Rocky shore habitat
This article describes the habitat of rocky shores in a tidal environment. It is one of the habitat sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an introduction to the type of biota that lives there, the problems and adaptations the habitat is facing with and the importance of it in the marine environment.
- 1 Introduction
- 2 Zonation
- 3 Stresses and adaptations
- 4 Why are rocky shores important?
- 5 Appendix Habitat classification of sea cliffs
- 6 Related articles
- 7 References
Rocky intertidal areas are a biologically rich environment that can include several distinct habitat types like steep rocky cliffs, platforms, rock pools and boulder fields. Because of the permanent action of tides and waves, it is characterized by erosional features. Together with the wind, sunlight and other physical factors it creates a complex environment, see Rocky shore morphology.
Organisms that live in this area experience large 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.
Because the physical conditions and associated stresses differ greatly for different elevation zones, there are also major differences in the species composition for different elevation zones. Distinct horizontal bands or zones on the rocks are populated with specific groups of organisms; this is called vertical zonation. It is a nearly universal feature of the intertidal zone.
The upper regions around the high-tide mark are exposed to air during most of the time. The organisms in this region are subject to severe stresses related to respiration, desiccation, temperature changes and feeding. This upper region is called the supratidal or splash zone. It is moistened by the spray of breaking waves and it is only covered during the highest tides and during storms. Organisms are exposed to the drying heat of the sun in the summer and to low temperatures in the winter. Because of these severe conditions, there are only few species that can cope with these extreme conditions. Common organisms are lichens. They are composed of fungi and microscopic algae living in symbiosis and sharing food and energy for their growth. The fungi trap moisture for both themselves and their algal symbiont. The algae on the other hand produce nutrients by photosynthesis. They are capable of surviving on the moisture of the sea spray from waves. During 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 adapted animals are isopods, barnacles, limpets,…
The intertidal zone or littoral zone is the shoreward fringe of the seabed 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, organisms living in this zone are subject to stresses related to temperature, desiccation, oxygen depletion and reduced opportunities for feeding. At low tide, marine organisms face both heat stress and desiccation stress. The degree of heating and water loss is determined by the body size and body shape. When the body size increases, the surface area decreases so the water loss is reduced. Shape has a similar effect. Long and thin organisms dry out 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 have evolved in an environment more exposed to the sun. Normally, respiration rates increase with temperature and so does the oxygen demand. However, marine organisms exposed to the air cannot feed or carry out gas exchange with seawater, so normal rates of aerobic respiration cannot be sustained. Therefore these organisms have evolved physiological mechanisms to tolerate a wide range of body temperatures, for example by reducing their metabolic rate (see the section on adaptation).
The intertidal zone can be divided in three zones:
- High tide zone or high intertidal zone. This region is only flooded during high tides. You can find here organisms such as anemones, barnacles, chitons, crabs, isopods, mussels, sea stars, snails,...
- Middle tide zone or mid-littoral zone. This is a turbulent zone that is dried 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 by abrasion and weathering of less resistant rock and scouring of fractures and joints in the shore platform. This leaves holes or depressions where seawater can be collected at high tide. 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 in the lower intertidal zone. When the tide retreats, the pool becomes isolated. The water does not remain stagnant, because new water enters the pool when the tide rises. 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 in composition between high and low located pools. Low-located pools are home to whelks, mussels, sea urchins and the common periwinkle (Littorina littorea). Rough periwinkles (Littorina rudis) are found in high located pools. Other organisms that are commonly found in pools are flatworms (polycladida), marine worms (oligochaetes), rotifers, water fleas, small crustaceans (copepods, ostracods, amphipods, isopods), barnacles, and larvae of flies (chironomids). Vertical zonation also has been documented in tidal pools.
The subtidal zone or sublittoral zone is the region below the intertidal zone and is continuously covered by water. This zone is far more stable than the intertidal zone. There are no strong fluctuations in temperature, water pressure and sunlight radiation. Organisms do not dry out as often as organisms higher on the beach. They grow faster and are better competitors for the same niche. They extract essential nutrients from the water and do not need to cope with extreme changes in temperature.  
Stresses and adaptations
In this section, stresses and adaptations are discussed in more detail. The regular strong fluctuations in environmental conditions imply that organisms have to be tolerant to the associated stresses, in particular stresses related to temporary aerial exposure. Adaptations are solutions to deal with these stresses and are necessary to survive.
Most intertidal animals depend on aerobic respiration by extracting oxygen from water. An exception are some limpet species that live high on the shore and that have a mantle cavity adapted to breathe air, similar to a lung. Other intertidal animals have gills and cannot tolerate prolonged air exposure. Since gills only function when they are moist, these animals need to avoid desiccation. In response to desiccation stress, some sessile species (periwinkles) have adapted their gills to allow gas exchange with the air. Other species (barnacles) store air bubbles in cavities in the gills that supply oxygen to the moisture around the gills. The main adaptation strategy of sessile animals to prolonged air exposure is to slow down their metabolism and associated oxygen consumption; some animals (snails) can temporarily switch to anaerobic respiration. Mobile animals (crabs, chitons) mainly adapt by moving with the tide to stay underwater.
Temperature differences can be very large in the intertidal zone. Most marine animals are ectothermic, that is, they cannot regulate their body temperature, but depend on the ambient temperature. As a result, they cannot tolerate large temperature differences. In water, temperature changes are buffered, but in the air, animals can be exposed to very cold or very hot temperatures. Especially animals with a small body weight have a hard time.
In most animals the metabolism accelerates at high temperatures and thus also the oxygen demand. However, in the intertidal area the animals can hardly absorb oxygen when the tide is low. One way of adaptation is regulation of the membrane fluidity (homeoviscous adaptation). At high temperatures, the fluidity increases, the saturated fatty acids decrease and thus the rates of metabolism and respiration. The opposite happens at low temperatures. Another adaptation to harmful high ambient temperatures is the production of heat shock proteins (HSP). These proteins, which protect important enzymes against heat damage, are produced by many intertidal molluscs such as mussels, limpets, top shells and periwinkles. Many intertidal animals can tolerate much greater temperature changes than their estuarine relatives. Possible adaptations are also light colors to reflect light or a large surface (ribbed shells) to dissipate heat. However, when cooled by evaporation, desiccation can lead to problems.
When the temperature is too low, the organisms must cope with physiological threats associated with cold stress. This can be the case in polar and temperate latitude coastal zones. The body fluids can then reach their freezing point and ice crystals develop. This causes damage to cell membranes and increase of the osmotic concentration of the nonfrozen fluid. Some organisms have developed antifreeze proteins (cryoprotectants). Increase of the concentration of osmolytes such as glycerol and sucrose in the body fluids increases the freezing tolerance. Another strategy is to control formation and spread of internal ice crystals. When the ice formation is intracellular, it is lethal but extracellular ice formation can be tolerated. Invertebrates found naturally in seawater of high salinity are more cold-tolerant than specimens inhabitating brackish waters. In molluscs, the cold tolerance can be increased by acclimating the animals to higher salinities. This is probably based on increased concentrations of intracellular solutes such as amino acids.
Mobile organisms can avoid extreme temperatures by migrating to more suitable places; this is also a response to other stresses associated with emersion.
Dehydration is the main environmental factor in the supralittoral and high intertidal zones, and the green macroalgae living in these zones are exposed regularly to air, yet still survive. Desiccation tolerance can be defined as the ability to survive drying to about 10% remaining water content. Dehydration-tolerance involves maintaining homeostasis during dehydration by minimizing or repairing any damage as fast as possible. Highly mobile organisms can avoid the desiccation by migrating to a region that is more suitable. Less mobile organisms restrict various activities (reduced metabolism) and attach more firmly to the substrate. Physiological features to tolerate water loss include adaptations such as: deployment of desiccation-resistant egg cases for embryonic development, reduction of the exposed surface areas across which water loss takes place (thus accepting reduced gas exchange and concomitant anaerobic respiration with accumulation of metabolic end products), temporary depression in metabolic and developmental rates, maintenance of intracellular osmolytes for water retention and macromolecular protection and differential gene expression for the production of protective macromolecules. Some sessile organisms can anticipate emersion by storing water in body cavities (e.g., anemones) or mantle cavities (e.g., barnacles, mussels) .
Many intertidal animals have a biological clock that allows them to anticipate changes as a result of tides (circatidal rhythmicity) or light (circadian rhythmicity). Different signals play a role in the setting of endogenous rhythmicity in some crustaceans and crabs: water agitation, hydrostatic pressure, immersion, light and temperature cycles. Once trained after a few tidal periods, the rhythmicity is maintained. Thanks to the biological clock, the animals can adapt in time, instead of waiting for an adverse situation to arise.
Sunlight is another parameter that influences the organisms. When there is too much sunlight, organisms dry out and the capacity to capture light energy can be weakened. The light that is not used or dissipated can cause damage to subcellular structures. Algae can protect themselves against an excess of sunlight by so-called non-photochemical quenching (NPQ): the light energy absorbed by the chlorophyll is dissipated in the form of heat or in the form of fluorescence. NPQ is a quick and effective way to prevent damage from excess sunlight. There are also several other mechanisms, such as scavenging or deactivating free radicals produced from an excess of light. Too little sunlight reduces the growth and reproduction of the organism, because photosynthesis is reduced.
Intertidal zone organisms can be subjected to varying salinity, especially those living in pools that are not regularly refreshed with new seawater. Rain can cause the salinity to drop and evaporation can cause the salinity to rise. Changes in salinity change the osmotic pressure in the cells of the body tissues, causing them to swell or shrink (see Osmosis). Organisms living in estuaries have adaptations to deal with this, such as adaptation of the cell membrane, salt storage in vacuoles or glands to secrete salt. However, most intertidal organisms are osmoconformers: they cannot control the salt content of their body. In some species (e.g., periwinkle), the salinity of their tissues is similar to that of normal seawater, which is the environment that they evolved in and are adapted to. Most intertidal organisms adapt to salinity variations by producing organic osmolytes that keep intracellular fluids at the same pressure as the marine environment to avoid cell shrinkage or dilatation.
A wide variety of strategies to escape from predation exists. The first strategy is calcification, which makes it more difficult for the predator to eat these organisms. This strategy is applied by algae. It makes them tougher and less nutritious. A second one is the production of chemicals, usually produced as secondary metabolites. These (toxic) chemicals can be produced all the time, but other chemicals are only produced in response to stimuli (inducible defence). Another way to avoid predation is to have two distinct anatomical forms within one life cycle. This can be e.g. an alternation between a crusty form when the predator is present and a more delicate form (e.g. blade) when the predator is absent. Also the shape of the body can be a distinct evolutionary advantage. Bioluminescence is another strategy to avoid predators. Many intertidal and subtidal predators forage visually. The light is used for warning, blinding, making scare, misleading or attracting the predator. A commonly used form of protection against predation is camouflage. This can be visually or chemically. Visual camouflage means that the prey becomes invisible to the predator by using the same colors as the environment. Chemical camouflage is the passive adsorption of chemicals. The predator does not smell the prey anymore, because the smell is masked. To escape seabird predation, some animals (periwinkles, chitons and apex shells) can hide in inaccessible crevasses or between seaweed. Others, such as beach crabs, bury themselves in the sediments that often accumulate under rocks.
One way to protect organisms from waves is permanent attachment. But this strategy cannot be used by organisms that have to move to feed themselves. These organisms make a compromise between mobility and attachment. Attachment can be done by different structures. Bivalves usually use threads (byssal threads) to attach to rocky surfaces or to other organisms, but they can also use a foot. Another one is cementation. This is the case for bivalves such as oysters, scallops and some other forms. They lay on their side, with the lower valve cemented firmly to the bottom. This can be combined by reduction or enlargement of certain muscles. Another way to be protected from waves is to burrow into the sediment or seek shelter, such as a crevasse.
Why are rocky shores important?
- They are home to many organisms
- They provide a nursery area for many fish and crustacean species
- They provide shelter in areas where seaweeds reduce the wave power
- They provide food for fishes
- Algal beds are an important food source for rare and threatened species like sea turtles
- The are a feeding ground at low tide for wading birds
- They protect the hinterland
Appendix Habitat classification of sea cliffs
In the habitat classification used by the European Union  there are four cliff types defined by the vegetation and their geographical location all considered to be composed of 'Hard' rock:
- 1230 Vegetated sea cliffs - Atlantic & Baltic, PAL.CLASS.: 18.21
- 1240 Vegetated sea cliffs - Mediterranean with endemic Limonium spp., PAL.CLASS.: 18.22
- 1250 Vegetated sea cliffs with endemic flora of the Macaronesian coasts, PAL.CLASS.: 18.23 and 18.24
- 4040 * Dry Atlantic coastal heaths with Erica vagans, PAL.CLASS.: 31.234
'Soft' rock sea cliffs are not classified although they can be considered to be included in 1230 above.
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