Download Tropical reef symbiosis

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
+
amensalism
commensal
0
+
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