Encyclopedia of Ecology || Rocky Intertidal Zone
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<ul><li><p>Ecosystems | Rocky Intertidal Zone 3107Rocky Intertidal ZoneP S Petraitis and J A D Fisher, University of Pennsylvania, Philadelphia, PA, USA</p><p>S Dudgeon, California State University, Northridge, CA, USA</p><p> 2008 Elsevier B.V. All rights reserved.Introduction</p><p>Physical Aspects of the Shore</p><p>Attached Organisms</p><p>Mobile Organisms</p><p>ZonationFigure 1 Closeup of predatory snails, mussels, barnacles, andbrown algae in Maine, USA. Photo by P. S. Petraitis.Rocky Intertidal Shores as an Important System in</p><p>Development of Ecology</p><p>Unresolved Problems and Future Directions</p><p>Further ReadingIntroduction</p><p>The British ecologist A. J. Southward described the</p><p>intertidal zone as the region of the shore between</p><p>the highest level washed by the waves and the lowest</p><p>level uncovered by the tide, and thus communities on</p><p>rocky intertidal shores are primarily defined by the</p><p>tides and the presence of hard surfaces. The types of</p><p>organisms, the number of species, and the distribution</p><p>and abundance of individual species found in a parti-</p><p>cular rocky intertidal community also depend on the</p><p>physical aspects of the shore, the supply of resources,</p><p>food and larvae from overlying water, the biological</p><p>interactions among the species present, and the regio-</p><p>nal pool of species. Although rocky intertidal shores</p><p>cover only a small fraction of the Earths surface, they</p><p>contain a large diversity of organisms ranging from</p><p>highly productive microalgae to transient vertebrate pre-</p><p>dators (Figure 1).Physical Aspects of the Shore</p><p>Tides</p><p>Tides are caused by the gravitational effects of the Moonand Sun, which ideally produce a cycle of two high tidesand two low tides per day. However, the amplitude andfrequency of the tides are altered by the phases of theMoon, the Earths orbit and declination, latitude, and theconfigurations of the shoreline and the seafloor. The tidalrange tends to be smaller toward the equator and can varyfrom several meters in high latitudes to less than tens ofcentimeters near the equator. Configuration of the coastand the ocean basin can cause harmonic resonances andcreate tides that vary dramatically in amplitude and fre-quency. In extreme cases, the reinforcing and cancelingeffects can produce a single high and low tide per day oralmost no change over the course of a day.</p><p>The timing of low tides can have a profound effect byexposing organisms to extreme conditions. For example,the lowest tides in the Gulf of Maine, USA tend to occurnear dusk or dawn, and so organisms are rarely exposed tomid-day sun in the summer but are often exposed tobelow freezing temperatures on winter mornings. In con-trast, the lowest summer tides in southeastern Australiaoccur mid-day and expose organisms to extraordinarilyhigh temperatures.Characteristics of the Shore</p><p>Any firm stable surface in the intertidal zone has thepotential to support the organisms that commonly occurin rocky intertidal communities, and at low tide, intertidalhabitats can range from dry rock to filled tide pools. Rocksurfaces can vary from very hard to relatively soft rocksuch as from granite to sandstone and can range fromsmooth platforms to irregular fields of stone cobbles andboulders. Topography, inclination, color, and texture ofthe rock affect rate of drying and surface temperature,which can limit the distribution and abundance of species.</p></li><li><p>Figure 2 Extensive brown algal beds in Maine, USA. Photo byP. S. Petraitis.</p><p>3108 Ecosystems | Rocky Intertidal ZoneMan-made surfaces such as rock jetties and wooden pierpilings and biogenic surfaces such as mangrove roots canalso support communities that are indistinguishable fromthe communities found on nearby rocky shores.</p><p>Tide pools can be very different than the surroundingshore because of thermal variability, changes in salinityfrom evaporation and runoff, and changes in pH, nutri-ents, and oxygen levels caused by algae. Pools oftensupport residents such as sea urchins, snails, and fishthat would otherwise be restricted to subtidal areas.</p><p>The amount of wave surge affects the types of organ-isms found on the shore and their distribution. Wavesurge and breaking waves tend to expand the extent ofthe intertidal zone and distribution of species by continu-ally wetting the shore and allowing species to extendfarther up the shore. Wave surge can also cause mobileanimals to seek refuge and can limit the distribution ofslow moving species, and the force of breaking waves candamage and sweep away organisms. Sand and debris suchas logs swept up by the waves can scour organisms off thesurface. In areas of low wave surge, sedimentation of sandand silt may bury organisms or clog gills and other filter-feeding structures.Attached Organisms</p><p>Unlike terrestrial habitats, which depend largely on localplant material to support resident animal populations,rocky intertidal assemblages are supported not only byalgal primary production but also by secondary produc-tion from suspension feeders, such as barnacles andmussels, which link the oceans productivity to the shore.Algae</p><p>The term algae refers to an extraordinarily diverse andheterogeneous group comprising about seven majorlineages, or roughly 41% of the kingdom-level branchesin the Eukarya domain. Most lineages consist of unicellularmicroalgae, but the multicellular macroalgae that dominatemany rocky shores worldwide occur in only three groups(Rhodophyta, Chlorophyta, and Phaeophyta) (Figure 2).</p><p>Microalgae are ubiquitous and although inconspicu-ous, they are important members of rocky intertidalcommunities. For example, diatoms are the primaryfood source of many grazing gastropods and form bio-films, which facilitate settlement of invertebrate larvaeand stabilize meiofaunal assemblages.</p><p>Benthic macroalgae (i.e., seaweeds) dominate manyrocky shores, especially the low- and mid-intertidalzones of temperate regions, and many exhibit morpholo-gies adaptive for life on wave swept shores. The idealizedbody plan of a seaweed consists of a holdfast, a stipe, andone or more blades. The holdfast usually attaches the algaeither by thin encrusting layers of cells tightly appressedto the rock surface or by a massive, thick proliferation oftissue that often produce mucilaginous glues to adherethe tissue to the rock. The stipes are analogous to plantstems and display remarkable material properties thatenable seaweeds to withstand the tremendous hydrody-namic forces imposed by breaking waves. The blade is theprincipal structure for the exchange of gases and nutri-ents, and the capture of light for photosynthesis. Bladesalso contain reproductive tissue, either within a vegeta-tive blade, or in sporophylls (i.e., special blades forreproduction). Some larger brown seaweeds, such asfucoids and kelps, have gas-filled floats called pneumato-cysts that buoy the blade so that it remains closer to thesurface where light intensity is greater.</p><p>The diversity and complexity of the life cycles of mostseaweeds contributes to their great abundance on rockyshores. The life cycle of most seaweeds consists of analternation of separate gametophyte and sporophytegenerations. The two generations can either look thesame (i.e., isomorphic) or different (heteromorphic). Insome species, the heteromorphic generations are so differ-ent that they were originally described as different species.Heteromorphic life histories are hypothesized to representan adaptation to grazing pressure, and heteromorphic gen-erations clearly show tradeoffs with respect to competitiveability, resistance to disturbance and longevity associatedwith upright foliose and flat encrusting morphologies.Sessile Invertebrates</p><p>Adults of many invertebrate species are attached perma-nently to the rock or other organisms (epibiota). Theseinclude members of the phyla Porifera (sponges), Cnidaria(hydroids and sea anemones), Annelida (tube-buildingpolychates), Arthropoda (barnacles), Mollusca (mussels</p></li><li><p>Ecosystems | Rocky Intertidal Zone 3109and clams), Bryozoa (moss animals), and Chordata (tuni-cates). Suspension feeding either by pumping waterthrough a sieve structure or trapping particles carried oninduced or external currents is a common feature ofsessile animals and serves to transfer inputs of energy andnutrients produced in the water column into the intertidalzone via the ingestion of plankton. Additionally, by feedingon locally derived detritus, suspension feeders capture someof the nutrients that are produced by neighboringinhabitants.</p><p>Sessile intertidal animals are often physically or che-mically defended against predation and display plasticphenotypes in response to changing environmental con-ditions because they are fixed in place and cannot move toavoid predators. For example, the presence of the preda-tory gastropod Acanthina angelica induces change in theshell shape of its barnacle prey Chthamalus anisopoma, andthe barnacle forms a curved shell making it more difficultfor the predator to attack.Mobile Organisms</p><p>Mobile invertebrates and vertebrates that are found onrocky intertidal shores are typically divided into twocategories based on the amount of time spent betweentidemarks. Resident species remain in the intertidal zonethroughout most of their life and face a large range oflocal physical conditions that they mitigate by a variety ofbehavioral and physiological adaptations. Many residentsfind shelter during low tides, either between rocks, underalgae, or in tide pools, while other species attach toexposed rock surfaces just ahead of the incoming tide.Transient species are those that spend only a small partof their life cycles in the intertidal zone (e.g., as juveniles)or are those that enter and leave the intertidal zone duringlow or high tide.Figure 3 Rocky shore in Central California, USA with elephantseals on the beach. Photo by S. Dudgeon.Invertebrates</p><p>Large, mobile invertebrate consumers are ecologically themost intensively studied guild on rocky shores andinclude species from Turbellaria (flatworms), Crustacea(e.g., crabs, shrimp, amphipods, and isopods), Annelida(e.g., polychaetes), Gastropoda (e.g., snails, nudibranchs,and chitons), and Echinodermata (sea urchins, brittlestars, and sea stars). Herbivores range from grazers ofdiatom films to browsers of macroalgae, and predatorsexploit a variety of methods (crushing, stinging, drilling,and partial consumption) to overcome the defenses oftheir prey.</p><p>Small mobile metazoans (roughly 0.11 mm and col-lectively termed meiofauna) thrive on and among thealgae, animals, and the trapped sediments on rocky shores.Meiofauna include consumers from many invertebratephyla, that due to their small sizes, extremely highabundances, and high turnover rates are an importantguild of consumers whose effects have largely beenneglected in comparison to studies of larger invertebrates.Vertebrates</p><p>Vertebrates tend to be transient species that use theintertidal zone to feed or hide and include fish and marinemammals that enter at high tide and birds and terrestrialmammals that enter at low tide (Figure 3). For instance,marine iguanas (Amblyrhynchus cristatus) of the GalapagosIslands, Ecuador forage extensively on intertidal algae onlava reefs during low tides. The major exceptions areresident intertidal fishes, which are often cryptic andless than 10 cm in length. Resident and transient fishesinclude hundreds of species from dozens of families,though members of the families Blenniidae, Gobiidae,and Labridae are the most common.</p><p>Birds and mammals, characterized by high endother-mic metabolic rates and large body sizes, have significantimpacts on intertidal communities even at low densities.Birds include locally nesting and migratory species andcan remove millions of invertebrates during a season. Inaddition, birds in some communities provide major inputsof nutrients via guano and prey remains. More than twodozen terrestrial mammals, mostly carnivores, rodents,and artiodactyls, have been reported as consumers orscavengers of rocky intertidal organisms on every conti-nent except Antarctica. Most recorded prey species aremollusks, crabs, or fish. Probably one of the most unusualcases is a population of feral rabbits on a small island offthe coast of South Africa that forage on seaweeds in theintertidal zone. Given the mobility of vertebrates, theirimpact on rocky intertidal shores has been difficult toassess and intertidal activity is often discovered by finding</p></li><li><p>3110 Ecosystems | Rocky Intertidal Zoneexclusively intertidal animals or algae in the gut contentsof otherwise pelagic or terrestrial species.</p><p>Little is known about the effects of harvesting byhumans in the rocky intertidal zone. Results from a fewlarge-scale studies in Australia, Chile, and South Africa,however, have demonstrated that harvesting has had sig-nificant effects on intertidal assemblages.Zonation</p><p>Patterns</p><p>Rocky intertidal shores often display a vertical zonationof fauna and flora associated with the strong environmen-tal gradient produced by the rise and fall of the tides. Forexample, most moderately exposed rocky shores of thenorthern hemisphere have kelps at the littoral sublittoralinterface, followed by rhodophyte algae dominating thelow intertidal zone, by fucoid algae, mussels, and barna-cles dominating the mid-intertidal zone, and bycyanobacteria, lichens, and a variety of small tufted,encrusting, or filamentous ephemeral seaweeds occurringin the high intertidal zone. While species from manyphyla may be found together, often a single species orgroup is so common; vertical zones are named accordingto the dominant group (e.g. the intertidal balanoid zonenamed after barnacles in the family Balanidae).</p><p>Combinations of various physical factors acting upondifferent inhabitants in intertidal zones that vary in theirexposure to waves can lead to complex patterns of dis-tribution and abundance along shorelines in a particularregion. Nevertheless, some general patterns are evident ata regional scale. Geographically, vertical zonation pat-terns are most pronounced on temperate rocky shoreswhere species diversity is high and tidal amplitudes tendto be greatest. On rocky shores in the tropics, biotic zonesare compressed into narrow vertical bands because ofsmall tidal amplitudes. In polar regions, annual ice scourand low species diversity tend to obscure any conspicuousvertical zonation.Causes</p><p>It is often stated that the upper limits of organisms are setby physical factors, whereas the lower limits are set bybiological interactions but there are many exceptions tothis rule. The specific causes of the zonation seen on mostrocky shorelines vary with geographic location, but zona-tion results primarily from behavior of larvae and adults,tolerance to physiological stress, the effects of consumers,and the interplay between production and the presence ofneighbors.</p><p>Adult movements and larval behavior during settlementfrom the plankton onto rocky shores have major effects onthe distribution of animals. For example, studies of barnacleshave shown that vertical zonation of larvae in the watercolumn cont...</p></li></ul>
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