When high tides aren’t very big, plants and animals which live high on the shore may be exposed to air for several days. Organisms which live very low on the shore may only occasionally be exposed to air.
Rocky shores as ecosystems
Many animals and plants live on rocky shores in the area between high and low tide called the intertidal zone. These organisms must be able to cope with problems of not one environment, but two. They are pounded by waves, exposed to extremes of temperature and salinity, and flooded by sea water and exposed to drying air twice every 24 hours. They also have to avoid being eaten by birds, molluscs and crabs at low tide, and by fish and other marine life at high tide.
Several distinct habitats exist in rocky shores, each with its own survival challenges for plants and animals living there.
Platforms
These are formed when waves, wind and rain carve rock into flat platforms. Often, the back of the rock (the bit which hasn’t been eroded yet), forms a cliff, while the ocean edge of the platform steps down into the water. This means one rock platform can support many different kinds of plants and animals, because some sections are almost always under water, while other parts are usually dry.
Rock pools
These are usually formed when a boulder lodges in a depression in the rock and grinds a hollow as it rolls around in the waves. After some time, the depression becomes deep enough to hold water during low tide. If the boulder stays in the pool it will gradually grind it deeper, but sometimes, a big wave washes the boulder out and the rock pool stays shallow.
Because pools trap grit, stones and boulders, only certain plants and animals can survive in them. The grit smothers some organisms, while stones and boulders rolling around in storms can smash delicate creatures.
Boulder fields
Groups of boulders are found where little wave action occurs, and the shore’s not too steep.
During storms, the boulders roll around and flip over, smashing any animals living on their underside or the rock bottom. Animals and plants which were on top of the boulder may find themselves having to cope on the bottom, in the dark and permanently under water. And anything which lived on the bottom will be exposed to air, sunlight and heat.
Since sand gathers inside boulder fields, abrasion increases, smothering some plants and grinding others.
Nature’s survival tricks
Special adaptations enable animals and plants to live in these conditions.
Many animals avoid sun, drying air and predators such as birds, by staying in cracks, under rocks or in their own burrows at low tide. Some sessile animals such as barnacles and oysters close their valves tightly to avoid drying when the tide goes out. They come out to feed when covered by water.
Others may be well camouflaged and appear to be invisible.
Organisms which are constantly pounded by waves are often very tough, so it doesn’t matter if they’re hit hard. Others are flexible or flat, so they bend instead of breaking when they’re hit, or don’t get hit at all.
Rocky shore dwellers
Seaweeds
Most plants found on rocky shores are seaweeds. They’re algae, which means they can live on hard surfaces where plants with roots wouldn’t be able to survive. Instead of roots, they have special suckers called ‘holdfasts’ which cling to rock, even in big waves. They don’t have flowers, or normal stems or leaves. The bit that looks like a stem on some seaweeds is called a ‘stipe’. This joins the holdfast to the ‘frond’, the leaf-like part. Seaweeds are mainly green, red or brown, depending on which wavelength of sunlight they’re trying to trap. Not all seaweeds have long, floaty fronds. The fronds can be tiny, so the seaweed looks like velvet covering the rocks. Other may look like tiny cabbage leaves.
