1. Chapter 14
Coral Reefs

2. Reef Building Organisms that produce calcium carbonate build reefs
Sediment fills in the gaps
Reef builders include
Various coral species
Coralline algae
Bryozoans
Sponges
Sea urchins

3. Corals Only some corals build reefs
Called hermatypic corals
The scleractinian corals are the biggest reef producers
Called stony or “true” corals
Most reef building corals have zooxanthellae
Dinoflagellates that provide food for shelter
The polyp stage builds reefs

4. Coral Feeding Coral polyps capture food using their tentacles
Nematocysts sting and incapacitate small prey
Mostly detritus is caught and consumed
Mesenterial filaments also help coral feed
Filaments extruded from the gut
Allow external digestion
Zooxanthellae give most of the coral’s nutrition
Some corals have lost their tentacles completely

5. Coral Polyps Reef building life stage of corals
Live in colonies in reef building corals
Start in planktonic larval stage
Planula
From sexual reproduction
Planula settles on hard surface
Starts dividing itself into colony
The colony shares
Nervous system
Digestive system

6. Zooxanthellae Live as part of a symbiotic relationship with coral
Produces organic material for the coral
Nutrients are recycled between the coral and its zooxanthellae

7. Coralline Algae Red algae that produce calcium carbonate
Thallus produces calcium carbonate
Thrive better in rougher waters than coral
Can build reefs and hold them together

8. Sediments Sediments glue the coral reef together
Fine sediment impedes coral growth
Coarse sediment good for reef growth
Organisms that produce sediment include
Coralline algae
Bryozoans
Sponges
Foraminiferans
Sea urchins

9. Coral Reef Growth Corals have specific requirements for growth
Require correct
Light
Temperature
Salinity
Sediment type
Are very sensitive to pollution

10. Light and Temperature
Need enough light for zooxanthellae to photosynthesize
Usually close to the surface
Clear water needed
Temperatures need to be in range for the zooxanthellae to tolerate
Optimal temperature around 20°C (68°F)
Too hot or too cold will stress out the coral
Stressed coral expel their zooxanthellae
Event called bleaching

11. Coral Bleaching
When temperatures rise over a coral’s tolerance, bleaching events happen
Coral expels their zooxanthellae
Zooxanthellae give the coral their color
Corals can recover from bleaching
Die if they cannot regain zooxanthellae

12. Salinity, Sediments, Pollution
Corals need highly saline conditions
Do not do well in estuaries
Sediment in the water impedes coral growth
Silt covers the coral, blocks light
Nutrient pollution from human activities affect corals
Corals grow in nutrient-limited areas
Influx of nitrogen from fertilizers causes algae to bloom
Coral are unable to outcompete blooming algae

13. Coral Structures Coral colonies form different shapes
Different shapes adapted for different needs
Flat shapes are good for catching more light in deeper waters
Taller, branching shapes are better at competing for light in shallow waters

14. Staghorn coral, branching
Brain coral, massive

15. Lettuce coral, foliaceous

17. Coral Reef Types Reefs are divided into 3 types
Fringing reefs
Barrier reefs
Atolls
Reefs can be in between types
But the categories are still useful

18. Fringing Reefs Most common type of reef
Form near shore lines in the tropics
Grow best on rocky surfaces
Coral can make their own hard surface

19. Fringing Reef Structure
Made of an inner reef flat and outer reef slope
Reef flat is wide and shallow
Can be exposed when the tide is out
Reef slope goes almost vertically down
Reef crest is in-between flat and slope
Often contains richest amount of corals

20. Barrier Reefs Grow farther from the shore than fringing reefs
Fringing reefs can lead into barrier reefs
Can follow the shore, but have a lagoon separating them

21. Barrier Reef Structure
Reef slopes on front and back
Reef flat can accumulate sand and other sediment
Called keys in the US, sand cays elsewhere
Outer reef crest has the richest coral growth

22. Atolls Rings of reefs surrounding a lagoon Often formed far from shore
Form islands big enough for people to live on
Often formed far from shore
Indo-West Pacific region
Rare in the Atlantic ocean
Far from land
No silt in the water
Clear conditions

23. Atoll Structure Front and back slopes
Outer and inner slopes
Wind affects the organisms on the reef
Coralline algae grow on the windward side

24. Atoll Formation Volcanic islands rise out of the water
Like the Hawaiian Islands
A fringing reef grows around the island shore
The island shrinks and disappears over time
Erosion weathers the rock away
In the end, only the ring of reef is leftover

25. Coral Reef Ecology Coral reefs are typically in low nutrient waters
No rivers feed into the reef, too much sediment
Corals rely on their zooxanthellae for nutrients
Corals breaking down add nutrients to the water
Feeds the rest of the reef
Large amounts of cyanobacteria live on corals reefs
Nitrogen fixation

26. Coral Reef Trophic Structure

27. Coral Reef Communities
Lots of interactions due to so many species being present
Interactions include
Competition
Predation on corals
Grazing on algae
Symbiosis

28. Competition Space is very limited in coral reefs
Competition for sunlight and hard surfaces
Encrusting algae and soft corals compete with hard corals
Grow faster than hard corals
Fish compete with each other on the reef
Many different species live on a single reef
Each species has a niche, specialized for a certain part of the reef

29. Predation on Corals Many organisms directly feed on corals
Adapted to eat different parts of the coral
Sea stars can decimate coral populations
Crown-of-thorns sea stars in particular are a problem
Sea stars bloom like algae
Human caused or natural?

30. Grazing on Algae Coral reefs are present in areas of high sunlight
Lots of algal growth
Many organisms eat algae
Keep population down
Prevent coral from being competitively excluded
Removal of grazers can cause algae to overtake corals and choke them out

31. Symbiotic Relationships
Many species live on coral reefs
Leads to many different relationships between them
Zooxanthellae live not only on corals
Giant clams
Sea anemones
Snails
Anemones and anemonefishes
Fishes immune to anemone stings
Live in anemone and protect it