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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
© 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
© 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
© 2006 Thomson-Brooks Cole
© 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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 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
© 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
© 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
© 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
© 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
© 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© 2006
and
accidental visitors
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
© 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
© 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
© 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
© 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
© 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
© 2006 Thomson-Brooks Cole
© 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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 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
• 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
© 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
© 2006 Thomson-Brooks Cole