Hello marine biology students.
In this video we're going to be learning about
some of the challenges to living in a marine
environment.
[Intro Music]
This week we're going to be talking about
challenges to life in the oceans, reproduction
of marine organisms, and an introduction to
the diversity of marine life.
The oceans include many habitats, each with
their own challenges and concerns.
So, habitats are areas where an organism lives.
Now, marine organisms will fall into different
groups based on where in the habitat they
live and also how they move.
Planktonic organisms 
are those that drift in the water.
They're subject to currents, they are not
strong swimmers.
They may have some mild swimming ability,
but the current will carry them where it's
going.
So, planktonic organisms, they are in the
water column and they are not strong swimmers.
Conversely, benthic organisms live at the
bottom and they may have varying degrees of
swimming ability, to being entirely sessile
and not able to swim at all, to otherwise
just fish that tend to spend most of their
time on the bottom itself, and also other
marine invertebrates.
And then, the last group are the Nekton.
These are the fast active swimmers.
So these are the three major categories of
marine organisms, again, the planktonic organisms
who just sort of float, the benthic organisms
who are down at the bottom, and then the nekton
who are the fast active swimmers.
They can resist currents or even break free
of currents when needed.
Now, regardless of which category an organism
finds itself in, one factor that impacts all
of the marine organisms is water/solute balance.
All organisms need to be able to have the
essential components of life within the cytoplasm
of their cells, whether it's the macromolecules
they need, the energy making molecules, even
their enzymes, they need to be present there.
But there also needs to be mechanisms for
cells to regulate the amount of water and
solutes within their cells.
The plasma membrane, which is a boundary around
the cell, is mostly permeable to water, but
impermeable 
to ionic and organic solutes.
Yet still, water balance becomes an issue
because water is going to attempt to move
to equalize concentration gradients across
that plasma membrane.
Diffusion is the property of liquids that
causes molecules to move from where they're
in a high concentration towards where they're
in a low concentration.
This is a property that occurs in all liquids
and gases due to the movement of their molecules.
Now, there's a specific category of diffusion
known as osmosis.
Osmosis is the diffusion of the solvent, or
the main dissolving liquid of a solution.
In the vast majority of cases, this is going
to be water, and it's usually the movement
of water from an area in where the water is
more concentrated to an area where there is
a lower concentration of water.
But a solution that's more concentrated with
solutes than with water, water is going to
attempt to move towards that solution.
So when we think of water as a solute, a variety
of different types of substances will dissolve
into that water and as those molecules dissolve
they will try to distribute themselves evenly.
This is the result of diffusion.
Diffusion is always working towards equilibrium.
Since marine organisms live in a very solute-rich
environment, they have a tendency to gain
solutes and lose water.
This can result in the death of the cells,
if the water loss or solute gain is significant.
So organisms living in a marine environment
need to find ways to deal with this solute-rich
environment.
We'll talk about some specific mechanisms
that organisms have as we start discussing
the diversity of life in the next few weeks.
Now, a few terms that are important for us
to understand as we're discussing osmosis
and diffusion are the terms we use to compare
solution concentrations between two different
solutions.
The first term that I'd like to introduce
you to is hypertonic.
The prefix “hyper-“ means above or more
than, and in this case tonic or tonicity is
referring to molecules dissolved in a solution
or the solute concentration.
So a hypertonic solution has a higher solute
concentration when compared to another solution.
The opposite, a hypotonic solution, is going
to have a lower concentration of solutes compared
to another solution.
And so in fact, by definition, since we only
use these terms in comparison, if one solution
is hypertonic that means the solution you're
comparing it to is hypotonic to that first
one.
Then lastly, if you have two solutions that
have the same ion concentration or the same
solute concentration, we would say that those
two solutions are isotonic and that prefix
“iso-“ just means the same.
Now, it turns out you can have two isotonic
solutions that have very different solutes
in them, but that ratio of solute concentration
to solvent concentration would be consistent
and so we would still say those two solutions
are isotonic with each other.
Again, these terms, even isotonic, cannot
be used when talking about a single solution.
We can only use these terms when we are comparing
solutions.
Now sometimes when we're using these terms
to describe an environment, like a hypertonic
environment, the other solution that's being
compared is the cytoplasm
within the organism cells.
So if the environment is hypertonic, that
means the cytoplasm is hypotonic.
Now, why do we care about the concentration
of solutions?
Well, it has to do with that property of osmosis.
If you recall, water is going to try to equalize
concentration differences.
Water can move through the cell membrane,
even if the solutes cannot.
And so, if an organism is in a hypotonic solution,
meaning the solution outside of the cell is
less concentrated, water is actually going
to start flowing into that cell.
In fact, enough water could flow into that
cell that the cell could rupture or lyse.
This would be the case of taking an organism
that has adapted to living in a marine environment
and putting it in a freshwater setting.
Whether you caught a saltwater fish on your
last vacation and plan to bring it home to
your freshwater fish tank, that's not going
to work out well for that organism.
A marine-adapted organism usually has a high
concentration of solutes in its cytoplasm.
If you put it in a freshwater environment,
the water will rush into those cells.
Those cells will swell up, rupture, and burst.
Conversely, if you decide you want to take
your pet goldfish to a vacation down to the
beach and you take this freshwater fish and
put it in a marine setting, well, that poor
goldfish is going to shrivel.
The water is going to leave the cells of the
goldfish, attempting to dilute the ocean,
and your goldfish is going to lose that battle.
It will take a lot more water to dilute the
ocean than is found in your goldfish and it
will shrivel and shrink and die.
Organisms are happiest when their environment
matches, at least to some degree, the concentration
in their cytoplasm.
But what we'll see is that some organisms
do have the ability to live in an environment
slightly different than their own cytoplasm
and still manage to survive, but it does take
special mechanisms to do that.
Now why is it that water flows towards a hypertonic
solution?
The concentration of a solution 
tells you about the ratio between the amount
of solute or the dissolved components and
the amount of solvent or the dissolving components.
So, the ratio between solute and solvent,
a hypertonic solution
has a higher ratio of solute to solvent compared
to another solution.
This also means that it has a lower solvent
concentration compared to that other solution.
So, there are more salts or there are more
dissolved materials, but there's actually
a lower concentration of water in a hypertonic
solution.
Now since osmosis is a form of diffusion,
diffusion always wants molecules to move from
where they're at a high concentration towards
where they're at a low concentration.
So with osmosis, the water will move from
where you have a lot of water or a high concentration
of water, a very dilute solution, towards
a more concentrated solution, towards the
solution that doesn't have a lot of water
in it or has a higher concentration of solutes.
So in that way, water will move from the hypotonic
solution into the hypertonic one.
Another way to think about this is that water
is always trying to dilute the more concentrated
solution.
Because diffusion is always working towards
equilibrium, diffusion is trying to make things
equal.
Now, when it comes to different organisms,
organisms need to maintain somewhat stable
internal conditions.
We call this concept homeostasis.
Now, how does this work?
Well, some organisms will try to match whatever
that ionic solution of their environment is.
We call these organisms osmoconformers.
So, for osmoconformers, their internal concentration
varies as the salinity in the water around
them changes.
They match their environment.
They do not attempt to control solute and
water balance.
That means that these organisms, their cytoplasm
concentration will match their environment.
These organisms can only tolerate very narrow
ranges of salinities.
Many marine invertebrates fall into this category.
Luckily, the open ocean and many marine environments
have very stable salinity levels.
Another strategy is to be actively different
than your environment.
These organisms are known as osmoregulators
because they regulate their water and solute
balance.
These are organisms that control their internal
concentrations of solutes and water.
This is true of most marine vertebrates and
also most freshwater aquatic vertebrates as
well.
Now, this can be done in a variety of ways,
like secreting very little urine or using
specialized glands 
to secrete salts.
In fact, as we'll see sharks and even some
of the giant squid, they use a different metabolic
product called urea to fill the cytoplasm
of their cells so that it matches the surrounding
salinity of the seawater, yet they are in
active control of those solute levels.
Osmoregulators can generally tolerate a wider
range of salinity than osmoconformers, yet
usually there are limits for these osmoregulators
as to the concentrations of environments that
they can live in.
But we see some amazing examples of fish,
such as salmon, that start their life entirely
in freshwater in rivers and streams and yet
they live most of their adult life in the
ocean, so a salty environment, and then they
return back to a freshwater environment to
spawn and eventually die.
When looking at saltwater balance in fish,
we’ll see different mechanisms for a marine
fish compared to a freshwater fish, because
these organisms are dealing with slightly
different environments, even though their
internal cytoplasm may be very similar to
each other, at about 14 PSU or about 14 parts
per thousand salinity compared to the ocean
at 35 or compared to freshwater which is at
0.
These fish need to have mechanisms to balance
their water and solute level within their
body.
Again, for a marine fish, it will be drinking
a lot of seawater, yet a lot of the salt in
that seawater will pass through its gut.
It won't be absorbed and it will be excreted.
Usually there's very small amounts but highly
concentrated urine.
Also, salt can be secreted by the gills.
Conversely, looking at a freshwater fish,
they don't drink the water.
They end up absorbing water through osmosis.
They are able to retain salt through their
gills and they usually produce large amounts
of very dilute urine without a lot of salts,
because they need to retain as much salts
as they can, so it's not lost to their environment.
Another aspect of homeostasis is temperature
control and you may be familiar with the major
categories of ectotherms versus endotherms.
Ectotherms generate body heat metabolically,
but that heat is lost rapidly to the environment.
So those organisms cannot maintain a constant
internal body temperature.
Their body temperature is simply going to
match their surrounding environment.
These are organisms we would refer to as cold-blooded.
So ectotherms, “ecto-“ means outside or
external.
So they get most of their body heat from outside
of their own body.
Conversely, endotherms, endotherms retain
most metabolic heat.
Their body temperature is generally higher
than that of the surrounding environment.
Now, another category beyond just ectotherm
versus endotherms are poikilotherms versus
homeotherms.
So, a poikilotherm is going to be an organism
whose body varies with the temperature of
their surrounding environment and this might
sound very similar to ectotherms versus endotherms,
which we just talked about, but as we're going
to see, there are actually some categories
of endotherms who’re also poikilotherms.
Conversely, homeotherms, these are going to
be organisms that their body temperature does
not change with their environment.
So, humans and in fact all mammals and birds,
we are endotherms and we are homeotherms,
but when we look at this chart looking at
marine organisms, there are poikilotherms
that are ectotherms and these are most marine
invertebrates even most fish and marine reptiles,
but there are some larger fish such as sharks
and tunas and even some bill fish like marlins
that are technically endotherms.
They generate body heat, their body is warmed
and they are warmer than their surrounding
environment, but they are poikilotherms, meaning
that if they're in a cold environment they'll
be slightly warmer than it, but they'll still
be cooler, compared to birds and mammals who
almost regardless of the temperature of their
surroundings their internal body heat is set
by their own mechanisms.
So that takes us to the end of this video.
Now, before our next video I want you to ponder
the question of how reproduction counteracts
death.
All right, see you in the next video!
