A seaweed of the upper shore, gutweed is able to tolerate very low salinities. Both common and Latin names come from the fronds, which are, in fact, tiny tubes. These inflate with the oxygen of photosynthesis and float in rock pools like curled intestines. It’s a rather boorish seaweed, fast-growing under normal conditions; its growth becomes overwhelming when the sea is polluted with nitrogen and phosphorous compounds. At Sandy Bay in south Devon, some years ago, I discovered that its mile of very wide sandy beach had disappeared completely beneath an 8in layer of this seaweed. The council had begun bulldozing it into several seaweed mountains, which produced no-go areas on the beach because of the impressive (and lethal) smell of hydrogen sulphide. Actually, there is another, nicer, smell associated with Ulva species—that of truffle. The chemical common to both is dimethyl sulphide. It is of critical importance for regulating climate by controlling cloud formation and most of it comes from seaweeds.