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Intertidal Communities

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Title: Intertidal Communities


1
Chapter 13
  • Intertidal Communities

2
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.

3
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.

4
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.

5
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

8
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

11
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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14
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

15
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

22
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

23
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

24
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

31
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

32
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

33
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

38
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

39
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

40
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

41
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

42
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

43
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

44
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

45
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

46
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

49
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

66
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

69
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

70
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

71
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
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