Hello marine biology students.
In this video we're going to talk about coral
reefs.
[Intro Music]
Corals are cnidarians, which are characterized
by the presence of stinging cells known as
nematocysts, which are found on their tentacles.
They are related to jellyfish.
In a coral, the coral polyp secretes a calcium
carbonate skeleton, and this builds up over
time and over many generations, growing the
coral colony.
Most reef-building corals are colonial, meaning
they are interconnected polyps that develop
asexually from a single polyp.
Two of the major categories when it comes
to corals are the hermatypic corals, which
are reef building, and the ahermatypic, which
do not build reefs.
So again, the hermatypic corals are reef builders
whereas the ahermatypic are not.
The hermatypic corals will contain zooxanthellae.
These are symbiotic, photosynthetic dinoflagellates,
a
type of phytoplankton.
The ahermatypic corals may or may not have
these zooxanthellae.
Hermatypic corals will be tropical in distribution,
meaning you'll find them in warm and shallow
waters, whereas some of the ahermatypic corals
can actually be found in cold, deep water.
These are some of the categorizations within
the corals.
In coral nutrition, the coral zooxanthellae
produce organic matter through photosynthesis.
Some of this organic matter is passed on to
the coral polyp itself.
The zooxanthellae can also help in the deposition
of calcium carbonate, making the foundation
that the polyp lives on.
Not only do coral polyps get some of the photosynthetic
product from the zooxanthellae, but they can
also feed on prey aided by the nematocysts
in their tentacles.
There's some debate as to whether the coral
polyps might not be collecting zooplankton
to provide nutrients for the zooxanthellae
to allow them to photosynthesize more.
Corals are also able to feed by using extensions
of the gut wall called mesenterial filaments.
These filaments can secrete digestive enzymes
and can be extruded through the mouth into
surrounding areas.
This allows the coral to feed and digest food
outside of the body, and as we'll see in our
next video, this can also be a form of defense
when competing with neighboring corals.
For a coral reef to grow, a few conditions
have to be met.
There initially needs to be a hard substrate
for settlement of the first larvae and polyps
of the colony.
There needs to be light for the zooxanthellae.
There's a narrow temperature range at which
reef formation occurs.
There's also a narrow salinity range.
There can't be a lot of sediments in the water,
and there can't be a lot of pollution either.
Corals are very sensitive marine organisms.
In the diagram, we see, after the initial
hard substrate, there ends up being loose
sediment and coral rubble that begins to accumulate
on the bottom.
Other reef organisms and the coral themselves
can actually consolidate that sediment, and
that increases the overall height of the reef,
allowing the reef to grow taller.
Corals reproduce both sexually and asexually.
The asexual reproduction allows for the growth
of a colony of polyps from a single polyp,
whereas the sexual reproduction results in
the release of planktonic planula larvae,
which is motile and can disperse into a new
location.
The mass spawning of planula often occur at
the same time in some coral reefs, and often
this is synchronized by the moon.
When we talk about the reef building, or hermatypic,
coral, these corals are very sensitive to
temperature.
They tolerate only a low temperature range
compared to most marine organisms.
They are limited by cold currents, and they
cannot survive and travel through them.
Coral can only reproduce and grow if water
temperatures are over 20 degrees centigrade,
or 68 degrees Fahrenheit.
Higher water temperatures are preferred for
optimal growth, but there is an upper limit
as well.
Temperatures above 30 to 35 degrees centigrade
cause coral stress.
Coral stress can lead to bleaching, an expulsion
of zooxanthellae from the coral polyp.
When we look at the temperatures which promote
coral growth, we can see it's warm water currents
which allow corals to be deposited and distributed
to other areas.
The cold currents prevent the transfer of
corals.
Now the expulsion of zooxanthellae is called
bleaching because the normally colored coral
is bleached white after expulsion.
Without the zooxanthellae, the coral is mostly
translucent, and the white color you see is
the calcium carbonate skeleton below.
Zooxanthellae are what give corals their distinctive
colors.
Other events besides high temperatures can
cause coral bleaching such as poor water quality,
increased sediment in the water, wave stress,
or disease.
If a coral remains bleached for too long,
it could result in death of the coral, but
coral bleaching does not always result in
coral death.
These corals can reacquire zooxanthellae from
the environment, and it's believed that some
zooxanthellae have greater temperature ranges
and tolerances, so it could be that coral
bleaching is actually a mechanism in which
the coral is attempting to get zooxanthellae
that will be better suited to the higher temperature.
Widespread bleaching often occurs as a result
of events such as El Niño, which is a change
in the circulation patterns of the upper ocean,
and can bring warmer waters to more areas.
Other requirements of coral reef building
is that corals do not normally occur in areas
where rivers deliver large amounts of fresh
water because they have very narrow salinity
tolerances.
Low salinity can result in coral bleaching
as well.
Rivers also usually bring with them suspended
sediments, which can be detrimental to the
coral if they settle on them and reduce the
water clarity, which reduces access to sunlight
needed for photosynthesis 
by the zooxanthellae.
High wave action can also increase the amount
of suspended sediment, limiting the formation
of coral reefs.
We have come to find that even low levels
of pollution can kill coral polyps.
High-nutrient runoff levels from land can
also allow seaweeds to grow and take over.
Increased seaweed growth shades the light-sensitive
coral and their zooxanthellae.
Now this photo is taken from Kaneohe Bay in
Hawaii.
The Kaneohe Bay coral reef has had several
challenges within the past few decades of
humans encroaching and living closer on this
shore.
The history of this bay is recorded in our
textbook and definitely something worth reading.
Coral colonies can come in a variety of shapes
and sizes with different species of coral
generating different colony shapes.
Taller, more branching corals tend to be more
common in shallower areas of the reef due
to intense competition 
for space and light.
This is also why high-rise skyscrapers and
apartment buildings are more common in areas
where a lot of humans are living—because
there's more competition for space.
Flatter forms of coral tend to be more common
in deeper areas of the reef, and yet they
need to be larger to help catch more light
in that deeper area.
Corals aren't the only organisms contributing
to the calcium carbonate of the reef as well.
There are calcareous 
green algae and coralline algae that are also
producing calcium carbonate.
These algae can also function to cement sediments
together by encrusting over them, adding to
the bulk of the reef.
There are some other cnidarians, such as hydrozoans,
that can produce a calcium carbonate skeleton
that might also contribute to reef formation.
Even sponges can play a role in cementing
the coral rubble together into a solid substrate.
Also bryozoans, which are colonial lophophorates,
they can end up contributing their calcium
carbonate tests to the sediments as well.
All of these can be combined together to allow
the reef to grow taller and taller.
So that completes our introduction to coral
reefs.
Before our next video, I would like for you
to think about what it would be like to compete
with many other individuals for limited resources.
We'll talk about that.
See you in the next video.
