Hello marine biology students.
In this video we're going to talk about the
intertidal zone and the challenges of living
in this marine environment.
[Intro Music]
The intertidal zone 
is the area between the mean low tide and
the mean high tide.
By contrast, the subtidal zone 
is the area that is always submerged, even
at low tide.
The intertidal zone will be exposed during
low tide.
There are a variety of different types of
substrate
or bottom that make up the intertidal zones.
The intertidal zone substrate can be rocky
or made of a softer material.
Soft bottoms can vary from sand to silt or
a mixture of mud and we can see the size gradient
from sand being a more coarse substrate down
to silt and clay, which make up mud.
Rocky substrates can vary as to the type of
rock and the slope of the angle.
In the lower picture we see a gravel beach.
A volcanic gravel beach in Hawaii.
The gravel is hard and much larger than sand,
yet far more movable than a solid volcanic
rock would be.
When we look at the organisms that are living
in the intertidal, they are either going to
be epifauna, living on the surface of the
substrate.
These could be snails that are living on the
surface of the mud or barnacles living on
the surface of rocks, also a variety of different
types of algae and seaweeds and other organisms
living on the surface.
Infauna will be organisms living buried within
the substrate.
These could be burrowing clams in a soft bottom
or even certain types of tube worms that are
interspersed down into the rocks themselves.
And then, the last category are the meiofauna.
The meiofauna are the organisms that are so
small that they actually live between the
grains of sand in soft substrate.
These organisms can be sessile or motile.
For a marine organism, the intertidal is a
very harsh place to live and there are many
challenges that must be faced living in this
environment.
These challenges are the result of being exposed
to air regularly and also the wave action
of the water.
These challenges can include desiccation or
water loss, temperature change, salinity changes,
interrupted feeding, wave action and tides,
oxygen availability, buildup of CO2 and also
just limited space.
Many of these challenges are not seen in other
marine environments.
They are unique to the intertidal and unique
challenges that intertidal organisms need
to be able to deal with.
And so, the first of these is desiccation.
When exposed, the intertidal organisms deal
with potential water loss.
Water loss will be more pronounced 
on hot dry or windy days.
Organisms can deal with the potential water
loss either by hiding or by clamming up.
So, as we look at these strategies, hiding
may involve moving to a tidepool or an area
with more moisture.
This would be the case for motile organisms.
Hiding may also mean that some organisms only
live in areas where moisture will remain,
even when the tide is out, such as crevices
in rock or deeper spots in soft bottoms.
So things like algae, these seaweeds, they
might only be growing in the little pockets
or cracks or crevices where at least some
water remains, even during low tide.
And the absence of these algae growing on
the surrounding rocks has more to do with
the inability for algae to survive on those
rocks during low tide.
Clamming up occurs with those organisms that
have protective shells or valves and they
can be closed within their environment.
In that way, they can conserve moisture.
Yet, while moisture can be conserved this
way, there is a downside.
No gas exchange can occur and no feeding can
occur, so oxygen might be used up, carbon
dioxide will build up, and these organisms
have a reduced amount of time during the day
to obtain the nutrients they need for their
survival.
Most marine environments are marked by very
stable temperatures, but that's not the case
for the intertidal.
Most marine organisms are not use to significant
temperature fluctuations, yet again, this
is not true for organisms in the intertidal.
The wide range of temperatures that must be
tolerated by organisms in the intertidal can
be severe.
And so, only those organisms that can handle
these temperature fluctuations will continue
to be found there.
Some organisms have mechanisms to help stay
cool, such as the white shell of this Littornia
snail.
Many other snails of this species have a darker
pigmentation to their shell, but these are
snails that will often be exposed during low
tide and their shell color might allow them
to stay cooler, even if it makes them more
obvious to predators.
Another factor that can change quite dramatically
in the intertidal is salinity.
This variation in salinity can be due to evaporation
within the tide pools or even freshwater precipitation
in the form of rain.
In fact, a heavy rainstorm can sometimes be
very damaging to organisms in the intertidal
if it occurs during low tide.
Normal ocean salinity is around 33 to 35 parts
per thousand.
Estuarian salinity can vary between 5 to 30
parts per thousand.
We can see that tide pools will vary even
more.
Organisms in the intertidal are normally euryhaline,
meaning they can tolerate wide varieties of
salinities.
Whereas subtidal organisms by contrast are
normally stenohaline
and they do not tolerate variations in salinity
well.
For an intertidal organism, the salinity may
vary greatly in the course of a single day,
starting at 20 to 25 parts per thousand due
to freshwater addition in the form of rain,
but then climbing dramatically throughout
the day due to evaporation and water loss
when the intertidal is exposed.
Salinity may also drop during the day due
to a sudden influx of fresh water provided
by a passing rainstorm.
A salinity change of possibly 20 parts per
thousand or more combined with the temperature
change of 40 or more degrees may occur.
These would be hellish changes for most marine
organisms and this is why the intertidal is
a very harsh environment for an organism to
survive and yet even though these are severe,
these aren't the only challenges for living
in the intertidal.
Interrupted feeding was mentioned previously.
If an organism clams up, feeding will stop.
Feeding will also stop for filter feeding
organisms when they're exposed to air.
In fact, there are very few organisms in the
intertidal that can continue to feed during
low tide, although snails are some of them.
For intertidal communities which experience
semidiurnal tides, an animal could spend nearly
half of their day exposed and not feeding.
The next challenge to consider has to do with
wave activity.
Wave exposure can be a problem for intertidal
organisms, either from physical damage of
the waves themselves or the waves actually
moving around parts of the substrate, such
as loose rock or gravel that can shear and
scrape and pulverize organisms living in the
intertidal.
Some organisms are able to deal with this
wave motion, but if the wave motion is too
extreme, they may not be able to feed properly
and they may not be able to behave in a manner
similar to neighboring intertidal organisms
living in a sheltered bay.
Rocky shores are often exposed to significant
wave action not just the animals but even
the algae has to be able to tolerate the intense
forces of wave action and only those organisms
that are sturdy and hardy enough to do so
will be able to live in the intertidal.
Gas exchange is another challenge to be aware
of.
Oxygen can be exhausted internally 
if an organism clams up during low tide.
It can also be exhausted in tide pools if
there's a high density of organisms, surprisingly,
even algae at night when there is no photosynthesis.
So if there's a lot of algae in tide pools
at night, they are actually using up the oxygen
instead of contributing oxygen and that can
result to very low oxygen levels in these
tide pools.
Carbon dioxide can also build up if there's
excessive respiration.
Some tide pool organisms have evolved the
ability to exchange gases in both air and
water, although for most marine organisms,
gas exchange happens exclusively within the
water, with non-fish marine vertebrates being
an exception there.
Another challenge of the intertidal has to
do with limited space.
In some intertidal communities, space may
be limited.
This is particularly true in the rocky intertidal,
where surface area is limited.
Space can also be limited in soft bottom communities
where many organisms select hard substrates
such as oyster reefs to prevent being washed
away.
This particular image actually shows an artificial
tide pool but the capacity at which it is
stocked shows just how tightly some tide pools
may end up being crammed.
Space is clearly a limiting factor for all
of these predatory sea stars and anemones.
And this completes our discussion of the challenges
of living in the intertidal zone.
We're going to see how these challenges result
in what's called zonation of the intertidal
zone.
And as a question to think about before our
next video, “What factors do you think go
into deciding where you live?”
Alright.
See you in the next video.
