This is a plumose anemone Metridium senile, says Matt Doggett. When exposed at low tide, it is unable to retain its shape and shrinks to resemble a soggy, deflated balloon. But when submerged, its feeding tentacles extend and it transforms into a rather beautiful creature.
The species occurs throughout the northern hemisphere, from the extreme lower shore to depths of at least 10om, and attaches itself to the hard surfaces of overhangs, caves, wrecks, pier pilings and boulders.
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It can grow to 30cm in height and is variable in colour. White, orange and dark green are the most common forms, but red and yellow varieties sometimes occur.
When extended, its many tentacles form a large, feathery plume on top of the smooth column, and the animal uses these appendages to filter food carried past on the current. Other prey includes small worms, fish and algal material.
Like many anemones, the plumose moves by creeping along the sea bed. It escapes predators It escapes predators such as sea stars and the sea slug Aeolidia by detaching its basal disc and drifting to a new location.
Reproduction can be sexual ot by 'basal laceration', whereby fragments of the base break off and develop into new anemones.