens considered vulnerable to bottom trawling?
Sea pens are relatively delicate, slow-growing colonial structures anchored in soft sediment, and heavy fishing gear dragged along the sea floor, such as bottom trawls, can easily damage, uproot, or destroy sea pen colonies in the area it passes through, which is why some regions have introduced protected areas or gear restrictions specifically aimed at preserving dense sea pen habitats and the smaller animals that shelter among them.