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What is a coral reef built from?

Built from the accumulated skeletons of calcium carbonate (CaCO3) secreting animals and plants


Where are tropical coral reefs found?

Along Australian and Indonesian coastline
Near the warmer currents of oceanic gyres


Problems of tropical surface waters

Stratified and nutrient-poor
Permanent thermocline
Strong pycnocline
Low biomass except in upwelling regions and coral reefs


What is ahermatypic coral?

Soft coral
Not reef-building
Do not produce a rigid calcareous skeleton
Can produce sclerites (calcium carbonate needles)


What is hermatypic coral? What is the dominant group?

Stony corals
Dominant group are Scleractinian coral
Produce massive skeletons of calcium carbonate


Features of Scleractinian coral

CaCO3 skeleton produced by coral polyps
Solitary or colonial
Colonies formed by asexual division - all polyps genetically the same
Polyps connect into a single living surface (tissue layer) above the exoskeleton


How do corals obtain the nutrients they need?

They have a symbiotic relationship with photosynthesising algae (zooxanthellae - dinoflagellates)
Holobiont = ecological unit of coral and it’s symbionts
Algae supplies up to 90% of energy required by tropical coral
Coral therefore requires well-lit marine environment
Nutrients cycled between coral polyp and symbiotic algae


What genus are zooxanthellae in?



Density of algae within corals

more than a million per cm squared of coral surface


How else does coral obtain nutrients besides its symbiotic relationship with algae?

-Prey capture of zooplankton using stinging cells called cnidocytes
-Mesenterial filaments: tubes attached to the wall of the gut that are extruded through the mouth to digest food outside the body
-Mucus threads secreted over colony surface to capture passing plankton, then gathered into mouth (mucus is food source for benthic bacteria)


How do symbionts affect calcification?

The skeletal growth of tropical coral with symbiotic algae is much faster than cold water due to symbionts
Calcification is 3x higher during the day when algae can photosynthesis


Cold water coral features

Long-lived, slow growing and extremely fragile coral
Below 40m water depth they are most common, as there is no symbiotic relationship with photosynthetic algae
Host to specific bacteria


What is ahermatypic cold water coral growth limited by?

Oxygen and food rather than water temperature and light


What is coral bleaching?

Breakdown of symbiosis between algae and coral host
Coral will bleach if temperatures reach 1 degrees above average seasonal maxima


When was the most extensive coral bleaching seen on the Great Barrier Reef?

February 1998
87% of inshore reefs saw bleaching occur


What are important alternative energy sources for coral during bleaching?

-Capture of plankton
-Lipid reserves
-Night-time mesenterial feeding on neighbouring algal turfs


How might coral adapt to shifts in thermal tolerance

Adaptive bleaching hypothesis
Shuffling the population of symbiotic algae to increase the threshold temperature of bleaching


What is another coral reef builder besides hermatypic coral?

Crustose coralline algae
Order Corallinales, in Rhodophyta/red algae
Dominant reef-building genus is Porolithion
Acts like a mortar for the reef by forming an encrusting cement of Mg-rich CaCO3
Surface aids the settlement of coral recruits and other reef invertebrates


What is another coral reef builder besides hermatypic coral and crustose coralline algae?

Calcareous green alga
95% of weight is CaCO3
Individual segments accumulate to form carbonate sediment on reefs


What are the two types of coral reef erodes?

1. Bioeroders
2. Chemical eroders


What are three examples of bioeroders?

Polychaetes (e.g. Christmas tree worms)
Crown of thorn starfish
Parrotfish (Scaridae)


Features of parrotfish

Fused dental plants to form a beak
Have a pharyngeal jaw (second set of jaws within the throat)
Very long intestine, no stomach
Graze reef in large schools
Adult invests more than 5 tonnes a year of structural reef carbonate


What is an example of a chemical eroder of coral?

Upwelling high CO2 (low pH) waters


How does upwelling of high CO2 water erode coral?

Low pH water has low carbonate concentration
The rate of calcification for both coral and coralline algae is proportional to the carbonate concentration in seawater
The stability of calcium carbonate is dependent on the carbonate concentration in seawater, so a lower concentration of carbonate causes the dissolution of the CaCO3 skeleton of coral


Where is it that reefs can occur?

Where calcification exceeds erosion


What is the single species of coral which makes up most cold water coral reefs in the North Sea?

Lophelia pertusa