No one who has been to the seaside can have failed to see well the known bladder wrack seaweed which is very common around the shores of Britain but there are actually five species of wrack seaweeds and, whilst bladder wrack is the most common, it is often the spiral wrack (Fucus spiralis) that one is more likely to see.
Bladder wrack gets its name from the air filled bladders that form on its fronds and one might think that spiral wrack has bladders too at the ends of its fronds but it has not. What apppear to be bladders at the tips of the fronds are, in fact, the fruiting bodies which are full of a jelly-like substance not air.
Each of the wracks has a tendency to grow in different habitats, some grow in the upper tidal reaches and so are dry for much of the time, others like the lower levels and to be covered in water most of the time and the bladder wrack grows midway between the two and needs the air bladders for flotation being under water for much of the time. Sprial wrack, however, grows at the high water level, is covered less frequently than bladder wrack and so has no need for the bladders.Fucus spiralis: the spiral wrack