Oyster thief

Looking suspiciously like  plastic waste, these semi-deflated, olive green bubbles  bob around on the strand line as the tide surges in and out.
They are a form of seaweed that arrived in the oyster beds of France  from  the USA in the late 19th century and thence spread to SW England in 1907. It's now found all around the coast of Britain and Ireland. It does no harm but its curious name is a literal description of its potential larceny where are oysters are concerned. If it attaches to shellfish it can, by reason of its buoyancy,  lift and float away with them,  although not, I think, with any malicious intent. However at one time it did some economic damage to oyster farming. Maybe the oysters were happy though.