Chapter 14 Coral Reefs

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

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

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

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

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

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

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

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

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

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

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

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

Staghorn coral, branching Brain coral, massive

Lettuce coral, foliaceous

https://www.youtube.com/watch?v=ribykabvLRA

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

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

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

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

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

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

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

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

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

Coral Reef Trophic Structure

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

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

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?

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

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