Intertidal Communities

Intertidal Communities
Chapter 13 Intertidal Communities

Key Concepts The intertidal zone is that part of the marine environment alternately exposed and submerged by tides. Organisms that inhabit intertidal zones must be able to tolerate wave shock, desiccation and radical changes in temperature and salinity.

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.

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.

Intertidal Communities
Found within the shoreline area reached by waters of the highest high tide and uncovered at the lowest low tide Stressful environment due to interaction of physical factors, e.g., wind, waves, sunlight Organisms must be able to withstand desiccation, searing heat and freezing Home to a diversity of organisms exceeding that of submerged habitats Well studied environment with insights that can be applied to other areas of ecology

Rocky Shores Composed of hard materials
More densely and have greater diversity of algae and animals than sand/mud intertidal regions Found from California to Alaska on the west coast and from Cape Cod northward on the east coast of North America

Rocky Shores Pacific coast lies on an active continental margin which geologically produces a coastline dominated by rock Atlantic coastline occurs on passive continental margin and is dominated by sediment

Adaptations to Life on Rocky Shores
Organisms on rocky shores are faced with challenges not encountered by organisms in the deeper ocean alternately submerged by incoming tide and exposed to air by ebbing tide exposed to salinity fluctuations due to rain and evaporation pounded by high energy waves

Avoiding Overheating and Desiccation
Organisms attached to high intertidal locations face greatest challenge to maintain suitable body temperature and avoid water loss – adaptations include: large body exposes less surface area, e.g., knobby periwinkle light color reduces heat gain special adaptations of kidneys aggregate in large clumps mobile animals (crabs) move down as tide retreats into crevices of sheltered areas close shells at low tide, e.g., barnacles and mussels clamping down tightly to rock, e.g., limpets and chitons produce gelatinous covering, e.g., rockweed

Coping with Cold Organisms in high latitudes produce anti-freezing type compounds during winter

Avoiding Wave Shock Animals exhibit compressed or dorsally flattened bodies or shells Adhere tightly to rock barnacles cement themselves limpets, chitons and snails have enlarged foot for attachment sea stars use tube feet rock urchins hollow out cavities mussels attach with byssal threads intertidal algae are flexible and attach with holdfast organ

Adaptations to Salinity
Rainstorms can flood intertidal tide pools decreasing salinity and evaporation on hot days can elevate salinity many organisms have adapted to tolerate these changes or move to other areas

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

Figure 13-7 ROCKY SHORE ZONATION.

COMMUNITY ZONATION PATTERN TIDE ZONES
Supralittoral zone Supralittoral Fringe EHWS Midlittoral Zone (barnacles, rockweed, mussels) Littoral zone Infralittoral Fringe ELWS Infralittoral (subtidal) zone Stepped Art Fig. 13-7, p. 362

Rocky Shore Zonation Zone system for rocky shore proposed by Alan and Anne Stephenson supralittoral fringe (splash zone): uppermost area covered only by the highest (spring) tide, usually just dampened by spray of crashing waves supralittoral (maritime) zone: area above high water that may extend several miles inland

Rocky Shore Zonation Zone system (continued)
midlittoral zone: the true or middle intertidal zone; extensive part that is regularly exposed during low tides and covered during high tides infralittoral fringe: area below midlittoral zone extending from the lowest of low tides to the upper limits reached by large kelps (laminarians) Infralittoral (subtidal) zone: the region of shore covered by water, even during low tide

Figure 13-8 (a) VERTICAL INTERTIDAL ZONES.

Figure 13-8 (b) VERTICAL INTERTIDAL ZONES.

Temperate Rocky Shores
Temperate region lies between the polar circles (66.5 degrees north and south latitudes) and the tropics (25 degrees north and south latitude) Supralittoral fringe 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, supralittoral zone also called littorina zone

Supralittoral fringe (continued) other inhabitants include limpets and isopods some periwinkles and isopods breathe air most inhabitants protected from predation by virtue of location

Figure 13-5 (b) COPING WITH WAVE SHOCK

Figure 13-5 (c) COPING WITH WAVE SHOCK

Figure 13-6 INTERTIDAL ALGAE AND WAVE SHOCK.

Midlittoral zone upper midlittoral zone typical organisms = acorn barnacles and rock barnacles barnacles permanently attach to surfaces and are common on shores pounded by heavy waves barnacle populations may be as dense as 9,000 individuals/m2

Midlittoral zone (continued) middle and low midlittoral zone oysters, mussels, limpets, and periwinkles dominate Bivalves open shells during high tide to feed on plankton 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 actively feed on algae during high tide but retreat to depressions in rock when tide recedes

Rocky Shore Zonation Midlittoral zone (continued)
rockweeds (brown algae) grow on rocks without full exposure to the sea, do not tolerate large waves some species use gas-filled chambers (air bladders) to buoy them during high tide when tide goes out, seaweeds form mats that trap water and provide haven for sea stars, brittle stars, sea urchins and bryozoans, reducing stress of increased temperature and desiccation on these animals

Temperate Rocky Shore 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 dense turf of seaweeds in cooler waters, molluscs, sea stars and brittle stars live among large kelps other animals include hydrozoans, anemones, sea urchins, spider and Jonah crabs

Figure 13-11 THE INFRALITTORAL (SUBTIDAL) ZONE.

Tropical Rocky Shores Tropics defined as areas between 23.4 degrees north and south latitudes Less temperature variation and seasonal rainfall than in temperate intertidal higher temperatures are more stressful less temperature variation and fewer storms are less stressful

Tropical Rocky Shores Supra littoral fringe
can be divided into white, gray and black zones White zone: the true border between the land and the sea hermit crabs, isopods, knobby periwinkles are common 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

Tropical Rocky Shores Supralittoral fringe (continued) 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

Tropical Rocky Shores Midlittoral fringe Yellow zone
usually divided into yellow and pink zones Yellow zone microscopic boring algae covering its surface give it a yellow or green color barnacles, limpets, fuzzy chitons, rock snails, irregular worm snails

Figure 13-13 (a) YELLOW AND PINK ZONES OF TROPICAL SHORES.

Tropical Rocky Shores Midlittoral fringe (continued) Pink zone
sometimes underlies the yellow zone characterized by widespread encrustation of coralline algae irregular worm snail, mats of anemones, keyhole limpets, gastropods

Figure 13-13 (b) YELLOW AND PINK ZONES OF TROPICAL SHORES.

Tropical Rocky Shores 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 and turf algae Boring urchins, anemones, sponges, bryozoans, sea cucumbers, keyhole limpets

Tropical Rocky Shores 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

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 residing in higher areas are more important means of avoiding predators in temperate areas Macroalgae have less impact on community structure in the tropics

Tide Pools Tide Pools depressions in the rocks which retain water during low tide Prevent organisms within them being exposed to air but have other challenges: 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

Figure A TIDE POOL.

Tide Pools tide pool organisms
e.g., algae, sea stars, anemones, tube worms, hermit crabs, molluscs many are filter feeders

Intertidal Fishes Can be divided into two categories: residents and temporary inhabitants Resident species typically have special adaptations for surviving harsh intertidal conditions, e.g., cling fish, blennies, gobies, sculpins 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

Intertidal Fishes Intertidal fishes (continued) Temporary inhabitants
tidal, seasonal and accidental visitors tidal visitors move in with the tide seasonal visitors move in for breeding accidental visitors are trapped by storms

Ecology of the Rocky Shore
Physical factors interplay with biological factors determining which organisms occur at a given location within intertidal Competition, grazing and predation competition for space is dominant biological factor in the organization of intertidal communities predation can vary over a predator’s geographical range, e.g., keystone predator (an animal that is responsible for maintaining species diversity within an area) Pisaster, or with environmental variables e.g., heavy wave action in tropical systems, predation is very strong and spread over a number of consumers, hence no keystone predator grazing by limpets and other molluscs can reduce algal cover and allow barnacles and others to colonize rocks

In New England, biotic interactions help structure tide pool communities e.g., Irish moss lives in tide pools where common periwinkles eat the green alga Enteromorpha, which normally outcompetes Irish moss

Figure 13-19 BIOLOGICAL INTERACTIONS IN TIDE POOLS.

Variation in Larval Settlement most intertidal organisms produce a larval stage which develops in the water column and is carried back to shore habitats with high water flow have more recruitment high levels of recruitment can increase competition resulting in classical zonation patterns where space is limited low recruitment may result in zonation patterns breaking down producing different distributional patterns

Positive Interactions Among Species group aggregations can buffer high temperatures and desiccation dense groups minimize water loss associations with noxious species allow some organisms to survive

Ecological Succession in the Intertidal Zone succession occurs when disturbances strip intertidal rocks of inhabitants usual sequence: diatoms or filamentous algae followed by perennial species of red algae followed by barnacles and mussels climax community = final stage in ecological succession sequence and final structure can vary depending on availability (seasonality) of plant propagules and larvae

Ecological Succession in the Intertidal Zone diversity = number of species living in an area – can be affected by consumers and physical factors patchiness = organisms occurring in isolated groups within a larger, contiguous habitat diversity is greatest when disturbance prevents dominant competitors from excluding other species and allows less successful competitors to become established

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

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 is determined by interaction of waves, sediment particle size, and relationship of swash and backwash

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, receive less wave action and have gentle swash reflective beach: wave energy is directly dissipated on the beach usually steep with course sediment deposited as swash and backwash waters collide

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 Coarse sand beaches drain well, dry out quickly, hence support relatively fewer organisms

Table 13-1 Characteristics of Sandy Beaches at Various Levels of Exposure

Comparison of Rocky and Sandy Shores
Sandy shores lack readily apparent pattern of vertical zonation and superficially appear devoid of life wave action is most important factor in determining organism distribution on exposed beaches, few large organisms, inhabitants are either mobile or deep burrowers temperature has less effect because of insulating properties of sand and water retention hence desiccation and salinity fluctuations are less severe than on rocky shores

Comparison of Rocky and Sandy Shores
oxygen levels may be low beneath the sand owing to lack of water exchange oxygen availability in sediment may be limited when respiration is high and exchange of water with the sea is reduced, e.g., sand flats with fine sediments anoxic conditions result below uppermost layer when oxygen is severely limited anaerobic bacteria are common in anoxic sediments and produce hydrogen sulfide gas

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)

Figure 13-21 SANDY SHORE ZONATION.

Lugworm Auger snail Mole crab Coquina Sand sea stars Ghost crab
Sea urchin Blue crab Killifish Heart clam Stingray Sand dollar Bristle worm Supralittoral Midlittoral Subtidal Stepped Art Fig , p. 376

Sandy Shore Zonation Supralittoral Fringe of Sandy Shores
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

Figure SAND DUNES.

Sandy Shore Zonation 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

Figure GHOST CRAB.

Sandy Shore Zonation Midlittoral Zone of the Sandy Shore
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

Figure 13-24 VERTICAL ZONATIONOF THE SANDY INTERTIDAL ZONE.

Zone Dry sand Drying sand Retention Resurgence Saturation Isopod
crustaceans Dry sand Drying sand Polychaete worms Retention Polychaete worms Coquina Tellin Amphipod crustaceans Resurgence Saturation Stepped Art Fig , p. 378

Sandy Shore Zonation Midlittoral Zone of the Sandy Shore (continued)
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

Figure ANIMAL TRAILS.

Figure (b) LUGWORM.

Figure 13-28 (a) FEEDING BEHAVIOR OF MOLE CRABS.

Figure 13-28 (b) FEEDING BEHAVIOR OF MOLE CRABS.

Figure 13-28 (c) FEEDING BEHAVIOR OF MOLE CRABS.

Stepped Art Fig , p. 380

Figure 13-30 SANDY SHORE FOOD WEB.

Sandy Shore Zonation The Infralittoral Zone of Sandy Shores
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

Sandy Shore Zonation 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

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

Figure MEIOFAUNA.

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

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

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 detritivores most exhibit brood protection because they produce a small number of offspring

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

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 Predation and disturbance important on protected sand flats Greater exposure = less influence of competition and predation, more influence of abiotic (physical) factors, e.g., waves, particle size and slope

Intertidal Communities

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