The littoral zone or rocky shore is the area between the highest and lowest limit of the tides. It is a unique environment, both terrestrial and marine as the tides transition from high to low. This provides extreme conditions for a vast array of plants and animals that live here. Hundreds of millions of years of evolutionary change has allowed these organisms to adopt a multitude of adaptations to cope with these extremes. The distribution of organisms is determined by a variety of interacting factors. These may be the non-living (abiotic) or the living (biotic) components of the marine ecosystem. A combination of these limiting factors determines the zones or bands of species that we see on the rocky shore. One of the clearest observations of this is bands of different species of brown seaweeds when moving from the upper to the lower shore.
Light does not travel as easily through water as it does the atmosphere. The harmful effects of radiation are counteracted by creatures having a form of hard shell. Seaweeds at depths receive less light for photosynthesis and some species have adapted to be buoyant near the surface. Red light which is most useful for photosynthesis does not penetrate much more than 5m into the sea.
Organisms living on the rocky shore are exposed when the tide goes out and this can lead to dehydration or dessication. There is a greater potential for water loss higher up the shore. An upper-shore master of survival is the Channelled Wrack (Pelvetia Canaliculata) which can survive 95% loss of its body water and return to thrive again from a shrivelled state. The Strawberry Anemone (Actinia fragacea) can retract its tentacles to form a blob that has a reduced surface area. Combined with a mucous secretion this means it can survive out of the water during low tide. Other organisms can cope with this stress by having waterproof shells with spaces that retain moisture and by hiding in microhabitats such as crevices and under rocks.
Waves can remove rocky shore organisms and affect their distribution. A shore that receives a lot of wave action is described as exposed and one that does not is sheltered. Creatures like limpets and Dog Whelks can vary in shape as an adaptation to the amount of wave action that the shore receives. Whelks on sheltered shores tend to have a much narrower opening which makes it harder to be removed from the shell by predators such as crabs. On a more exposed shore the larger opening allows more room to house the muscular ‘foot’ that gives it firm gripping on wave battered shores. Floating air sacs on Bladder Wrack increase the surface area of the seaweed’s fronds and its potential to be ripped from rocks by the waves. Bladders on this seaweed tend to be smaller, less in number or absent on exposed shores where there is more wave action. Limpets exposed to more wave action tend to be smaller and more conical. The shell is tall and narrow due to repeated muscular contractions of the ‘foot’ to maintain position on the rock. On sheltered shores they are generally more flattened.
Most rocky shore animals are ectothermic, meaning they cannot regulate their body temperature but rely on the external temperature. The further up the shore the greater the temperature fluctuations that organisms will be subjected to. Air temperature on bare rock can fluctuate from -12 to 30 degrees Celsius. Creatures like limpets are particularly sensitive to temperature when it comes to spawning, which generally occurs when temperatures drop below 11 degrees Celsius and waters are rough.
If a marine creature is exposed to fresh water they will absorb water and swell up. If the rock pool water becomes too concentrated with salt due to evaporation on a warm day, then water is lost from the creatures body. Some animals are adapted to be able to tolerate changes in their tissues and are called osmoconformers. Osmoregulators such as the Common Shore Crab can avoid changes in tissue salinity by moving ions in and out of its body. It can survive inland in estuaries and where rivers enter the sea. Green seaweeds, particularly Ulva species are able to cope with reduced salinity in locations where a river enters the sea. In Charmouth this is evident at low tide where the river Char flows into the sea near the Heritage Coast Centre. It is also likely that these species thrive due to reduced grazing pressure from herbivores that cannot tolerate the freshwater environment.
Geology and topography
The exposure of the shore to wave action is affected by its shape. Slopes, platforms and outcrops of rocks can provide shelter on what is otherwise an exposed area of coast. The geology will determine the shape of the shore but the rock type can also influence what communities can attach to the substrate. Stable, non-erosive rock can sustain barnacles, limpets, mussels, snails and algae. Softer rock like chalk, limestone and shale may have more limpets and short-lived green algae. Soft rock can also favour rock-boring animals such as the Piddock which is prevalent in the shale of the Jurassic Coast. At Charmouth there is a large population of Honeycomb Worms (Sabellaria alveolata) on the rocks. These reef-building worms create tubes from sand and shell fragments and create a rare habitat listed on the UK Biodiversity Action Plan.
Lower down the shore as more time is spent underwater the abiotic environment becomes more favourable. On the flip side there is greater diversity of species and hence more competition for space, resources and enhanced predation. Bladder Wrack (Fucus vesiculosus) prevent young sporelings from developing beneath it by blocking out light and their fronds ‘sweep’ the rock to prevent the establishment of other organisms. Two species of barnacles compete for habitat space. The Acorn barnacle (Semibalanus balanoides) is a northern species which extends as far south as Britain. Chamalus barnacles are generally mediterranean with Britain being their most northerly range. The northern species does better in cooler areas further down the shore but the southern species can survive further up the shore with high summer temperatures. Where the range overlaps Chthamalus grows faster with bigger calcareous plates and competitively excludes Semibalanus.
Rocky shore creatures are exposed to predators on two fronts. When the tide is in they are hunted by animals like crabs and fish. When the tide is out there are birds and people to contend with!