Is That a Feather Standing on the Seabed? It Is Hundreds of Animals
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What looks like a quill pushed into the sand is a colony of tiny polyps sharing one fleshy stem, able to deflate into the sediment and to flash light when disturbed.
What the organism is
The structure is a colonial animal related to corals and sea anemones, made of many individual polyps that are not independent and cannot survive alone. One polyp at the base enlarges enormously into a muscular stalk that anchors in soft sediment, and the rest bud from the upper part in rows, giving the feathered appearance that supplies the common name. Different polyps in the colony do different jobs, with some feeding and others pumping water through the shared internal cavity, so the division of labour is real rather than decorative.
How it lives
The lifestyle follows from being rooted in mud with no hard skeleton:
- •The base is buried in sand or mud and grips by swelling
- •Feeding polyps catch plankton drifting past on the current
- •Colonies orient themselves across the prevailing flow
- •Water pumped into the colony inflates it to standing height
- •Expelling that water collapses it into the sediment
- •A stiffening rod of protein and calcium runs up the stalk
The light they make
Many species produce light when touched, and the display is unusual in form because the colony is a connected unit. A disturbance at one point triggers waves of greenish light that travel along the rows of polyps, so the whole structure pulses rather than glowing steadily, and repeated contact produces repeated waves. The purpose is not settled and the leading explanations are the familiar ones for the phenomenon, either startling a predator or advertising its presence to something larger that might eat it. The effect is bright enough that trawlers hauling them up at night have reported the nets glowing.
The relatives worth separating
The wider group these belong to is easy to confuse with stony corals and the difference is structural. Both are anemone relatives and both build colonies, and the group here has polyps with eight feathery tentacles rather than a multiple of six, and builds a flexible internal skeleton of protein rather than a rigid external one of limestone. That group also contains the soft corals, the sea fans and the precious red coral used in jewellery. None of them build reefs in the way stony corals do, and most lack the photosynthetic partners that tie reef corals to shallow sunlit water.
Where they matter
Fields of them form a recognised habitat on soft muddy bottoms from shallow water to great depths, and that habitat is fragile in a specific way. Because they are anchored in soft sediment and rise well above it, a trawl net passing over flattens an entire field in one sweep, and recolonisation is slow since growth is measured in years. Several jurisdictions list such fields as vulnerable marine ecosystems and close areas to bottom trawling on that basis. The colonies also shelter other species, giving small fish and crustaceans structure to hide in on a seabed that otherwise offers none.
The takeaway
A colony of specialised polyps shares one fleshy stem, with the basal polyp enlarged into a muscular anchor and the rest arranged in feeding rows. Water pumped in raises it and expelling water collapses it into the sediment. Disturbance sends waves of green light travelling along the rows. Fields of them are flattened by a single trawl and take years to return.