Intertidal Communities
Karleskint
Turner
Small

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

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

• 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

• 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 antifreezing 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

Supralittoral
Fringe
Midlittoral
Zone
(barnacles, rockweed, mussels)
Infralittoral
Fringe
COMMUNITY ZONATION PATTERN TIDE ZONES
EHWS
Supralittoral zone
Littoral zone
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

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

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

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

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, 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

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

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)

Lugworm
Auger snail
Blue crab
Sea urchin
Killifish
Mole crab
Stingray
Ghost crab
Supralittoral
Sand sea stars
Coquina
Midlittoral
Heart clam
Sand dollar
Bristle worm
Subtidal
Stepped Art
Fig. 13-21, 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

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

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

Zone
Dry sand
Drying sand
Retention
Resurgence
Saturation
Isopod crustaceans
Polychaete worms
Polychaete worms
Coquina
Tellin
Amphipod crustaceans
Stepped Art
Fig. 13-24, 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

Stepped Art
Fig. 13-28, p. 380

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

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