Corals & Coral Reefs

“The Forests of the Sea”Fringe 1/6th of the world’s coastlinesLargest reef is the Great Barrier Reef in

Australia (2000km or 1200mi)Member of the Cnidaria phylumOnly have a polyp stage

Have stinging tentacles and a soft body column, but they also have hard skeletons of calcium carbonate

Coral Growth Start out life as a tiny planktonic larva,

floating through the waterWhen it finds a hard substrate it attaches

itself and begins secreting a hard skeleton towards the base

Coral Parts:Exoskeleton: chalky white skeleton on that is

located on the outsideCalyx: the cup-shaped depression directly

beneath the polyp; the polyp can retreat into itPolyp: the soft tissues on top of the calyx;

often very brightly colored

Tentacles

Body column

Solitary v. ColonySolitary Corals

A single polypSome secrete a skeleton that produces a

vertical columnSome produce a skeleton that grows outward

(like the Mushroom Coral)Few species are solitary corals

Polyp

Skeleton

Mouth

Solitary v. Colonial cont’dColonial Corals

Often described as “coral heads”Some grow by budding (the new polyps will

form around the original)Some reproduce sexuallyThe new polyps begin secreting skeletal matter

and adding to the thickness, diameter, and mass of the coral skeleton

All polyps are connected by a thin, often brightly colored layer of surface tissue

Classified as Branching or Encrusting corals

Branching v. Encrusting CoralsFig 4-1Branching Corals

Formed by polyps that produce buds at the tips of small branches Forms a complex, branching shape More delicate than the encrusting type

Encrusting CoralsFormed by polyps that only bud at the edges;

forms a “crust” over the base

Encrusting coral Branching Coral

Mutualistic SymbiontsZooxanthellae

A single-celled algae that is commonly found in the tissues of most cnidarians

3 major purposes Zooxanthellae photosynthesize, producing oxygen for the

coral The coral uses the O2 and produces CO2 and other wastes,

which the algal cells use to photosynthesize and nourish themselves

Zooxanthellae also stimulate skeletal secretion – they enhance the ability to extract calcium carbonate (CaCO3) from the water

*Reef-building corals (generally) only grow and thrive where there’s enough sunlight for the algae to photosynthesize

Reef FormationReef – a massive deposit of coral skeletons

Step 1 – A coral larva floats until it finds a hard substrate

Step 2 – The polyp begins to secrete a skeleton under the thin surface of soft tissue

Step 3 – The corals bud or sexually reproduce to form “buddies” to help with the growth of the coral head

Repeat steps 1-3 several million times… Reefs can be massive!

Ex: On the coral islands of Bikini and Enewetak, the researchers drilled through more than 2 km of skeletal matter before they hit the volcanic base!

Reef Formation cont’dReef Parts

Reef Flat The flat upper surface of the reef This is the part that is mostly exposed during low tide Tides limit the height the reef grows Prolonged exposure to air causes the coral to die, producing the

skeletons that compose the flat Few corals actually live on the flat

Reef Face The outer, seaward surface of the reef The reef grows fastest on this side due to the currents that bring

food particles to the living polyps As this part grows, the reef flat is expanded The corals on the shallower outer edges are exposed to high

wave energy and the abrasive force of moving sand particles A few hardy corals and coralline algae grow on the face

Reef Formation cont’dCoralline Algae

Marine algae that produce large amounts calcium carbonate that forms robust skeletons

Better adapted wave action than coral They make up about 90% of the shallow outer reef

face Their skeletons resemble coral skeletons, but they

don’t have calyces Algal Reef

The part of the reef mostly made up of coralline algae

Encrusting coralline algae cement parts of the reef together; making it very resistant to heavy wave action

Reef Formation cont’dThink of the reef face as being divided into

sectionsShallow Outer Reef Face

Mostly coralline algae because of the heavy wave action

Midwater Outer Reef Face This is where coral growth is excellent There can be as many as 200 species of corals

competing with each other

Deepwater Outer Reef Face (depths greater than 30m) Coral growth limited by lack of light

Coral Reef EvolutionThink back to how reefs form… (look @ Fig4-7)Fringing Reef

The early stage of the reefIt is small and close to shore

Barrier ReefThe reef has grown as the island has started subsiding

LagoonA region of (usually shallow) water between the reef and the

islandCoral Atoll

Formed when the island sinks below the surface of the ocean and the coral reef grows over it

This is all that can be seen at the surface of the waterMotu(s)

Mounds of coral rubble on the reef faceFormed when something like a storm breaks off a large chunk of

reef and deposits it on top of the face