Chapter 11
Between the
Tides
Intertidal or Littoral Zone
• Best known
• Narrow fringe along the shoreline that lies
between the highest high tide and the lowest low
tide
• Easy to study
• No specialized equipment needed
• Can return to the exact same spot again
A unique environment
• Regularly exposed to air – organisms must have
a way to cope
• Emersion – being out of water and exposed to air
• Immersion – being submerged
• Bottoms vary and determine the community that
lives there
• Substrate or substratum – material on or in
which organisms live
• Can be hard and rocky or soft and muddy or
sandy
Rocky Shore
Communities
• Occur on steep coasts
without large amounts of
sediment
• Active margins – west
coast
• Glaciers moving over land
and scraping away
sediments
• Waves and currents can
carry sediments away
• Hawaii – rocky because it
is geologically young
The Organisms
• Live right on the rock’s
surface
• Epifauna – live on the
surface of the substrate
• Sessile – stay attached to
the rock
• Living on the rock’s
surface, the organisms in
the rocky intertidal are
fully exposed to the
elements – great physical
stress
Exposure at Low
Tide
• Low tide – left high and dry, exposed to air which is
much harsher environment than water
• Highest part of the intertidal (only wet on high spring
tides) is almost never immersed and kept wet by wave
splash
• Low intertidal – submerged most of the time
• So the higher the organisms live in the intertidal,
then, the more time they have to spend out of the
water
Water Loss
• Desiccate – dry out if left out of the water too
long
• To live in the intertidal an organism must be
able to prevent desiccation, tolerate it or both
• They either run and hide or “clam up”
Organisms that run and hide
•
•
•
•
Shore crabs
Hermit crabs
Snails
Huddle in moist shady
crevices
Tide Pools
• Depressions in the rocks that hold sea water after
the tide goes out
• Good places to hide at low tide
Organisms that can’t run and hide
• Seeweeds and sessile animals
• Must live in moist areas all the time
Organisms that Clam up
• Have a protective covering like
a shell that they can close to
hold water
• Barnacles
• Mussels
• Limpets – clamp to the rock
• Limpets can also carve out
shallow depressions or “home
scars” to make a more
effective seal – use their
radula
• Littorina or periwinkles
clamp to rocks and also
seal the opening of their
shelf with their operculum
Organisms that do nothing
• Chitons – just dry up – can survive a loss of 75%
• Intertidal seaweeds – Fucus – can withstand a water
loss of 90% - get practically crunchy
Temperature and
Salinity
Temperature
• Sea temperatures are relatively constant and mild
because of high heat capacity
• Air temperatures can be much more extreme –
hot and cold
Dealing with Temperature
• Moist hiding places also have lower temperatures
• Nerita plicata – tropical snail has pronounced ridges to
help radiate heat
• Color can help deal with high temperatures
Salinity
• Fluctuates widely
• When it rains salinity decreases, even have to
deal with fresh water
• Tide pool residents – high salinity when there is a
lot of evaporation – low salinity when it rains
Restriction of Feeding
• Little sediment accumulates so deposit feeders
are rare
• Deposit feeder – animals that eat organic matter
that settles to the bottom
• Most are sessile filter feeders – can not feed
when the tide is out
• Filter feeders – animals that actively filter food
particles from the water
The Power of the
Sea
• When the tide is in life is still hard in the intertidal
• Ocean waves expend tremendous energy as they
crash on the shore
• Rocky intertidal organisms are exposed to the full
power of the sea
• There is tremendous variation in the intensity of
wave impact (wave shock) from place to place
along the shore
• Exposure to waves strongly affects intertidal
organisms
Coping with
Wave Shock
• Some organisms can’t withstand wave shock and
are only found in sheltered locations
• Some deal with wave shock by firmly anchoring
themselves to the rocks
Ways to Anchor
• Seaweeds use their
holdfasts or crust on the
rock
• Mussels hold on with
byssal threads (strong
fibers made of protein)
• Limpets and Chitons use
their muscular foot as a
suction cup
• Intertidal fishes lack swim
bladders so they sink and
stay on the bottom
Other Adaptations to deal with Wave
Shock
• Thicker shells
• Compact shape to reduce
impact
• Low profiles that keep
them close to the rocks
(barnacles, mussels,
limpets and chitons)
• Seaweeds are flexible and
can go with the flow
The Battle for
Space
• The intertidal has plenty of food
• Shallow coastal water provide lots of light and
nutrients – photosynthesis – lots of food for
animals
• Plankton rich water moves in at high tide
• Detritus moves in with the tide
Limiting Factor
• Space
• Nearly all of the space is occupied
• Organisms may attach to each other because
space is so limiting
Competition for Space
• A dominant biological factor
• Several ways to compete for space
• One – get there first - be the first to colonize the
area (which means an organisms must have an
effective means of dispersal)
• Two – take over a space that is already occupied
– uncut or bulldoze the off (barnacles and
limpets)
• Three – grow over the competitors making them
vulnerable to waves, smothering them, or blocking
their sunlight
Vertical Zonation
• The rocky intertidal community is usually divided
into distinct bands or zones at characteristic
heights in the intertidal
• Vertical zonation - A given species is only found in
a particular vertical range
Zonation is Caused by:
• Complex interaction of physical and biological
factors
• Upper limit determined by physical factors
• Lower limit determined by biological factors
Organisms of the Upper Intertidal
• Lichens
• Cyanobacteria (Calothrix)
• Filamentous green alga
(Ulothrix)
• Periwinkles (Littorina)
which graze on algae
• Limpets – hardy grazers
• Predators- birds raccoons
and rats from land
Organisms of the Intertidal
•
•
•
•
Acorn barnacles
Little gray barnacles (Chthamalus)
Rock Barnacles (Balanus, Semibalanus)
Little gray barnacles live higher up than rock barnacles – upper
limit physical factor – emersion
• Lower limit – biological factors (competition and predation) – rock
out compete little gray – rock are eaten by Whelks
• Mussels
• Gooseneck barnacles
• Brown seaweeds –rock
weeds (Fucus, Pelvetia)
• Sea stars (Pisaster,
Asterias) eat mussels
• Lower limit of mussels is
set by predation of sea
stars
• Spiny lobsters – eat
mussels
Pelvetia
Gooseneck Barnacles
Pisaster
Lower Intertidal Organisms
•
•
•
•
•
•
•
Seaweeds
Light and space are important resources
Green alga (Entromoropha)
Irish moss (red alga) Chondrus crispus
Kelps
Coralline algae (Corallina, Lithothamnion)
Sea urchins
Chondrus crispus
Corallina
•
•
•
•
•
Sea anemones (Metridium, Anthopleura)
Polychaete worms (Spirorbis, phragmatopoma)
Snails (Tegula, Nucella)
Sea Slugs (Aplysia, Dendronotus)
Gobies, clingfishes, sculpins, pricklebacks and
gunnels
Spirorbis
Metridium
Spirorbis
Sculpin
Aplysia
Soft-Bottom
Intertidal
Communites
• Soft bottom – bottom composed of sediment –
organisms can burrow in it easily
• Whether and what type of sediment accumulates
depends on how much water motion there is and
on the source of the sediment
• The type of sediment strongly influences the
community
The Shifting
Sediments
•
•
•
•
Unstable, constantly shifting
Organisms do not have a solid place to attach
Few seaweeds, some areas have sea grass beds
Most burrow in the mud – infauna (live in the
sediment)
Important Physical Factor of Soft
Bottom Communities
• Kind of sediment
• Size of the grains (gravel, sand, silt, clay)
• Sediment composition is directly related to the degree
of water motion
• Fine sediment remains suspended longer and can be
kept suspended with a small amount of water motion
Living in the
Sediment
Advantages
• Stays wet when the tide goes out
• Desiccation is not as critical
• Course sand however will dry out quickly
Oxygen Availability
• Amount of organic matter in bottom sediments is
particularly important to deposit feeders
• Few primary producers, detritus is the main
source of food
• Deposit feeders extract this organic matter from
the sediments
• Smaller grain sizes contain more organic matter
• Grain size also affects the amount of oxygen
available in the sediments
• Oxygen is used through respiration and decay
bacteria
Problems for Muddy Bottoms
• More organic matter to decay and use up oxygen
• Flow of water that brings in new oxygen is reduced
• Except for the upper few centimeters of mud the
interstitial water (water between the grains) is
deficient of oxygen
• Anoxic – sediments with absolutely no oxygen
• Anaerobic bacteria can break down organic
matter without oxygen – hydrogen sulfide is
produced as a byproduct
• H2S – smells like rotten eggs and turns things
black
Adaptations of Infauna to low oxygen
levels
• Pump oxygen rich water from the sediment
surface with siphons or through their burrows
• Adaptation of hemoglobin to low oxygen levels –
can extract as much oxygen as possible
• Sluggish – reduces the need
Getting Around
•
•
•
•
Muscular foot pulls the organism into the sediment
Burrow or plough along
Jointed appendages to dig – crustaceans
Eating their way through the sediment – sea cucumbers
and certain worms
Feeding
• Detritus is the main food source
• Diatoms sometimes form highly productive mats
• Plankton brought in by the tides
• Deposit feeding – burrowing through the
sediments and eat them
• Tube feet to pick up particles – sand dollars
• Siphons to filter feed
Plankton
Diatoms
Sand Dollar
Zonation
• Not as obvious as it is on the rocky intertidal
• Best seen on sandy beaches where water drains
quickly
• Hardiest to see in muddy areas where the area is
flat
The End ……