Corals are colonies of tiny polyp animals that build reef structures by secreting limestone, while also hosting symbiotic algae called zooxanthellae that provide food through photosynthesis. However, when corals experience stress like higher water temperatures, they expel the zooxanthellae in a process called bleaching which can damage or kill the coral. While bleaching stresses corals, recovering is possible if the stressful conditions do not last too long and the corals are able to regain their symbiotic algae.

1. Corals
Landlords of the Reefs

3. What is a coral?
A “coral”is actually a “coral colony”
Rocky limestone base,Surface is covered
by thousands of tiny coral animals, called
“polyps”.
Polyps are filled with microscopic algae

4. What should you know about corals?
 They are animals
 Plants live inside of them
 Two kinds
Soft corals
Hard corals (These build reefs!)
 They are made of tiny polyps (which look like upside-
down jellyfish)

5. Hard Corals :-The Reef Builders
 Polyps build hard
limestone cups around
their bases.
 The cups cement
together to make a
coral Colony.
 Reefs are made of
hundreds of hard coral
colonies next to and on
top of each other.

6. Anatomy of a
coral polyp.
- Each polyp looks like a
tiny sea anemone
- Ring of stinging
tentacles around a
central mouth
- Rests in a “cup” on the
surface of the colony

7. What’s a Polyp?
 Tentacles release stinging
cells when something
brushes by them.
 Polyps make their own
limestone cup to hide in
during the day.
 At night, polyps come out to
catch plankton floating by.

8. Like a sea
What do corals eat? anemone, the polyp
can capture small
prey (mostly
zooplankton) with
its tentacles & Also
get nutrition from a
Symbiotic
relationship with
zooxanthellae.….

9. Fish What are zooxanthellae?
Algae
Shrimp

10. What are zooxanthellae?
-Algae that live in the
coral polyp’s surface
layer
-Algae get nutrients and a
safe place to grow
-Corals get oxygen and
help with waste removal
-Corals also get most of
their food from the algae.

11. Symbiosis:
So Happy Together
Two organisms living together and helping each other
is called symbiosis.
Zooxanthellae make oxygen, remove the
polyp’s wastes, and make food for the polyp
from photosynthesis.
Coral polyps protect the zooxanthellae, release
CO2, and provide it with necessary nutrients
from their own waste.

12. What is coral bleaching?
Healthy Bleached
coral coral
Coral bleaching is caused by stress
Poll question: what might be stressful for a
coral? We’ll get back to the answer later….

13. What turns the coral white?
As a stress response, corals expel the
symbiotic zooxanthellae from their tissues
The coral tissue is clear, so you see the
white limestone skeleton underneath.

14. What can stress a coral?
High light or UV levels
Cold temperatures
Low salinity and high turbidity from coastal
runoff events or heavy rain
Exposure to air during very low tides
Major: high water temperatures

15. Thermal stress
Corals live close to their thermal maximum limit
If water gets 1 or 2°C higher than the summer average,
corals get stressed and bleach
NOAA satellites measure global ocean temperature and
thermal stress.

16. How warm is warm?
How hot do you think the ocean has to get
before corals start to bleach?

17. Can corals recover?
Yes, if the stress doesn’t last too
long Some corals can eat more
zooplankton to help survive the
lack of zooxanthellae.Some
species are more resistant to
bleaching, and more able to
recover

18. Can corals recover?
Corals may eventually
regain color by repopulating
their zooxanthellae
Algae may come from the
water column
Or they may come from
reproduction of the few cells
that remain in the coral
Corals can begin to recover
after a few weeks

19. Does bleaching kill corals?
Yes, if the stress is
severe
Some of the polyps
in a colony might die
If the bleaching is
really severe, whole
colonies might die
Bleaching in Puerto
Rico killed an 800-
year-old star coral
colony in 2005

20. What else can stress do to corals?
Question: what is something that happens to
people when they are highly stressed?
Gain weight Get sick Turn orange
Answer: More likely to get sick

21. Bleaching and coral disease
Coral diseases are
found around the
world.
High temperatures
and bleaching can
leave corals more
vulnerable to
disease.
Can quickly kill
part or all of the
coral colony.

22. Bleaching and bioerosion
We have seen that
bleaching can kill part
or all of a coral colony.
Areas of dead coral
are more vulnerable to
bioerosion (when
animals wear away the
coral reef’s limestone
structure.)

23. Hurricanes & coral bleaching
2005:
Most named storms
Most hurricanes
Most damage in US
The same warm water that causes corals to
bleach can also lead to strong hurricanes.

24. Hurricanes can cause direct
physical damage to coral reefs
However, the storms also cools the
water
Hurricane Katrina led to less
bleaching in the Florida Keys, 2005

25. Hurricanes: a mixed blessing
Sombrero Reef , FL
DHW (degC week)
SST (degC)
Dennis
9 July
Katrina
26 Aug
Rita Wilma
21 Sept 24 Oct
Each passing hurricane in 2005 cooled the
water in the Florida Keys.

26. Can we protect corals from bleaching?
Shade reefs.
Cool reefs.
Improve water quality.
Reduce other stress.
Reduce overfishing.