Bladder wrack seaweed Fucus vesiculosus growing in the middle zone of the shore on Washing Ledge, Kimmeridge Bay, Dorset. The name of the seaweed derives from the paired air bladders visible on either side of the midrib of the fronds. The air bladders keep the seaweed blade buoyant and near the surface of the water when the tide is in, so that photosynthesis can take place. Seaweed is a marine algae, some species of which float freely in the sea & some, as here, are attached to the shore.