How realistic can we be

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					Coral/algal Reefs III
     The future?
Utilitarian justification for reef conservation

• Therapeutic compounds from marine
  species
  – Anti-virals from sponges, seagrass
  – Anti-tumor compounds from tunicate, dogfish,
    bryozoan, sea hares, cyanobacteria, sponge
  – Compounds to promote bone grafts from stony corals
• Tourism
• Food
• Impact on global climate, carbon exchange
• Models for scientific study
Processes important in reef dynamics –
what maintains the reef structure?
• Symbiotic mutualism (and dissolution of
  associations)
• Competition

• Predation and grazing

• Disturbance & recovery
Questions:

• Are symbiotic relationships
  increasingly disrupted?
• What are the
  consequences?
• How are organisms linked
  through symbiosis?
• Are changes reversible?
Coral bleaching (Hoegh-Guldberg)
Coral bleaching – dissolution of symbiosis

• zooxanthellae
  expelled from tissue
• polyp can persist –
  for a while
• new associations can
  be formed
• responses to stresses
A general introduction to “symbiosis”

• De Bary (1850’s) – “The living together of
 different species for an extended period of
 time.”

• Proximity, not outcomes, define symbiosis

• Variation in characterizing some
 associations, e.g., pollination
Symbiosis has many dimensions

• Form of physical association
• Types of organisms involved
• Nature of the exchange or influence
• Outcomes of the interaction (+, 0, -)
• Degree of dependence
• Evolutionary derivation of the association
Physical nature of the association

• Close proximity,
 but physically
 independent

• External contact


• Internal
What taxa are associated?

• Algae-invertebrate

• Among animals

• Bacteria/archaea -
 animals
  What is exchanged?

Capability            Donor                Recipient
photosynthesis        algae, bacteria      protists, inverts
chemosynthesis        bacteria             invertebrates
added nutrients       bacteria             many animals
methanogenesis        bacteria, protists   anaerobic protists
cellulose digestion   bacteria, protists   herbivores (terrest.)
luminescence          Vibrio, Photobact.   molluscs, fish
protection            cnidaria             fish
What are the outcomes of symbiotic
associations?
                         –Recipient

                -            0           +
     -
     0                   amensalism   commensal

     +      parasitism                mutualism
Outcomes: nutrient exchange

• What is the evidence for exchange with
  endosymbiotic dinoflagellates?
• Experiment: remove zooxanthellae
  – ammonium content of polyp rises
• For Tridacna clams
  – experimentally enrich with ammonium
  – algal symbiont increases in density
What is the degree of dependence?

• Facultative

• Obligate (often has very specialized
 morphology and life history)

• Symmetry is not necessarily found
What is the evolutionary origin of the
association?

• Parasite-host may evolve to be mutualistic

• Predator-prey (coral/dinoflagellate)

• Close proximity may lead to coevolved
 relationship
How can we evaluate importance?

• Removal
 experiments,
 e.g., cleaner fish


• Alter background
 conditions –
 Chlorella/Hydra
 experiment
Bleaching occurs with high SST
    How does heat (& light) disrupt
            mutualism?
• Symbiodinium is damaged by oxidative
 stress
              Coral Responses

• Polyp responds immunologically
  – Apoptosis & autophagy
• Zooxanthellae can be expelled
• Polyp switches to heterotrophy
  – This is a short-term strategy
            Sensitivity to SST varies

•   Among genotypes of Symbiodinium
•   Among colonies within coral species
•   Between different coral species
•   Geographically for the same coral species
Variation in Florida Keys corals, 2005
Brandt, M. E. 2009. The effect of species and colony size
on the bleaching response of reef-building corals in the
Florida Keys during the 2005 mass bleaching event. Coral
Reefs 28:911-924.


• Background
   – Summer & fall, 2005 – high SST in ne
     Caribbean
   – Mass bleaching documented
• Methods
   – Monitor corals for 191 colonies in permanent
     quadrats
Bleaching was correlated with heating
Bleaching prevalence varied among spp
Bleaching incidence varied with colony size
          Why and what’s next?

• Symbiont “clades” vary genetically
  – Corals can switch
  – Symbiodinium communities can vary across
    environmental gradients
  – Degree of flexibility is debated
• Hosts (corals) also vary
  – Different fluorescent proteins for protection
  – Different abilities in heterotrophy
  – Coral structure affects the light environment
Competitive dynamics
• Exploitation competition (for light)
  – Upright, branching corals can shade massive
    corals
  – Encrusting algae can spread over corals
• Interference competition (for space)
  – External digestion by some corals
  – “Sweeper” tentacles for some species
• Hierarchy of competitive dominance
  – Algae easily overgrow most corals
  – Among corals Pocillopora is nastiest
Dynamics of predation on coral reef species

• Coral-feeding fish are present but usually
 not devastating
  – Territorial damselfish create safe zones (up to
    60% of surface area)
  – Coral-feeders have their own predators
• Starfish, such as “Crown-of-Thorns” can
 be problematic
  – Population “outbreaks” can damage living
    corals
Dynamics of grazing on algal reef species
• Urchins are major
    consumers (e.g., Diadema
    antillarum)
•   Grazing by herbivorous fish
    can be specialized on algae
    (more impact than fish
    feeding on corals)
•   Grazing can suppress
    competitively dominant
    algae
•   Indirect effects can become
    important

				
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posted:8/17/2012
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