© 2006 Thomson-Brooks Cole 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

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

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

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

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

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

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

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

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

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 and accidental visitors

© 2006 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

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

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

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

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

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

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

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 –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

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