Lecture 18: Coral Reefs

• Most diverse and complex marine communities

  Contain 25% of marine species
• Complex habitat: structural and biological
• Basis for productivity: symbiotic relationship between dinoflagellates (zooxanthellae) and coral
• Efficient nutrient cycling: good at recycling nutrients brought into the system.

  Once nutrients are introduced they stay within the system.
• Coral in the phylum Cnidaria; Anthazoa class
• Order Scleractinian Corals: stony or true corals

  Hermatyptic (reef building)
• Order Alcyonacea coral: soft corals

  Ahermatypic (non reef building)
• Coral Morphology: made from calcium carbonate.

  Gut cavity brings food in and exports waste.

  Carnivorous animals and use nematocyts to capture small prey.

  Colonies of thousands of individuals for large feeding areas.
• Many corals only feed at night due to the zooplankton only coming out at night
• Coral have a symbiotic relationship with zooxanthellae which dwell within surface tissues of corals.
• Coral benefits from zooxanthellae:

  1. oxygen production
  2. organic photosynthetic products
  3. aid calcification, synthesis of lipids
• The relationship between coral and zooxanthellae is very strict; once coral is unhappy, it rejects the zooxanthellae.
• Coral growth forms: massive, platelike/tabletop, columnar, foliaceous, branching
• Branching coral reef growth in shallow waters and platelike growth in deeper waters. This is due to sunlight patterns.
• Asexual reproduction: new individuals bud/fragment off the parent

  Sexual reproduction: results in free-swimming planula larvae (dispersal)
• Coral sexually matures in 7-10 years and is primarily hemaphroditic.
• Majority of coral reefs are broadcast spawners and some are brooders.
• Synchronous spawning occurs with cues from the moon, temperature, light
• Types of coral reefs: fringing, barrier, and atoll reefs
• Darwin's Subsidence Theory Steps: fringing reefs, barrier reefs, and atolls represent successive stages in an evolutionary sequence.
• Structure of a Barrier Reef: windward side has less coral growth than the leeward side.
• Factors limiting coral distribution:

  1. temperature (optimal range is 23-25 C)
  2. light/depth (optimal is less than 25m)
  3. Stenohaline (optimal is 32-35 ppt)
  4. Subtidal range is not optimal due to no tolerance of aerial exposure
  5. Sedimentation (too much sediment is harmful)
• Diversity and distribution of corals:

  More coral reefs in the Pacific Ocean due to upwelling in the Atlantic Ocean that makes it hard for coral to survive.
• Grazing from herbivores on coral reefs promotes the survival of coral by eating the microalgae.
• Coral predators that consume patches of corals: gastropods, nudibranchs, polychaetes, and several crabs
• Coral predators that destroy entire colonies: crown-of-thorns sea star, variety of fishes
• Corallivore fishes predate on coral reefs: puffers, filefishes, triggerfishes, and butterfly fishes
• Maintenance of high diversity/abundance of fish though the competition model (promotes small niches for speciation), recruitment limitation model (lots of areas to live in), predation disturbance model (no one species can dominate due to predation), and the lottery model (too much randomness for a species to dominate)
• Symbiosis on Coral Reefs:

  Zooxanthellae in coral and giant clams

  Clown fish and Anemonies
• Cleaner animals as a form of symbiosis: cleaner shrimp and wrasses remove parasites and diseased tissues from fish
• Mimicry of cleaner fish seen in fang-toothed blenny, which will eat the tissue of fish.
• Threats to Coral Reefs:

  1. Direct mechanical damage
  2. Degrading water quality
  3. Loss of grazers
  4. Crown of thorns
  5. Climate change