Coral Reefs

Definition: Compacted and cemented assemblages of skeletons and sediment from sedentary organisms

Importance

  • Biological: Biological structure, high diversity

  • Geological: Often massive

  • Economic: Shoreline protection, harbors, fishing tourism

Cover 0.17% of the Earth’s surface yet contain 25% of the earth’s species

Most diversity per unit of area for an ecosystem

Sequester 700 billion kg of carbon per year

Exist in the most nutrient poor waters

Distribution

  • Close to the equator, most occur between 25 degrees North and 25 degrees South

  • Mostly in warm waters (>23-25 degrees C) and shallow waters

  • Bound to high salinities

  • Produce “sunscreen” aka Amino Acids that filter out UVA and UVB

Constructional, wave-resistant features

Built up principally by corals, coralline, algae, sponges, and other organisms, but also cemented together

Reef building corals belong to the Scleractinia (phylum Cnidaria)

  • Possess endosymbiotic algae known as zooxanthellae

  • Exhibit a high calcification rate

Topographically complex, high diversity

Limiting Factors

  • Warm sea temperatures

  • High light (symbiosis with algae)

  • Fully marine salinity is needed

  • Low turbidity… corals do poorly in areas with suspended sediment

  • Strong sea water currents, wave action

  • Reef growth → growth vs. bioerosion

  • Reef growth must respond to rises and falls of sea level

  • pH - increasing ocean acidity

Biogeography

  • Current division between Pacific and Atlantic provinces - used to be united by a connection across Tethyan Sea. Disappeared in Miocene.

Reef Types

  • Coastal reefs - wide variety of reefs from massive structures (Great Barrier Reef) to small patches (Eilat, Israel)

  • Atolls - horseshoe or ring-shaped island chain of islands atop a sea mount

Reef-Building (Hermatypic) Corals

  • Phylum Cnidaria, Class Anthozoa, Order Scleractinia

  • Secrete skeletons of calcium carbonate

  • Colonies of many similar polyps

  • Can be divided into ‘branching’ and ‘massive’ forms

  • Have abundant endosymbiotic zooxanthellae

Hermatypic vs Ahermatypic Corals

  • Hermatypic: Reef framework building, have many zooxanthellae, high calcification rates (with symbionts)

  • Ahermatypic: Not framework builders, low calcification rates (without symbiont)

Growth Forms

  • Branching: Grow in linear dimension rapidly, 10cm/year

  • Massive: Produce lots of calcium carbonate but grow more slowly in linear dimensions, about 1cm/year

Measures of coral growth

  • Label with radioactive calcium

  • Spike driven into coral, measure subsequent addition of skeleton

  • Use of dyes to create a reference layer in coral skeleton

  • Natural growth bands

Zooxanthellae

  • Found in species of anemones, hermatypic corals, octocorals, bivalve Tridacna, ciliophora (Euplotes)

  • At least 10 distinct dinoflagellate taxa (no flagellum)

  • Occur in coral tissue (endodermal), concentrated in tentacles

Coral Benefits

  • Nutrition

  • Facilitates calcification - uptake of carbon dioxide by zooxanthellae enhances calcium carbonate deposition

  • Zooxanthellae gains nutrients from coral excretion and grazer protection

Bleaching

  • Causes expulsion of zooxanthellae, stress (temperature and disease)

  • White band disease - affects acroporid (polyped, stony) corals. Potentially caused by gram-negative bacteria

  • White Plague - Rapid degradation of corals, gram negative bacterium, cultured in lab and infects corals

  • Black band disease - affects non-acroporid corals, consortium of microorganisms, leads to sulfide accumulation and toxicity to corals

Mass Spawning on Coral Reefs

  • Most corals have planktonic gametes

  • On Great Barrier Reef and reefs off Texas: Many coral species spawn at the same time

  • Facilitates gamete union, perhaps a mechanism to flood the sea with gametes to avoid all being ingested by predators

Depth zonation on Reefs

  • Reefs dominated by different coral species at different depths

  • Factors causing this are possibly similar to rocky shores, also changing light conditions

  • Deepest to shallowest: Boulder, Staghorn, Buttress, Elkhorn

Biological Interactions

  • Competition: shading, overgrowth, interspecific digestion, sweeper tentacles, allelopathy

Growth Factors: Competition, Predation and grazing, Disturbance, Larval recruitment, Disease

Predation and grazing

  • Pacific Ocean: Crown-of-Thorns Starfish

Climate change and coral reefs

  • Increased bleaching due to rising sea surface temperature

  • Acidification: Carbon dioxide addition to atmosphere results in reduction of seawater pH

  • Corals secrete aragonite (unstable form of calcium carbonate), which becomes harder to secrete under acidic conditions and causes lower skeletal density over time