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Between the Tides

Of all marine habitats, it is the narrow zone where the sea meets the land, the intertidal region, which is most familiar to us. The tide goes out twice daily and reveals whole communities of marine organisms which we can see without even getting our feet wet.

Rocky shore habitat usually occurs on exposed coastlines, such as headlands, where wave action is strong and removes any loose sediment. The bedrock provides a hard surface for many seaweeds to attach to and they can thrive in the presence of sunlight and nutrients from the sea. Rocky areas are also inhabited by many sessile animals which adhere to the rocks and exploit the massive food supply the sea provides, filtering the water for suspended particles and tiny organisms. These animals include, for example, sea anemones, sea squirts, sponges and mussels. The attached life in turn attract mobile creatures such as crabs, starfish, snails and fish which feed on them.

Problems of Survival

Despite the abundance of food in the intertidal zone, it is actually a very stressful environment to live in. The daily movements of the tide expose the marine organisms to the threat of desiccation, drying up when the tide is out. Rocky shore inhabitants have adapted to cope with this problem in a variety of ways.

Mobile animals such as fish, shrimps, crabs, sea urchins and starfish can move to rock pools in the lower shore and hide beneath shady seaweed or stones. However, rock pools are also potentially stressful. If the air temperature is warm they quickly heat up and evaporation increases the salinity. Conversely, rain reduces salinity in the pools. Rock pool creatures are very resistant but can also move to find another pool - for example the blenny, a small fish, may use its pectoral fins to crawl over the rocks.

Sessile animals cannot move to find better conditions and must stick them out until the tide returns. Here are some examples of how they cope:

Limpet" Limpets clamp down on the rock, always in the same place, eventually forming a depression which exactly fits their shell-shape and seals against the rock. They also retain water within the shell.

" Periwinkles and other marine snails can close up their shell to protect them against desiccation, with a trap door, or operculum, which draws across the opening of the shell.

" Barnacles live within a calcified shell made up of a number of plates which also has an operculum to allow moisture to be retained inside.

" Mussels group together on the rocks, attached by means of byssus threads at one end of the shell. They close up to avoid drying out.

" Beadlet Anemones close up their tentacles to hold in water, becoming jelly-like blobs on the rock.

Patterns on the Rocky Shore

The position of plants and animals along the intertidal zone is far from random - there are distinct patterns in the distribution of certain species. The causes of these zonation patterns have been thoroughly investigated and were previously thought to be the result of the organisms' ability to survive the changes in tide level and extended periods out of water. However we now know that the distribution of species is governed not only by physical factors, such as the length of time species are left out of water, but also by biological factors, or the interactions between other species on the shore.

Out of Water

The hardiest organisms, able to survive longer periods exposed to the air, tend to be found higher up the shore in the drier areas. Thus the rocky shore community is divided into distinct bands characterised by certain species.

Right at the top, just below the land plants, there is a 'splash' zone of black, yellow and grey lichens. These plants are rarely covered by the tides, but are frequently splashed with salt water by waves.

Below the lichens there is usually a greyish or white band of barnacles, and these may extend down over most of the mid-shore. On rocky or stony shorelines, various types of tough brown seaweeds, called wracks, grow alongside the barnacles, and may largely replace the barnacles on very sheltered rocky shores.

Channelled wrack grows near the high tide mark, and may go for several days without being covered by seawater. The channel in its frond usually faces downwards, helping it to hold on to its moisture, and to prevent it from drying out.

Bladder wrack(22) and egg or knotted wrack, come next down the shore, and nearer the low tide mark grows serrated wrack, with a saw-like edge to its frond.

The upper limit of each band is set, therefore, by the longest amount of time the organisms can survive out of water. Since living lower on the shore would obviously be preferable to organisms (being covered by water allows prolonged feeding time), it was established that the lower limit of a species' distribution must be set by biological factors.

Competition and Predation

Space is a premium on the rocky shore and organisms which can grow fastest and compete successfully for space will crowd out other, slower growing species. For example, mussels are usually dominant in the middle shore. They establish themselves in large clumps very quickly and force other organisms, like barnacles, higher up the shore where mussels cannot survive. The lower limit of barnacle distribution on the shore is therefore dependent on the mussels' uppermost limit.

Mussels are the favourite food of starfish. Predation by starfish keeps mussel numbers in check and prevents them from taking over the whole community. However, starfish distribution is also set by the tide, and this corresponds to the lower limits of mussels. Areas with fewer starfish are generally mussel zones.

Rocky shores of the Moray Firth

The majority of rocky shores occur on the outer, more exposed coasts from Duncansby Head to Brora, the Tarbat Ness peninsula and from Burghhead to Fraserburgh. There are a few sheltered rocky sites protected by harbours such as Wick and Macduff, or where littoral rocky areas are very wide as at East Helmsdale, Cullen and Pennan. Generally, there is a restricted range of rocky intertidal habitats available and for this reason the rocky shore flora and fauna of the east coast of Scotland is considerably less rich in species than the west coast.



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