Conflicting constraints on the form of intertidal algae

Plants that live astride the margin of the sea seem to inhabit the worst of all worlds. Seaweeds are, as their name implies, true inhabitants of the ocean, yet on the shore the water is an intermittent visitor and when the tide withdraws they are exposed to the full rigours of life in air. Even when the water returns, bearing moisture and nutrients, it comes armed with waves. Algae in exposed sites may experience wave velocities of 14–15 m s-1 (Denny, 1988; Jones & Demetropoulous, 1968). Allowing for the greater density of water, this is equivalent in its effects to a wind speed of almost 430 m s-1 (950 miles h-1) on land. No oak tree could withstand the storms of the sea. It is how the seaweeds have developed a structure that does not merely survive, but flourishes under the rigours of intertidal life, that I will explore in this paper.