Bladder wrack seaweed (Fucus vesiculosus) thallus in a rocky shore. 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 while others, as here, remain attached to the shore.