MadSci Network: Zoology

Re: How do animals live when the tides are out?

Date: Thu Nov 4 11:55:26 1999
Posted By: June M. Wingert , RM(NRM), Research Associate, Comparative Pathology Department, Baylor College of Medicine
Area of science: Zoology
ID: 941478459.Zo

This was a great question.  The following should explain everything you 
wanted to know about what happens to sea animals when the tide goes out. 
The information was taken from the Fitzgerald Marine Site located at
If the following does not satisfy your curiosity try going to the following 
California boasts over 1,100 miles of coastline and with it some of the 
world's richest tidal life.

Tide Pools

 Woods Hole's "Coastal Brief" series: Tide pools by Kimberly Amaral This is 
a map of plants and animals typically found in a Massachusetts tide pool

Oceans cover more than 70% of the earth's surface.
Everywhere that the ocean meets the land, a unique and
brutal habitat exists. Waves pounding at the rocks
constantly reshape the coastline. In this narrow band
between water and land, strange animals and plants thrive.
Within the confines of the shore, tidal plants and animals
have developed specializations which allow them to survive. 

The tidal zone is continually redesigned by three actions:
wind, water and rock. Winds howl across the vast oceans
which, over time, slowly erode the land as the waves crash
against rocks stirred up by the wave action. Imagine
pounding one fist into an open hand over and over again.
This is life in the tidepool. 

When the tide retreats the sun beats down relentlessly.
Exposed animals and plants need to conserve water to survive until the tide 

Because the tides slowly invade and retreat, distinct
sub-habitats exist within the tidepool, each with unique
qualities. In the deeper parts of the tidepools, more water is
present. Animals and plants that need constant moisture
thrive in this zone. Animals and plants that need constant
exposure to air live higher in the tidepool, where the waves
simply splash. In between, unique critters straddle both
worlds, surviving both in the air and underwater.

The intertidal zone (the area between high tide and low tide)
is rich with nutrients which are replenished by each incoming
tide. As the waves crash down, they carry food necessary
for survival of the tidepool inhabitants. Each wave carries a
plankton soup which is the main diet of mussels and
barnacles. Also, dead plants and animals wash into the
tidepool feeding the numerous scavengers and opportunists,
like hermit crabs, shore crabs, sea gulls and even anemones.

Lifestyles of the Spineless and Slimy

Shore crabs peek from protected rocky nooks as the tide slowly
retreats. During the six-hour transition from high tide to low tide,
sea stars search for mussel meals. Hermit crabs scavenge in
shallow pools for food and new shell-homes. As the tide
continues to drop, anemones close tightly, their flowery tentacles
protected inside their jelly-like bodies. Limpets, periwinkles, and
chitons wait for the tide's return to continue grazing on
algae-covered rocks. At high tide, it's meal time. Plankton flows
into the pools on each wave, feeding barnacles, mussels and
worms. Seaweed, or marine algae, glistens through the waves, in
bright greens, pinks, browns and reds. The watery world comes
alive with fish, octopi, urchins, anemones, snails, crabs and sea stars. 

Most of the animals that live in the tidepools, or intertidal zone, are 
invertebrates; they have no
backbone--they are spineless. Instead of a backbone, some animals have 
rigid exoskeletons that
protect them from drying out, being eaten and being smashed to pieces by 
the pounding surf.
Barnacles, sea stars, limpets, crabs, chitons and sea snails all have hard 
shells for protection.
Anemones, sponges and nudibranchs have very soft and flexible bodies that 
absorb wave impact.
When the waves crash it's not certain death, it's mealtime! 

Marine algae have developed similar characteristics as
land-dwelling plants. They photosynthesize light and nutrients into
sugar and oxygen. Many types of algae grow in the tidepools:
green, brown and red. The color of the algae gives away their
name. Marine algae contain a gummy substance, called agar,
which allows them to be very flexible as the tides change. The
leaf-like blades sometimes have small attached air sacs which
allow the blades to float to the surface facilitating photosynthesis.

Because of the changing nature of the tidepools, some areas get
more water and others get more exposure to the air. As a result,
plants and animals have adapted to each specific region: the
splash zone, the high zone, the mid zone, and the low zone.

During an ordinary low tide, the splash zone, high zone and part of the mid 
zone are exposed and
accessible for exploration. Only during the spring and neap tides is the 
low zone exposed. During full
and new moons, there are often very low tides which offer an excellent 
glimpse into this fascinating

Some animals survive best exposed to the air, with only an occasional 
splash. In the splash zone, for
example, animals find most of their food in the driest part of the 
tidepool. There are only a few
species that live in this area. The next zone is the high zone, or 
supralittoral fringe. This is the upper
limit the water reaches during high tide. Most of the plants and animals 
here need a minimum of
exposure to water. Barnacles are common inhabitants of this region. Below 
the high zone is the mid
zone, or infralittoral fringe. This is covered most of the time by water 
and receives exposure to air
only during low tides. The most diversity is found in this region. The 
lowest part of the tidepool is
called the low zone, or the infralittoral zone. Animals and plants live 
here that can survive a minimum
of exposure to air. Urchins, abalone and brown algae are common.

The Low Zone

The low zone receives the most consistent amount of water.
Only during the lowest tides is this area exposed to air. As a
result, compared to other regions of the tidepools, low zone
animals need the most water. Two of the most interesting
animals found in the low zone are nudibranchs and sea

Sea Urchins 

Purple sea urchins are common in the lowest part of the
tidepool. They protect themselves from drying out by carving
a hole in the rocks with their tough spines. Inside this little
cave, the urchins are protected from crashing waves. During
low tides, the holes fill up with water, providing necesssary
moisture for their survival. To feed, the herbivorous urchins
break off pieces of algae. They have rows of teeth located
inside their mouths. Sometimes, the urchins grow too large to
escape from this cave. To survive, they catch pieces of algae with their 
tube feet which can extend
beyond their spines. 


Nudibranchs are small, brightly colored slug-like creatures that live
in the deepest parts of the tidepools. They have developed exposed
gills (nudi=naked and branch=gill) which they use to breath
underwater. Nudibranchs are carnivores that prey on sponges,
bryozoans, hydroids and sometimes anemones. Those that eat
anemones are unaffected by the anemone's stinging cells. They have
developed a mechanism whereby they incorporate the anemone's 
stinging cells into their own bodies
to defend themselves against being eaten.

The Mid Tide Zone

The mid zone is the most active zone in the tidepool. Many animals cross 
over and feed in this region
as the tides change. Mussels, sea stars and anemones are commonly seen in 
this region. 

Anemones - Cousin of the Jellyfish 

Anemones, recognized by their flower-like shape, have
stinging cells on their tentacles. Like their cousin the jellyfish,
anemones paralyze their prey and draw them into their
mouths to be digested. Anemones are carnivores and will
eat just about any animal that comes close enough to be
caught by its tentacles. Giant green anemones eat crabs, sea
stars, mussels, limpets and fish. The smaller anemones eat
small fish, shrimp and even plankton. 

Anemones have soft, jelly-like bodies that absorb the
impact of battering waves. Most anemones, like barnacles,
remain in one place for their lives. Anemones attach to the
rocks with strong suction cups. Sometimes if food is scarce,
they will let go and pump water through their hollow body (like the 
jellyfish) and move to a more
suitable home. 

When the tide retreats, anemones close up their soft bodies, folding their 
tentacles inside to preserve
moisture. When tidepooling, take care not to crush anemones. They can be 
well-camouflaged with
small pieces of rocks and shells attached to their outside. 

                                Sea Stars 

Sea stars are a diverse group of spiny-skinned animals,
related to sea urchins (echinoderms). The most common
sea star in Northern California is the ochre star, which is
usually orange, black or purple with a tough exterior.
Leather stars, bat stars, six-rayed stars, sunflowers star
and brittle stars are also present.

Sea stars have a malleable exterior which allows them to
wrap tightly around rocks' algae during tidal changes. Their
internal hydraulic system allows them to cling tightly to
rocks. They suck in water through their tube feet that run
along their undersides, creating suction which they then
attach to the rocks. This helps them stay connected during
the roughest water.

                                Sea stars have a fascinating method of 
eating. They are
also carnivores, but they particularly enjoy mussels. The ochre star can 
often be seen during low tide
just below a belt of mussels. As the tide retreats, the sea stars move 
across the mussel bed for a
meal. They attach their strong feet along each of the two shells of a 
mussel. They then pull the shells
apart. Once the shell is opened, even a fraction of an inch, the sea star 
starts to eviscerate its
stomach (pull it outside its body). It then pushes the stomach into the 
shell of the mussel. The
stomach contains strong digestive juices which it secretes to make a mussel 


Chitons are pre-historic-looking mollusks. Their
eight-segmented flat bodies are attached to the rocks by a
strong foot. There are many types of chitons in Northern
California, from the large gumboot chiton to the beautiful lined

Chitons are, for the most part, herbivores. They graze on
marine algae during high tide. They also have a scraping
radula which shears algae from the rocks. 


Mussels, like barnacles, gather in dense clusters in the
tidepools. Their blue-gray bodies contrast against the brightly colored sea 
stars that prey on them.
Mussels are bivalves; they filter plankton from the water they suck into 
their bodies. An
average-sized mussel can filter up to three quarts of water in an hour! 
Like many of the other animals
in the tidepool, mussels remain in one place for their whole lives and they 
start their lives as plankton.
As they mature, they begin to secrete a thread-like substance called a 
byssal thread. These byssal
threads hold the mussels firmly to the rocks and other mussels. Once 
mature, mussels are unable to
regenerate their byssal threads.

The High Zone

Many of the animals that live in the high tide zone have
hard, flat shells to disperse the impact of the waves.
Barnacles, limpets, crabs and snails thrive in this region.
When the tide is in, they busily move around searching
for food. When the tide is out, they must protect
themselves from drying out, or dessication. All of the
high zone animals have incredible abilities to stay
attached to the rocks as the waves crash down. 

Crabs - A Tough Exterior 

Crabs, like all crustaceans, have a hard protective shell.
The tough exterior can withstand crashing waves. Their
flat shape allows them to squeeze into tight crevices,
further protecting them from the waves and predators.
Crabs can survive out of water for extended periods of time. As they wander 
around during low
tide, they scavenge for bits and pieces of plants and animals. 

Crabs have five pairs of strong claws which allow them to grasp the 
slippery rocks. The first pair is
noticeably larger than the rest. With these front claws the crabs can tear 
food from the rocks and
pinch probing fingers. A large crab can easily break the skin and 
potentially break bones. Beware! 

Barnacles - Cousin of the Crab 

Barnacles are the most common inhabitant of the high tide zone. Although 
small and camouflaged,
they cover tidal rocks by the hundreds of thousands. Like crabs, barnacles 
are protected by a hard
shell. Unlike crabs, however, barnacles do not move once they reach 
adulthood. After hatching from
eggs, barnacles are planktonic. They float around the oceans moved by wind 
and ocean currents. As
they mature, they develop a glue-like substance on their heads. As a 
result, they become stuck by
their heads to rocks. They eat by kicking the feather-like feet into the 
water to capture plankton. 

Limpets & Snails

Limpets and snails are soft-bodied animals that
produce a hard, often very decorative shell. They
scrape algae from the rocks with their sandpaper-like
tongue, or radula. Limpets and snails only graze while
the tide is high. Limpets always return to the same
place when the tide retreats. Some limpets carve
themselves a place in the rock which perfectly fits their
shells. This protects them from being eaten and drying
up when the tide is low.

                            The Splash Zone

During high tide, crashing waves send salty
mist onto the splash zone. This uppermost
region of the tidepool is seldom covered by the
ocean. Instead, it gets moisture from wave
splash and rain. As a result, an interesting mix
of plants and animals survive here. Land-based
(or terrestrial) animals and plants survive here
if they can tolerate some sea water. Marine
animals and plants survive if they can tolerate
exposure to air. As a result, relatively few tidal
animals live in this zone. Terrestrial plants, like
lichen, which can tolerate some salt water, are
found here. Isopods, related to the pill bug,
also scavenge in the splash zone for fragments
of plants and animals.

Thanks for taking the time to send in a question to the Mad Sci Network

June Wingert
Baylor College of Medicine
Houston, Texas

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