Bladder wrack (Fucus vesiculosus, centre) with other strands of dead seaweed, washed up on a shoreline. The air bladders (round) keep this seaweed buoyant and near the surface of the water when the tide is in, so that photosynthesis can take place. Seaweed is a marine alga, some species of which float freely in the sea while others remain attached to the shore. Photographed in November, on the Isle of Skye, off the coast of Scotland.