Hello marine biology students.
In this video we're going to talk about the
challenges of living in the epipelagic.
[Intro Music]
Living in the epipelagic means finding ways
to stay afloat.
This can be accomplished by increasing buoyancy
by means of accumulating lipids or air, or
by increasing surface area or drag.
Sharks often concentrate lipids within their
large liver, which allow them to stay buoyant.
Yet often sharks do still require forward
motion to avoid sinking.
Some organisms increase their surface area
by being flat and similar to dropping a sheet
of paper in air, this flat sheet will cause
horizontal movement and a rocking motion,
which can allow for continued exchange of
materials with the water.
Other organisms have a variety of spines or
appendages to increase their surface area.
In both cases, increasing the surface area
promotes drag or water resistance which helps
keep bodies from sinking.
Some organisms increase buoyancy by storing
droplets of lipids, which tend to float because
they are less dense than water.
In this image we can see a variety of different
diatoms and their lipid storage particles
can clearly be seen within their cells, applying
more buoyancy.
Organisms that do this include: diatoms, copepods,
and many planktonic larvae.
Some organisms also have pockets of air that
increase buoyancy, such as certain cyanobacteria,
cnidarians, and fish that have swim bladders.
Another designation of types of plankton are
those that float and whether they float right
at the surface or above the surface.
The plankton that are right at the surface
are known as neuston and these include snails
that prey upon Portuguese Man O’ War and
other floating cnidarians.
The Portuguese Man O' War and pictured below
by-the-wind sailor are both considered to
be members of the pleuston, because part of
them float above the water surface.
So, the Portuguese Man O' War, Physalia and
the predatory snail Janthina
can be seen in the top picture.
The snail is preying on the Portuguese Man
O’ War, and we can see barnacles attached
to the shell of the snail.
By-the-wind sailor is another type of pleustonic
cnidarian.
Because pelagic animals have virtually no
place to hide, it must have other means for
finding prey or to avoid being eaten.
Here in this image we see a flying fish, which,
while it cannot fly for long distances, it
can leap out of the surface of the water and
to its predators appear to have vanished,
until it lands a certain distance away.
Fast swimming, protective coloration, vertical
migrations and a variety of sense organs are
adaptations to accomplish this task of finding
prey and avoiding being eaten.
Eyes are very useful in the epipelagic, where
there is a lot of light.
Eyes can be used to form images for some organisms
or simply to sense the presence of light or
dark or patterns.
Most pelagic animals have well-developed eyes.
Eyesight is used to capture prey, avoid being
eaten, find mates, and for some, to stay in
groups.
Cartilaginous and bony fish also have a lateral
line for sensing prey or predators.
This lateral line can also be used in schooling
fish to remain a part of their group.
Dolphins and other cetaceans can use echolocation
as a way of getting more information about
their surroundings.
To blend in with their environment, organisms
can have different types of protective coloration.
This can include countershading, camouflage
and also transparency.
You can hardly see this cubomedusa in the
wild, and yet it has a very powerful neurotoxin
within its nematocysts.
Countershading is when a fish or another marine
organism will be darkly colored on its dorsal
side and lightly colored on its ventral side.
It may also have reflective silvery sides
as well.
And this countershading can help the organism
be disguised regardless of whether it's predator
or prey is seeing it from above or from below.
Another way to hide is to lack coloration
and become transparent.
This is the case with many jellyfishes, comb
jellies, salps, larvaeceans and some zooplankton
as well.
Another adaptation for some organisms in the
epipelagic is to have impressive swimming
ability.
Epipelagic predators must be able to swim
quickly to capture prey and some of the fastest
recorded swimmers are found in the epipelagic,
such as billfish and sailfish.
This is accomplished by several adaptations
in pelagic fishes such as in tunas.
They have a streamlined body to reduce drag,
they have a strongly forked caudal tail to
increase thrust and stiff pectoral fins for
lift and to decrease drag.
These fast swimming fish will also usually
be laterally compressed
to increase streamlining, reduce drag, and
to apply more force with their swimming.
Some of these fish are actually warm-blooded
because the heat energy generated by muscle
activity is carried back into the returning
blood and their body temperature is a few
degrees warmer than their surroundings.
Now, they are still poikilotherms not homeotherms,
but they are warmer than their surroundings.
These fast swimming fish often have red muscles,
which have a high concentration of myoglobin,
which are able to store extra oxygen.
Some organisms of the epipelagic and mesopelagic
migrate vertically throughout a 24-hour period,
spending the night in surface waters and daylight
deeper down.
So some pelagic animals move into deeper waters
during the day to avoid predators.
At night they move back in shallower waters
to feed on plankton.
This predator avoidance comes with a cost,
because it takes quite a bit of energy to
migrate then it would be to stay in one place.
In the epipelagic, typically an animal will
not feed on the same type of organism throughout
its life.
Here we're specifically looking at the feeding
patterns of a herring, and as a young herring
we see it is a prey to many and feeds on a
few different types of organisms.
As it grows larger, its diet changes and as
it is consumed by fewer, yet other members
of the epipelagic.
And soon, as an adult herring, none of the
other plankton consume it, except maybe for
some jellyfish.
Unlike the diatom - krill - baleen whale food
web in the polar regions, which is quite simple,
we can see quite a number of connections within
this food web.
While the epipelagic supports photosynthesis,
there will be limits to production based on
certain properties of the water.
Occasionally nutrients will be limited, particularly
nitrate and phosphate.
One of the reasons for this nutrient limitation
in the epipelagic is that sometimes organic
matter will be lost from the environment,
sinking down into lower levels and not being
replaced.
So at least some new nutrients need to be
added from other sources.
Occasionally light will be the limiting factor,
even though there's normally plenty of light
in the epipelagic, light is not present during
the night and also in high latitudes during
their winter.
When we look at the equatorial waters, the
total amount of primary production stays pretty
low throughout the year, and this is normally
nutrient limited.
When we look in the temperate regions, we
see that during the winter, primary production
is light-limited but upwelling allows there
to be large amounts of nutrients.
Those nutrients are then going to allow increased
photosynthesis and primary production during
the spring, but those nutrients will be consumed
and production will drop.
In the polar regions, light limiting is the
most common and nutrients can build up in
waters such as during the polar summer, there
will be intense primary production.
Bacteria, viruses, and other microorganisms
are important recyclers of nutrients in the
epipelagic, and this is known as the microbial
loop.
From decomposition of detritus and other organisms,
to lysis by viruses and release of dissolved
organic matter, this microbial loop often
plays an important step in moving from the
phytoplankton to the zooplankton.
Nutrients are often depleted in the surface
waters.
And so, often it requires water coming in
from the deep ocean in order to replace some
of these lost nutrients.
The water at the surface is less dense than
water deeper down, which is why it's the surface
water in the first place.
So, to bring new deep water up, the density
of that surface water needs to change.
The cooling of surface waters can cause deep
water to be propped to the surface in certain
areas.
This is known as upwelling.
Upwelling brings vital nutrients to the surface,
nutrients that had been lost from the pelagic
as dissolved organic molecules, fecal matter,
and mucus.
Primary production is higher in areas of upwelling.
Sometimes this upwelling can occur along the
coast.
When winds are blowing parallel to the coast
that causes a net movement of water, known
as Ekman transport, and the displaced water
will be replaced by deep water and that deep
water brings with it many nutrients.
This also occurs at the equator, known equatorial
upwelling.
As the trade winds are blowing along the equator,
Ekman transport will cause water to move in
different directions due to the Coriolis effect.
This parting of the waters will bring up deep
water, allowing for higher productivity at
the equator than might otherwise be expected.
And that takes us to the end of our discussion
of the challenges of living in the epipelagic.
Now, before our next video, I want you to
think about “What do you do when it's too
dark to grow food?”
We'll talk about that in the next video.
See you then.
