Rocky shores
This article describes the habitat of the rocky shores. It is one of the habitat sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an overview about the biota that lives there, the problems and adaptations the habitat is facing with and the importance of it in the marine environment.
Introduction
A rocky shore is an intertidal area that consists of solid rocks. It is often a biologically rich environment and can include many different habitat types like steep rocky cliffs, platforms, rock pools and boulder fields. Because of the continuously action of the tides, it is characterized by erosional features. Together with the wind, sunlight and other physical factors it creates a complex environment. Organisms that live in this area experience daily fluctuations in their environment. For this reason, they must be able to tolerate extreme changes in temperature, salinity, moisture and wave action to survive.
Formation
Sufficient loose or unconsolidated material and a suitable coastal environment to allow the sediments to accumulate are key features in the formation of beaches. The sediments can be terrigenous (land-derived), transported by rivers and streams or through erosion of coastal cliffs. In tropical regions or places with a lack of land-derived material, sediments may be biogenous and consists of broken corals or shells. Beaches on volcanic islands may be black and consists of broken lava and volcanic minerals. Rocky shores are usually steeper than sandy shores. These differences are the result of the different permeability of the sediments and the balance between waves that retreat and come up. Rocky shores have a high permeability and much of the upcoming wave percolates into the bottom. This reduces the outwash of sediment toward the sea. For this reason, coarse sediment can accumulate on the beach and can be transported further on the beach by stronger waves.
Zonation
Each region on the coast has a specific group of organisms that form distinct horizontal bands or zones on the rocks. The appearance of dominant species in these zones is called vertical zonation. It is a nearly universal feature of the intertidal zone.
Supratidal zone
When the tide retreats, the upper regions become exposed to air. The organisms that live in this region are facing problems like gas exchange, desiccation, temperature changes and feeding. This upper region is called the supratidal or splash zone. It is only covered during storms and extremely high tides and is moistened by the spray of the breaking waves. Organisms are exposed to the drying heat of the sun in the summer and to extreme low temperatures in the winter. Because of these severe conditions, only a few resistant organisms live here. Common organisms are lichens. They are composed of fungi and microscopic algae living together and sharing food and energy to grow. The fungi trap moisture for both themselves and their algal symbiont. The algae on the other hand produce nutrients by photosynthesis. Green algae and cyanobacteria can also be found on the rocks of the North Atlantic coasts. They are capable of surviving on the moisture of the sea spray from waves. During the winter, they are found lower on the intertidal rocks. The algae growing higher on the rocks gradually die when the air temperature changes. At the lower edge of the splash zone, rough snails (periwinkles) graze on various types of algae. These snails are well adapted to life out of the water by trapping water in their mantle cavity or hiding in cracks of rocks. Other common animals are isopods, barnacles, limpets,…
Intertidal zone
The intertidal zone or littoral zone is the shoreward fringe of the sea bed between the highest and lowest limit of the tides. The upper limit is often controlled by physiological limits on species tolerance of temperature and drying. The lower limit is often determined by the presence of predators or competing species. Because the intertidal zone is a transition zone between the land and the sea, it causes heat stress, desiccation, oxygen depletion and reduced opportunities for feeding. At low tide, marine organisms face both heat stress and desiccation stress. The degree of this water loss and heating is determined by the body size and body shape. When body size increases, the surface area decreases so the water loss is reduced. Shape has a similar effect. Long and thin organisms dry up much faster than spherical organisms. Intertidal organisms can avoid overheating by evaporative cooling combined with circulation of body fluids. Higher-intertidal organisms are better adapted to desiccation than lower-intertidal organisms, because they encounter more hours of sun. The organisms are exposed directly to the air or they are enclosed in burrows. This results in oxygen depletion, so they can’t get rid of their metabolic waste. A solution for this problem is to reduce the metabolic rate.
The intertidal zone can be divided in three zones:
- High tide zone or high intertidal zone. This region is only flooded during high tides. Organisms that you can find here are anemones, barnacles, chitons, crabs,isopods, mussels, sea stars, snails,...
- Middle tide zone or mid-littoral zone. This is a turbulent zone that is (un)covered twice a day. The zone extends from the upper limit of the barnacles to the lower limit of large brown algae (e.g. Laminariales, Fucoidales). Common organisms are snails, sponges, sea stars, barnacles, mussels, sea palms, crabs,...
- Low intertidal zone or lower littoral zone. This region is usually covered with water. It is only uncovered when the tide is extremely low. In contrast to the other zones, the organisms are not well adapted to long periods of dryness or to extreme temperatures. The common organisms in this region are brown seaweed,crabs, hydroids, mussels, sea cucumber, sea lettuce, sea urchins, shrimps, snails, tube worms,…
Tidal pools are rocky pools in the intertidal zone that are filled with seawater. They are formed where hard or resistant rocks occur and where wave action and other erosional processes have eroded parts of the exposed rocks. This leaves holes or depressions in the substrate where seawater can be collected at high tide. It is important that the rocks are hard enough to survive the regular wave actions, but be sufficiently weak (fractures, bedding differences) to be partly eroded. They can be small and shallow or deep. The smallest ones are usually found at the high intertidal zone, whereas the bigger ones are found near the water. When the tide retreats, the pool becomes isolated. Because of the regular tides, the pool is not stagnant and new water regularly enters the pool. This is necessary to avoid temperature stress, salinity stress, nutrient stress,…
Pools that are located higher on the beach are not regularly renewed by tides. These pools are basically freshwater or brackish water communities. It has different characteristics in comparison with other coastal habitats. Several taxa are more abundant in pools than the surrounding environment. These taxa are members of the algae and gastropods. There is also a difference between high and low located pools for the composition. In low located pools, whelks, mussels, sea urchins and Littorina littorea are common. Periwinkles and Littorina rudis are found in high located pools. Other organisms that are commonly found in pools are flatworms, rotifers, cladocerans, copepods, ostracods, barnacles, amphipods, isopods, chironomid larvae and oligochaetes. Vertical zonation also has been documented in tidal pools.
