Oyster Thief (Colpomenia peregrina)

This is a thin walled brown algae with broad semi spherical thallus (algal body). This free-floating specimen was found in a rock pool having been torn off its "moorings". The algae look like shower caps with puckered edges.
These spherical algae are usually epiphytic on other algae or grow on oysters or mussels attaching themselves with filamentous growths. The sphere is inflated with gas and when there is enough gas, the algae floats away with the oyster that it is attached to - hence the name "oyster thief" - an interesting story !
Natural to the Pacific ocean, they spread through Europe in oyster catches and now are found as far south as the southern Australian Coast.