oyster thief

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oyster thief The common name for the brown seaweed Colpomenia peregrina (order Scytosiphonales). The thallus is a thin-walled, hollow sphere up to 20 cm in diameter; it is yellowish-green to olive-brown, and smooth but not shiny. It grows attached to seaweeds or shells on the middle to lower shore. The common name derives from the fact that at low tides the spheres tend to fill with air and act as floats, raising from the sea-bed any oysters that are attached to them.