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Chapter 13 Intertidal Communities © 2006 Thomson-Brooks Cole Key Concepts • The intertidal zone is the coastal area alternately exposed and submerged by tides. • Organisms that inhabit intertidal zones must be able to tolerate radical changes in temperature, salinity, and moisture and also be able to withstand wave shock. © 2006 Thomson-Brooks Cole Key Concepts • Organisms on rocky shores tend to be found in definite bands, or zones, on the rocks. • In contrast to sandy shores, rocky shores provide a relatively stable surface for attachment. • Tide pool organisms must be able to adjust to abrupt changes in temperature, salinity, pH, and oxygen levels. © 2006 Thomson-Brooks Cole Key Concepts • Biotic factors are most important in determining the distribution of organisms on rocky shores, but physical factors are most important on sandy shores. © 2006 Thomson-Brooks Cole Characteristics of the Intertidal Zone • Daily fluctuations of the environment – organisms must tolerate radical changes in temperature, salinity and moisture, and endure the crushing force of waves • Inhabitants are most active during high tide, when area is submerged – water provides food for filter feeders • As the tide retreats, organisms adjust to exposure to air and sunlight © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Rocky Shores • Composed of hard materials • Found from California to Alaska on the west coast and from Cape Cod northward on the east coast of North America • Coasts may be recently uplifted, formed from lava flows, or highly eroded by wind and waves © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Zonation—separation of organisms into prominent horizontal bands defined by color or distribution of organisms • Rocks provide a stable surface for attachment of organisms • As tide retreats... – upper regions exposed to air, changing temperatures, solar radiation, dissication – lower regions exposed only a short time before tide returns to cover them © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Zone system for rocky shore proposed by Alan and Anne Stephenson – supralittoral (maritime) zone—area above high water that may extend several miles inland – supralittoral fringe (splash zone)— uppermost area covered only by the highest (spring) tide, usually just dampened by spray of crashing waves © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Zone system (continued) – midlittoral zone—the true intertidal zone; extensive part that is regularly exposed during low tides and covered during high tides – infralittoral fringe—area extending from the lowest of low tides to the upper limits reached by large kelps (laminarians) – subtidal (infralittoral) zone—the region of shore covered by water, even during low tide © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Supralittoral fringe of rocky shores – receives very little moisture – exposed to drying heat of the sun in summer and extreme cold in winter – few organisms inhabit this harsh area – gray and orange lichens composed of fungi and algae are common – sea hair – a filamentous alga – most common animal = periwinkles, molluscs of Littorina and associated genera © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Supralittoral fringe (continued) – other inhabitants include limpets and isopods – some periwinkles and isopods breathe air • Midlittoral zone of rocky shores – inhabitants must avoid desiccation, maintain gas exchange, and deal with temperature extremes as the tide moves in and out – wave shock—force of the waves as they crash against the rocks during low tide © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Midlittoral zone (continued) – upper midlittoral zone • typical organisms = acorn barnacles and rock barnacles • barnacles permanently attach to surfaces • barnacles open their shells to filter feed during high tide, and close them to trap water inside during low tide • barnacles cool themselves by opening the shell slightly and allowing a little water to evaporate © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Midlittoral zone (continued) – middle and low midlittoral zone • oysters, mussels, limpets, and periwinkles dominate • oysters and mussels survive low tides by trapping water in their shells • limpets and chitons graze algae at high tide • common periwinkles bury themselves in seaweed to retain moisture during low tide • rock urchins survive wave shock by hollowing out a space in the rock and wedging into it © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Midlittoral zone (continued) – seaweeds of the midlittoral zone • rockweeds (brown algae) grow on rocks without full exposure to the sea • rockweeds compete with barnacles for space by sweeping the rocky surface with their blades, preventing cyprid larvae of barnacles from settling • some rockweeds are toxic to deter grazing • rockweeds produce a gelatinous covering that retards water loss and prevents desiccation • they form large mats that trap water and provide a haven for animals during low tide © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Midlittoral zone (continued) – tide pools—depressions in the rocks which retain water during low tide • water loses oxygen as it heats in the sun • salinity may change owing to rainfall or evaporation of water by the sun • oxygen in tide pools containing algae may change drastically – high during the day when algae are active, low (+ low pH) at night • salinity, temperature, pH abruptly returned to ocean conditions when tide reaches the pool © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Midlittoral zone (continued) – tide pool organisms • e.g. algae, sea stars, anemones, tube worms, hermit crabs, molluscs • many are filter feeders • biotic interactions help structure tide pool communities • e.g., in New England, Irish moss lives in tide pools where common periwinkles eat the green alga Enteromorpha, which normally outcompetes Irish moss © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Rocky Shore Zonation • Infralittoral fringe of rocky shores – transitional area submerged except at spring tides – rich flora and fauna of organisms that can tolerate limited air exposure – rocks may be covered with seaweeds – in cooler waters, molluscs, sea stars and brittle stars live among large kelps – other animals include hydrozoans, anemones, sea urchins, spider crabs © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Tropical Rocky Shores • Less temperature variation and seasonal rainfall – higher temperatures are more stressful – less temperature variation and fewer storms are less stressful • Zonation patterns on intertidal rocks – supralittoral fringe and midlittoral fringe each have major subdivisions evident in tropical areas © 2006 Thomson-Brooks Cole Supralittoral Fringe • White zone—the true border between the land and the sea – hermit crabs, isopods, knobby periwinkles • Gray zone – knobby (and other) periwinkles – nerites – an exclusively tropical group that tends to replace limpets in higher intertidal zones – farthest zone from low tide line where macroscopic marine algae grow © 2006 Thomson-Brooks Cole Supralittoral Fringe • Black zone – immersed only at the highest spring tides – lacks knobby periwinkle – several species of algae and cyanobacteria dominate – smaller periwinkles, other nerites, fuzzy chitons © 2006 Thomson-Brooks Cole Midlittoral Fringe • Yellow zone – microscopic boring algae covering its surface give it a yellow or green color – barnacles, limpets, fuzzy chitons, rock snails, irregular worm snails © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Midlittoral Fringe • Pink zone – sometimes underlies the yellow zone – characterized by widespread encrustation of coralline algae – irregular worm snail, mats of anemones, keyhole limpets, gastropods © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Infralittoral Fringe • surf zone includes the edge of the lower rocky platform and parts of the reef • rocks may be covered with algae such as Sargassum • boring urchins, anemones, sponges, bryozoa, sea cucumbers, keyhole limpets © 2006 Thomson-Brooks Cole Subtidal Zone • Relatively barren compared to subtidal zone in temperate climates • Small, turf-forming red algae dominate – lack of larger algae present in higher zones thought to be owing to herbivory – in experiments, exclusion of herbivores permitted erect algae to establish themselves where they were not found previously © 2006 Thomson-Brooks Cole Comparison of Temperate and Tropical Rocky Intertidal Systems • Higher stress and predation in tropical environment • Abundant mobile invertebrates, but fewer sessile ones in the tropics • Large body size or residence in higher areas more important means of avoiding predators in temperate areas • Macroalgae have less impact on community structure in the tropics © 2006 Thomson-Brooks Cole Intertidal Fishes • Resident species – typically have special adaptations for surviving harsh intertidal conditions • small size; absent, reduced or firmly attached scales; compressed/elongate or depressed body shape; absent or reduced swim bladder; greater body density • tolerant of temperature and salinity changes • some intertidal fish can leave the water to feed • Temporary inhabitants – tidal, seasonal© 2006 and accidental visitors Thomson-Brooks Cole Ecology of the Rocky Shore • East coast rocky shores – barnacles dominate upper zones – below the barnacles are mussels – algae and consumers survive on protected shores • West coast rocky shores – barnacles compete with algae; mussels displace barnacles by growing over them – balance maintained by ochre sea stars, the keystone predator © 2006 Thomson-Brooks Cole Ecology of the Rocky Shore • Rocky shores in the tropics – total predation is strong, and control of competitively dominant species is spread over a number of consumers • Top-down and bottom-up factors – top-down factors—factors whose effects flow down the food chain: competition, herbivory and predation – bottom-up factors—factors that affect the basal level of food chains: nutrient availability, recruitment © 2006 Thomson-Brooks Cole Sandy Shores • Many temperate and tropical shorelines consist of sandy beaches • Extend almost continuously from Cape Cod south to the Gulf Coast on the east coast of North America © 2006 Thomson-Brooks Cole Role of Waves and Sediments • Sediment particle size influences the beach’s nature, porosity of sediments, ability of animals to burrow • Wave action influences sediment type: – heavy wave action = coarse sediments – little wave action = fine sediments • Beach slope results from interaction of waves, sediment particle size, and relationship of swash and backwash © 2006 Thomson-Brooks Cole Role of Waves and Sediments – swash—water running up a beach after a wave breaks – backwash—water flowing down the beach • Types of beaches: – dissipative beach—wave energy is strong but is dissipated in a surf zone some distance from the beach face • usually flat with fine sediment – reflective beach—wave energy is directly dissipated on the beach • usually steep with course sediment © 2006 Thomson-Brooks Cole Role of Waves and Sediments • On all sandy beaches, a cushion of water separates the grains of sand below a certain depth – especially true on beaches with fine sand where capillary action is greatest • Fine sand beaches have a greater abundance of organisms – greater water retention – sediment is more suitable for burrowing © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Comparison of Rocky and Sandy Shores • Sandy shores lack readily apparent pattern of zonation – wave action is most important factor in determining organism distribution – temperature has less effect because of insulating properties of sand and water retention – oxygen levels may be low beneath the sand owing to lack of water exchange © 2006 Thomson-Brooks Cole Sandy Shore Zonation • Less defined pattern of longitudinal zonation • Vertical zonation exists among organisms buried in the sand – depends on amount of water trapped at each level • 3 major zones: – supralittoral – midlittoral – subtidal (infralittoral) © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Life above the High Tide Line • Supralittoral fringe stretches from the high tide line to the point where terrestrial vegetation begins – sand dunes may border uppermost extent • Below is zone of drying sand, where moisture reaches only during the highest tides and gradually evaporates – inhabited mostly by infauna—organisms that burrow in the sand to survive dry periods and intense heat from the sun © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Life above the High Tide Line • Temperate inhabitants: insects, isopods, amphipod crustaceans • Ghost crabs and fiddler crabs replace amphipods in the tropics • Ghost crabs have gills, but only make short forays into water to wet them • Ghost crabs live in burrows, and are nocturnal scavengers © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Life in the Sandy Shore Midlittoral Zone • Vertical zonation – less extensive zones of dry and drying sand than supralittoral zone – zone of retention—retains moisture at low tide because of capillary action of water • inhabited by isopod crustaceans worldwide – zone of resurgence—water is retained at low tide • supports crustaceans, polychaete worms – zone of saturation—constantly moist; supports greatest diversity of organisms © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Life in the Sandy Shore Midlittoral Zone • Animals of the midlittoral zone – echinoderms (e.g. sand dollars, sea stars) – snails (e.g. moon snails, olive snails) • moon snails feed on bivalves by drilling a hole in the shell and inserting their proboscis – lugworms • deposit feeders that leave coiled, cone-shaped casts during low tide © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Life in the Sandy Shore Midlittoral Zone • Relationship between tides and the activity of midlittoral organisms – during high tide, bivalves project their siphons to filter feed and bathe their gills – carnivorous snails hunt bivalves – echinoderms emerge in search of food – mole crabs and coquinas move up and down the beach with the tide, filter feeding – predators come in with the tide as well © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Life below the Low Tide Line • Subtidal zone is truly marine, exposed only during lowest spring tides • Variety/distribution of organisms primarily influenced by sediment characteristics • Seagrass beds occur in the subtidal zones of some coasts • Many species of fish live here • Pace of life is constant relative to that on the beach © 2006 Thomson-Brooks Cole Meiofauna • Meiofauna – microscopic organisms that inhabit the spaces between the sediment particles of intertidal and subtidal zones – pass though a 0.5-mm screen but are retained by a 62-mm screen © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Meiofauna • Factors affecting the distribution of meiofauna – grain size: • coarse-grain sediments have greater interstitial volume that allows larger organisms to move between the particles • fine-grain sediments have less space and exhibit more burrowing forms – water circulation • fine sediments can inhibit water flow and produce anoxic conditions © 2006 Thomson-Brooks Cole Meiofauna • Factors affecting distribution (cont.) – oxygen availability • also lower at greater depths – temperature • upper layers are more variable – salinity – wave action • can suspend sediments along with organisms, making them more vulnerable to predation © 2006 Thomson-Brooks Cole Meiofauna • Characteristics of the meiofauna – invertebrates from many phyla – generally elongated with few lateral projections – many are armored to protect them from being crushed by moving sand grains – include predators, herbivores, suspension feeders and detrivores – most exhibit brood protection because they produce a small number of offspring © 2006 Thomson-Brooks Cole Meiofauna • Factors affecting the size of meiofaunal populations – seasons (peak during summer months) – protection from wave action = greater abundance – predation can have severe effects in the upper layers of sediments © 2006 Thomson-Brooks Cole Ecology of the Sandy Shore • Fauna is less abundant than in rocky shores, and does not occupy all available space • Competition is not a major factor in determining distribution • Predation is less important – fewer predators among invertebrates there • Predation and disturbance important on sand flats • Greater exposure = less influence of competition and predation, more influence of abiotic (physical) factors © 2006 Thomson-Brooks Cole