CORAL REEFS
What is a
Coral Reef?
• Structure built
from skeletons
of corals and
crustosecoralline algae.
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Corals: Two Types
• Reef-building hermatypic
(contain zooxanthellae)
• Non-reef-building –
ahermatypic
(no zooxanthellae)
Reef-Building Corals
• Zooxanthellae
- Intracellular, photosynthetic, symbiotic
dinoflagellates
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– Carbon fixed by
zooxanthellae
transported to host
tissue
– Nitrogenous wastes
from coral recycled
into nutrients for
zooxanthellae
– Recycling of CO2
and O2
– Important in
calcification (cloudy
days – 50% of that
of sunny days)
Other Forms of Coral Nutrition
• Planktivory
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Coral Morphologies
Surface / Volume
Autotrophy/Heterotrophy
versus Morphology
Highly branched, small polyps
Autotrophic
Massive, large polyps
Heterotrophic
Polyp Diameter
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Coral Life Cycle
Coral Reproduction
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What Physical Factors Limit
Coral Reef Development?
Temperature
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Other Physical Factors Limiting
Coral Reef Development:
• Light
• Salinity
• Sedimentation & Turbidity
Types of Coral Reefs
• Fringing reefs
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• Barrier reefs
• Atoll reefs
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Subsidence Theory
(Evidence from Enewetak Atoll)
Reef Growth & Erosion
• Growth in coral colonies
Branching corals:
1-26 cm/yr in height
Massive corals:
0.5-2.5 cm/yr in height;
1-2 cm/yr diameter
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Growth of Whole
Coral Reef
• Whole Reef
accretion rate
– 4-8 cm yr
• Organisms
contributing calcium
carbonate to reefs
–
–
–
–
corals
coralline algae 
calcareous algae
molluscs
Reef Erosion Processes:
storms (esp. hurricanes)
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Reef Erosion Processes:
bio-erosion
Ecological Interactions and
Community Structure
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Competition
Among Corals
• Compete for space
Coral Predators
• Continuous, low-level predation
Puffers, parrotfishes – can actually increase
coral diversity
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Episodic, intense predation:
Acanthaster planci outbreaks
Grazers and Interactions
Among Sessile Taxa
• Sea urchins and roving herbivorous
fishes play an important role in
mediating competitive interactions
between corals and algae.
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Grazers & Maintenance of
Community Structure
• Large carnivorous fishes eat sea urchins and other
herbivorous fishes, keeping their populations at
moderate levels. This results in high coral diversity.
• Jamaica: Overfishing has removed most of these
large carnivorous fish predators as well as many
herbivorous fish grazers (e.g., parrotfishes)
parrotfishes)
• Initial Consequence: High abundance of sea urchins
and coralline algae.
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Subsequent
Effects:
• Diadema decimated by disease (1983)
• Jamaican reefs decimated by hurricanes.
• Lack of grazers (due to overfishing of
herbivores, Diadema disease) prevents
recovery of reefs from hurricanes – algae
prevents coral recruitment & growth
Ecological Threats to Coral
Reefs
• Direct destruction (dynamite fishing,
mining of coral)
• Land use practices (sedimentation,
nutrient pollution)
• Diseases
• Overfishing
• Global warming – coral bleaching
events
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Coral Bleaching
• Breakdown in mutualism
between coral & symbiotic
algae (zooxanthellae)
• Caused by stress,
e.g. high temperature
• Major threat to coral reefs
globally
Coral Reefs are likely to be
severely impacted by global
warming by 2050
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