Intertidal Communities

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Chapter 13

• Intertidal Communities

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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.

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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.

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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.

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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

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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

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Rocky Shore Zonation

• Zonationseparation 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

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Rocky Shore Zonation

• Zone system for rocky shore proposed by Alan and
  Anne Stephenson
• supralittoral (maritime) zonearea 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

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Rocky Shore Zonation

• Zone system (continued)
• midlittoral zonethe true intertidal zone
  extensive part that is regularly exposed during
  low tides and covered during high tides
• infralittoral fringearea extending from the
  lowest of low tides to the upper limits reached
  by large kelps (laminarians)
• subtidal (infralittoral) zonethe region of shore
  covered by water, even during low tide

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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

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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 shockforce of the waves as they crash
  against the rocks during low tide

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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

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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

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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

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Rocky Shore Zonation

• Midlittoral zone (continued)
• tide poolsdepressions 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

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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

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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

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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

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Supralittoral Fringe

• White zonethe 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

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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

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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

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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

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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

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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

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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

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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

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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

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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 factorsfactors whose effects flow down
  the food chain competition, herbivory and
  predation
• bottom-up factorsfactors that affect the basal
  level of food chains nutrient availability,
  recruitment

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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

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Role of Waves and Sediments

• Sediment particle size influences the beachs
  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

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Role of Waves and Sediments

• swashwater running up a beach after a wave
  breaks
• backwashwater flowing down the beach
• Types of beaches
• dissipative beachwave energy is strong but is
  dissipated in a surf zone some distance from the
  beach face
• usually flat with fine sediment
• reflective beachwave energy is directly
  dissipated on the beach
• usually steep with course sediment

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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

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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

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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)

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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 infaunaorganisms that burrow
  in the sand to survive dry periods and intense
  heat from the sun

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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

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Life in the Sandy Shore Midlittoral Zone

• Vertical zonation
• less extensive zones of dry and drying sand than
  supralittoral zone
• zone of retentionretains moisture at low tide
  because of capillary action of water
• inhabited by isopod crustaceans worldwide
• zone of resurgencewater is retained at low tide
• supports crustaceans, polychaete worms
• zone of saturationconstantly moist supports
  greatest diversity of organisms

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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

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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

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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

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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

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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

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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

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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

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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

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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