MB: Coral Reefs

Coral Reefs

• Coral reefs are extreme examples of organisms building the physical structure of their environment

What are reefs built by?

• Reefs are built by organisms that create physical structure

Reef-building corals- characteristics & phylum

• Phylum Cnidaria

• have radial symmetry (like jellyfish and anemones)

• each coral is made of many polyps connected by tissue- almost all are colonial

• each polyp deposits a calcium carbonate skeleton, which builds the ENTIRE reef

Zooxanthellae

• single-celled dinoflagellates

• required symbionts for corals

• corals cannot survive long-term without zooxanthellae, this is why bleaching is deadly

corals host tiny algae called zooxanthellae inside their tissues, these algae:

• provide corals with most of their energy (via photosynthesis)

• help them deposit calcium carbonate quickly

Other Organisms that help build reefs

• Sponges: glue skeletons together by filtering and trapping tiny particles

• Encrusting coralline algae: act like cement, growing over coral skeletons, building structure

• Sponges and encrusting coralline algae contribute to the building of a reef by binding together individual coral skeletons into a solid structure

• these organisms increase reef strength & stability

Many Organisms break down coral skeletons:

• Parrotfish biting while feeding

• worms and clams burrowing

• sponges dissolving skeletons

• This generates sediment and helps create sediment around reefs

Feeding

Corals feed in two ways:

1. Capture zooplankton with their nematocyst tentacles

2. Photosynthesis from zooxanthellae (dominant energy source)

The recycling system (trading nutrients for organic nutrition) between corals and their algae is extremely efficient

Asexual Reproduction/Propagation

Two major types:

1. Budding: new polyps added to the colony = colony grows

2. Fragmentation/regeneration: broken pieces can settle and become new colonies

(this contributes to reef recovery after storms)

Sexual Reproduction

Two Types:

1. Brooders: fertilization occurs inside the mother polyp; larvae released later

2. Broadcast spawners: eggs and sperm released at the SAME time into the water

• usually synchronized on one night

• fertilization occurs in the water column

• increases gene flow and larval dispersal

Environmental Requirements for Corals

• high amount of light

• normal marine salinity

• clear water (low sediment)

• warm water 20-30 C

• moderate water movement (brings nutrients, removes waste)

any violation of these conditions=no reefs

Why do corals reefs grow best on the western sides of ocean basins?

1. Due to global wind patterns & rotating ocean gyres, the western side of basins has:

• warm water moving clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere

• less upwelling (water is clearer and warmer)

2. Eastern sides usually have strong upwelling» colder, nutrient rich water» too cold for reefs

Ecological Interactons in Coral Reefs

• EXTREMELY high species diversity

• complex interactions

• many mutualisms (both sides benefit)

EX:

• corals & zooxanthellae

• corals & crustaceans (crabs defend corals; coral gives home)

High diversity leads to complex, multi layers food webs

Causes of Coral Reef Decline

1. Bleaching (loos of zooxanthellae due to heat stress)

2. disease

3. sediment (shade of corals, blocks feeding)

4. excess nutrients (promote algae that smother corals)

5. Physical damage

6. invasive species

7. loss of herbivores (overfishing parrotfish)

8. loss of sponges (weakens reef structure)

Human Activities that Impact Coral Reef Decline

• adding carbon dioxide to the atmossphere by burning fossil fuels» warming & ocean acidification

• Coastal development» increases erosion» sediment

• addition of excess nutrients due to lack of sewage treatment & irresponsible use of agriculture fertilizers» nutrient pollution

• Overfishing» removal of herbivores & predators

• Destructive fishing (dynamite)

• achors & tourism-related damage

Growth of Coral Reefs involves:

• growth of corals

• growth of other calcium carbonate producers

• cavity & sediment production by excavating organisms, rubble production by storms

• sediment production by grazers

• stabilization of rubble by sponges and algae, and filling of gaps with sediment

• cementation of stabilized rubble by growth of encrusting coralline algae

• colonization by small corals