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The Intertidal Zone
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Background Information
The intertidal zone, also known as the littoral zone, in marine aquatic environments is the area of the
foreshore and seabed that is exposed to the air at low tide and submerged at high tide (the area between
tide marks).
Conditions in the Intertidal Zone
In the intertidal zone the most common organisms are small and most are relatively uncomplicated
organisms. This is for a variety of reasons; firstly the supply of water which marine organisms require to
survive is intermittent. Secondly, the wave action around the shore can wash away or dislodge poorly
suited or adapted organisms. Thirdly, because of the intertidal zone's high exposure to the sun the
temperature range can be extreme from very hot to near freezing in frigid climates (with cold seas).
Lastly, the salinity is much higher in the intertidal zone because salt water trapped in rock pools
evaporates leaving behind salt deposits. These four factors make the intertidal zone an extreme
environment in which to live.
Predators
Animals that live in the littoral zone have a wide variety of predators who eat them. When the tide is in,
littoral organisms are preyed upon by sea animals (like fish). When the tide is out, they are preyed upon
by land animals, like foxes and people. Birds (like gulls) and marine mammals (like walruses) also prey on
intertidal organisms extensively.
A typical rocky shore can be divided into a spray zone (also known as the Supratidal Zone, which is above
the spring high-tide line and is covered by water only during storms, and an intertidal zone, which lies
between the high and low tidal extremes. Along most shores, the intertidal zone can be clearly separated
into the following subzones: high tide zone, middle tide zone, and low tide zone.
High tide zone (upper mid-littoral)
The high tide zone is flooded during high tide only, and is a highly saline environment. The abundancy of
water is not high enough to sustain large amounts of vegetation, although some do survive in the high
tide zone. The predominant organisms in this subregion are anemones, barnacles, brittle stars, chitons,
crabs, green algae, isopods, limpets, mussels, sea stars, snails, whelks and some marine vegetation. The
high tide zone can also contain rock pools inhabited by small fish and larger seaweeds. Another organism
found here is the hermit crab, which because of its portable home in the form of a shell does extremely
well as it is sheltered from the high temperature range to an extent and can also carry water with it in its
shell. Consequently there is generally a higher population of hermit crabs to common crabs in the high
tide zone. Life is much more abundant here than in the spray
Middle tide zone (lower mid-littoral)
The middle tide zone is submerged and flooded for approximately equal periods of time per tide cycle.
Consequently temperatures are less extreme due to shorter direct exposure to the sun, and therefore
salinity is only marginally higher than ocean levels. However wave action is generally more extreme than
the high tide and spray zones. The middle tide zone also has much higher population of marine
vegetation, specifically seaweeds. Organisms are also more complex and often larger in size than those
found in the high tide and splash zones. Organisms in this area include anemones, barnacles, chitons,
crabs, green algae, isopods, limpets, mussels, sea lettuce, sea palms, sea stars, snails, sponges, and
whelks. Again rock pools can also provide a habitat for small fish, shrimps, krill, sea urchins and
zooplankton. Apart from being more populated, life in the middle tide zone is more diversified than the
high tide and splash zones.
Low tide zone (lower littoral)
This subregion is mostly submerged - it is only exposed at the point of low tide and for a longer period of
time during extremely low tides. This area is teeming with life; the most notable difference with this
subregion to the other three is that there is much more marine vegetation, especially seaweeds. There is
also a great biodiversity. Organisms in this zone generally are not well adapted to periods of dryness and
temperature extremes. Some of the organisms in this area are abalone, anemones, brown seaweed,
chitons, crabs, green algae, hydroids, isopods, limpets, mussels, nudibranchs, sculpin, sea cucumber, sea
lettuce, sea palms, sea stars, sea urchins, shrimp, snails, sponges, surf grass, tube worms, and whelks.
Creatures in this area can grow to larger sizes because there is more energy in the localised ecosystem
and because marine vegetation can grow to much greater sizes than in the other three intertidal
subregions due to the better water coverage: the water is shallow enough to allow plenty of light to reach
the vegetation to allow substantial photosynthetic activity, and the salinity is at almost normal levels. This
area is also protected from large predators such as large fish because of the wave action and the water
still being relatively shallow.
Advantages To Living In Intertidal Zones
1.
There are a number of advantages to living in a tide pool ecosystem.
o Algae and other intertidal plants grow in the abundant sunlight and support an entire food chain of animals.
o Constant wave action supplies the tide pool with nutrients and oxygen.
o Food is abundant.
o A varied substrate provides hiding places and surfaces to cling to.
Challenges To Living In The Intertidal Zone
1.
The rapidly changing conditions of a tide pool make survival a challenge. Exposure to surf and sun varies considerably.
o Plentiful sunlight, which helps intertidal plant life grow quickly, can also rapidly dry up precious moisture and increase the
water temperature.
o Waves that bring in much-needed nutrients and moisture can also carry unprotected animals out to sea.
o As the tides rise and fall, the salinity (salt concentration) constantly changes. Animals living in the intertidal zone must be
able to tolerate wide salinity variations.
o If sufficient nutrients are available, intertidal animals reproduce rapidly, so they constantly compete for space, light, and
food.
o Animals are also exposed to predators while the tide is out.
Adaptations To The Variable Environment
1.
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Small animals that live in the splash zone can avoid desiccation by closing their shells tightly to seal in moisture.
Some animals, like crabs and marine snails and bivalves, have thick, tough outer coverings to slow evaporation. Others, such as
mussels and leaf barnacles, cluster together to reduce individual exposure.
One main problem intertidal animals face is the constant pounding of waves. These animals have developed different adaptations to
keep from being washed away. Some, like sea stars, cling fast to the rocky surfaces; others find shelter in crevices or hide under
thick mats of seaweed when the tide is out.
Most intertidal life centers in the low intertidal level, which normally remains under water. Most of these inhabitants can only
tolerate exposure to air for short periods. It is here and in the subtidal zone (below the intertidal) that marine plants provide fish and
invertebrates with protective cover and food.
Animal Adaptations To Intertidal Life
1.
Tide pool animals and plants are well adapted to the intertidal zones. Some adaptations include:
o The ochre sea star can tolerate a longer time period exposed to air than many other sea stars. They regularly withstand up
to eight hours of exposure during low tides. In laboratory conditions, they have tolerated up to 50 hours out of the water
with little harm. Yet, they are not found in high intertidal pools due to their lack of ability to withstand high water
temperatures or low oxygen levels.
o Some abalones, limpets, and turban snails can smell approaching ochre stars and will move away to avoid being eaten.
o Sea cucumbers have few known predators, other than humans and sea stars. If disturbed, some species may eviscerate
(expel their entrails), leaving the entrails to the predator while the sea cucumber escapes. Its organs will regenerate after
several days.
o When the tide is out, periwinkle snails cluster in crevices, secrete a gluelike mucus to stick to the rock's surface, and
withdraw into their shells to avoid drying out.
o Many fishes that inhabit tide pools, such as tide pool sculpin and young opaleyes, can breathe air at the surface—an
adaptation that enables them to survive in oxygen poor water when the tide is out.
Tide Pool Food Chains
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A food chain is a diagram showing "who eats what" in an ecosystem. A single tide pool contains many food chains. Algae and other
plants are eaten by plant-eating zooplankton; this plankton is eaten by larger, carnivorous plankton; these are eaten by a mussel,
barnacle or other marine invertebrate; the mussel is then eaten by an ochre star, which may be eaten by a gull or a sea otter.
All the food chains in an ecosystem can be interconnected to form a food web, a complex diagram showing all the interconnecting
predator-prey relationships in an ecosystem.
Many marine animals rely on tide pools for food and other resources. Gulls and other seabirds, as well as some mammals, forage in
tide pools. Tide pools even serve as "nurseries" for some fish species.
The limpet is a gastropod, a soft-bodied invertebrate (an animal without a
backbone) that is protected by a very hard, flattened conical shell. This
mollusk is found in cooler waters of the Atlantic Ocean and the Pacific
Ocean. Limpets cling tightly to a rock (using the muscular foot). During the
day, limpets move around by rippling the muscles of the foot in a wave-like
fashion, looking for food. They return to the same place on their rock each
night. No one knows exactly how they find their way back to the same spot
each time.
Diet: Limpets are herbivores (plant-eaters) that eat during the day. They eat
marine algae and other marine vegetation. Limpets eat using a radula, a rough tongue-like organ that has thousands of tiny denticles (toothlike protrusions).
Anatomy: The soft body is divided into the head, the visceral mass, and the foot (which is small). The limpet has a long pair of tentacles on
the head; it has a light-sensitive eyespot located on the base of each tentacle. The biggest limpets are up to 4 inches (10 cm) wide, but most
are smaller. The limpet's shell is not coiled.
Predators: Many fish, starfish, birds, and people eat limpets.
Clams are animals that burrow under the sea floor. They are bivalves, mollusks that have
two shells that protect a soft body. There are over 15,000 different species of clams
worldwide. The biggest clam is the Giant Clam, Tridacna gigas; it is up to 4.8 feet (1.5 m) long
and weighs up to 550 pounds (250 kg). Most clams are only a few inches long.
Anatomy and Diet: Clams come in many colors, including shades of brown, red-brown,
yellow, cream, etc. The two shells are attached by a muscular hinge (the adductor muscle).
When a clam is threatened, most clams will pull their soft body into into the shells and close
the shells tightly for protection. The foot is used to burrow into the sand. Clams use their
tube-like siphon to draw in water, from which they extract oxygen and filter plankton (tiny
plants that they eat).
Predators of the Clam:Many animals eat clams, including eels, sea stars, whelks, and people.
Sea stars (also known as starfish) are spiny, hard-skinned animals that
live on the rocky sea floor. These invertebrates are NOT fish; they are
echinoderms. Sea stars move very slowly along the sea bed, using
hundreds of tiny tube feet. There are over 2,000 different species of sea
stars worldwide.
Reproduction: Most species of starfish expel enormous numbers of
eggs and sperm into the ocean; fertilization is external. After
fertilization, the tiny, transparent, bilaterally-symmetrical larvae (baby
sea stars) travel many miles as they are swept along by ocean currents
for about two months. As they develop, the tiny larvae swim in the sea,
eat phytoplankton, and are a component of zooplankton.
Diet: Sea stars are carnivores (meat-eaters). They eat clams, oysters,
coral, fish, and other animals. They push their stomach out through their
mouth (located on the underside of the sea star) and digest the prey.
Anatomy: Most sea stars have five arms (or a multiple of five) that radiate from a central disk. Sea stars do not have a brain; they have a
simple ring of nerve cells that moves information around the body. Eyespots (primitive light sensors) are at the tip of each arm. If a sea star's
arm is cut off, it will regenerate (regrow).
Classification: Kingdom Animalia (animals), Phylum Echinodermata (echinoderms), Class Asteroidea (sea stars), about 2,000 species.
Crabs are 10-legged animals that walk sideways. There are almost 5,000
different species of crabs; about 4,500 are true crabs, plus about 500 are hermit
crabs (hermit crabs don't have a very hard shell and use other animals' old
shells for protection). Most crabs live in the oceans, but many, like the robber
crab, live on land.
The Biggest Crabs: The biggest crab is the Japanese Spider crab
(Macrocheira kaempferi), which lives on the floor of the north Pacific Ocean;
it has a 12 ft (3.7 m) leg span. The biggest land crab is the Coconut crab
(Birgus latro), which lives on islands in the Pacific Ocean; it has a leg span up
to 2.5 ft (75 cm).
Diet: Many crabs are omnivores (plant- and meat-eaters), others are carnivores
(meat-eaters), and some are herbivores (plant-eaters).
Anatomy: Crabs are invertebrates, animals without a backbone. They have an exoskeleton (also called a carapace), an outer shell that both
protects them from predators and provides support. These crustaceans have ten jointed legs, two of which have large, grasping claws (called
pincers or chelipeds). They have a flattened body, two feelers (antennae), and two eyes located at the ends of stalks.
Breathing: Marine crabs breathe underwater using gills, which are located in a two cavities under the carapace. True land crabs have
enlarged, modified cavities that act like lungs so that the land crabs can breathe air.