Coral Reefs
Coral Reefs
Coral Reef Formation
Scleractinian Coral Polyps
• Scleractinia = stony, hard corals
• Coral polyps can be solitary or colonial
– Solitary forms are one polyp and one corallite.
• Corallite = hard skeleton under the epidermis
– Colonial forms can reproduce asexually by
cloning, and the new polyp forms another
corallite that is attached to the first corallite.
• Some produce horizontal, mounding or plate-like
corals
• Others produce branching, tree-like structures
Solitary Corallites
Solitary coral corallite, side view (left). Solitary coral corallite top view (right).
Solitary Corals
• Solitary corals can be found all over the Earth
• These species are called ahermatypic.
– Ahermatypic corals must capture prey as described
for most other cnidaria.
Colonial Corals
Calyx
Septa
Colonial mounding coral, side view of corallites (left). Colonial
mounding coral, top view of corallites (center). Colonial mounding
coral, close up of corallites from a dividing polyp (right).
Colonial branching coral, side view of corallites (left). Colonial
branching coral, top view of corallites (center). Colonial branching
coral, close up of corallites from a dividing polyp (right).
Coral Growth Forms
Different arrangements of coralites within a colony
– Incomplete list
Coral Growth Forms
1. solitary (e.g., mushroom corals, Fungia &
Cycloseris)
2. dendroid = spreading branches of single
corallites
3. phaceloid = distinct parallel-arranged corallites
arising from a common source
4. plocoid = corallites with distinct walls united to
each other by the coenosteum (e.g., Leptastrea
bottae)
Dendroid
Coral Growth Forms
5. cerioid = prismatic corallites closely
appressed and sharing common walls (e.g.,
crust coral, Leptastrea pupurea, and finger
coral, Porites compressa
6. meandroid = several corallites forming a
series within the same walls giving the
appearance of meandering valleys (e.g.,
corrugated coral, Pavona varians)
7. thamnasterioid = corallites with confluent
septa and lacking defined boundaries (e.g., flat
lobe coral, Pavona duerdeni)
i
Colonial Corallites
Coral polyps have retracted into their corallites in the
yellow-green coral. You can easily see the radiating
septa (walls of calcium from the outside of the circular
corallite pointing to the middle) of each corallite.
(Plocoid form)
Alcyonacea = Soft corals
Factors Influencing Coral
Colony Morphology
• Water motion
• Light
• Biological Factors
– Genetic
– Coral symbionts
Low light = platelike
High light = branches
Coral Growth
44 year old coral growth
rings.
Cross-sections reveal coralgrowth rings that can be used
to determine their age. These
growth rings can also reveal
information about ocean
conditions during the coral’s
life span.
Reef Building Corals
• Reef building corals are colonial
– create large 'coral heads' that may exist for
thousands of years
– produce tons of calcium carbonate that
becomes the base for the coral reefs growing
up and out over time.
• Reefs are found in nutrient poor tropical
waters that have low productivity.
– So no phytoplankton
– So no zooplankton
– So no food for corals
Reef Building Corals
• Zooxanthellae are photosynthetic, single-celled
dinoflagellates, living in the endodermal tissues
of stony corals polyps (intracellularly).
• Often, zooxanthellae are concentrated in the
polyps’ gastrodermal cells and tentacles.
• Deep water and some cold water corals lack
zooxanthellae, but virtually all reef-building
corals possess them.
Reef Building Corals
• Reef building corals are hermatypic
– Have a symbiotic relationship with photosynthetic
zooxanthellae (type of dinoflagellate).
– Zooxanthellae provide sugar to the coral polyp and
use the metabolic wastes (N & P) from the animal for
its own growth.
• Most zooxanthellae cannot survive outside of the
coral animal's body because there are not enough
nutrients in the tropical ocean to allow proper
photosynthesis.
– Zooxanthellae often give coral species their
characteristic color.
Hermatypic
Ahermatypic
Blue-green coral (left) and orange popcorn coral (right) are
just a few examples of the variety of colors found in tropical
corals. This color may be primarily from their symbiotic
zooxanthellae (as in the blue-green coral) -or- produced by
the coral animal itself (as in the orange popcorn coral).
Role of Zooxanthellae
• 94 to 98% of all the organic nutrients are
produced by the zooxanthella; they are the main
source of the polyps’ organic nutrient
requirements.
• Thanks to the photosynthetic activities carried
out by the zooxanthellae, the calcareous
skeleton of hermatypic coral is able to develop 2
to 3 times more quickly in the light than
in darkness.
Role of Zooxanthellae
• Through photosynthesis, zooxanthellae
convert carbon dioxide and water into
oxygen and carbohydrates.
– The coral polyp uses carbohydrates as a
nutrient.
– The polyp also uses oxygen for respiration
and in turns, returns carbon dioxide to the
zooxanthellae.
– Through this exchange, coral saves energy
that would otherwise be used to eliminate the
carbon dioxide.
Role of Zooxanthellae
• During photosynthesis, zooxanthellae “fix” large
amounts of carbon, part of which they pass on to
their host polyp.
• This carbon is largely in the form of glycerol but
also includes glucose and alanine. These
chemical products are used by the polyp for its
metabolic functions or as building blocks in the
manufacture of proteins, fats and carbohydrates.
• The symbiotic algae also enhance the coral’s
ability to synthesize calcium carbonate
Role of Zooxanthellae
• Nitrogen and phosphorus are cycled between
zooxanthellae and coral polyps.
– For example, zooxanthellae take in ammonia given off
as waste by the polyp, and return amino acids.
• Zooxanthellae also promote polyp calcification by
removing carbon dioxide during photosynthesis.
– Under optimum conditions, this enhanced calcification
builds the reef faster than it can be eroded by physical
or physical or biological factors.
Calcium Carbonate Formation
• In the coral Pocillopora
damicornis, 35-50% of the
total photosynthetic product is
excreted by the zooxanthellae, primarily as glycerol.
• In the coral tissue, the glycerol
is converted largely to lipids,
and the skeletal organic
matrix has a substantial lipid
component, consisting mostly
of cetyl palmitate
Calcium Carbonate Formation
1.
Symbionts synthesize molecules that are essential to
the calcification process
2.
Symbionts affect the dissolved inorganic carbon
equilibrium within coral tissues by taking up CO2 for
photosynthesis or by secreting hydroxyl ions that are
the product of Carbon Concentrating Mechanisms
3.
Symbionts produce energy and O2 that can accelerate
the calcification process
4.
Symbionts play a role in the removal of substances
that would otherwise inhibit calcification (phosphorus).
Coral Bleaching
• If the zooxanthellae leave or die, the coral
becomes 'bleached' (without color) and can die if
it does not recover its zooxanthellae.
• Causes of coral bleaching
– Insufficient light - Zooxanthellae require high amounts
of light for photosynthesis.
– Silty deposits from land can suffocate a reef.
– Zooxanthellae are very sensitive to heat so global
climate change is another danger for reefs as sea
temperatures rise.
Sexual Reproduction
This colonial coral is getting ready to spawn, you can
see the pink spawn right below the surface of the mature
polyps. 75% of corals are broadcast spawners.
Sexual reproduction of coral reefs begins with a
spawning event. Coral animals reproduce sexually by
releasing their gametes (eggs and/or sperm) into the
ocean through their mouth (broadcast spawning).
Coral spawn are usually tiny pink balls that are
released through the mouth of the spawning polyp.
Once spawning starts, the ocean is full of these pink
bundles.
Sexual Reproduction
Close up showing zygotes
right after fertilization
Close up showing several zygotes
that have begun cell division. It also
shows a micrometer (in millimeters)
so you can see the actual size of
the spawn.
Sexual Reproduction
• Brooding species are most often
ahermatypic (not reef-building) in areas of
high current or wave action. Brooders
release only sperm, which is negatively
buoyant, sinking on to the waiting egg
carriers who harbor unfertilized eggs for
weeks. Synchronous spawning events
sometimes occurs even with these
species. After fertilization, the corals
release planula that are ready to settle.
Sexual Reproduction
Red Coralline
Algae
Coral Reef Formation
Coral Reef Formation
Aerial view of the stages of coral reefs (fringing, barrier, atoll).
Darwin originally developed the theory of reef formation,
(mid-1800’s) and his hypothesis has been supported by the
data collected using modern oceanographic technologies.
A fringing reef forms along the shoreline of
most new tropical islands.
A barrier reef forms as the oceanic island begins
to sink into Earth's crust due to the absence of
volcanic island building forces, the added weight of
the coral reef, and erosion at the surface of the
island.
An atoll forms when the oceanic island sinks
below the surface of the ocean but the coral reef
continues to grow upward. Eroded reef material
may pile up on parts of the reef, creating an area
above sea level, an island called a 'cay.'
The three stages of coral reef formation fringing, barrier, and atoll.
Coral Reef Zonation
Biogeographic Profile of Reef
A seamount or guyot (gē-ōh)
forms when the coral reef
cannot keep up with the
sinking of the island.
Seamounts and guyots are
below the surface of the
ocean and may be home to a
large number of species
depending on their location
and depth.
Beautiful
Valuable
Valuable
Biodiversity
Threatened
•
Natural disturbances:
–
–
–
Hurricanes, tsunamis
Predation (e.g., coralivorous snails, starfish,
parrotfish)
Competition with other corals
Hurricanes, tsunamis
A wave battering a coral reef
Parrotfish Predation
Predation
The crown of thorns sea star on a South Pacific coral reef
Competition
Coral competition between brain coral and cavernous star
coral (left). A close view of two coral species fighting for space
(right). The pink species is winning at this time - notice the new
white corallites overgrowing the brownish species.
Aggressive Behavior
• Extruded digestive filaments and sweeper
tentacles
– Typically, an attack by an aggressive coral on a
subordinate neighbor will result in the death of some
of the subordinate’s polyps
– Experiments also revealed that each coral species
attacked only certain species, and each was attacked
itself by certain other species, suggesting an
“aggressive pecking order” among the corals
– Thus, it appears that at least on Jamaican reefs, fastand slow-growing coral species can coexist because
the speed at which branching corals grow is balanced
by the aggressive nature of massive corals.
Threatened
• Many natural challenges to corals appear
to be accelerated by anthropogenic factors
– Competition with macroalgae / blue-green
algae (overgrowth / settlement space) accelerated by nutrient input from sewage,
fertilizers
– Diseases of corals and other reef organisms
(fungal, viral, bacterial; white pox disease of
once-dominant branching corals now known
to be a bacterial agent derived from the
human gut)
Bleaching
Seaweed bloom after
bleaching
Algal growth
Recovery
White pox disease on Elkhorn coral.
Caused by Serattia marcescens
Algal Overgrowth
Threatened
• Many natural challenges to corals appear to
be accelerated by anthropogenic factors
– Loss of herbivores that keep algal populations
reduced
• Loss of herbivorous fish due to overfishing
• Loss of Diadema antillarum, long-spined sea urchin,
from virus (of unknown origin; susceptibility to
diseases greater with additional stresses)
• Potential switches in diet of herbivorous community
due to changes in algal populations (e.g., potential
increase of corallivory by parrotfish as a result of an
increase in unpalatable algal species due to loss of
grazing by Diadema)
Diadema antillarum
Threatened
• Coastal development
• Sewage
• Dredging
Threatened
•
•
•
•
Coastal development
Sewage
Dredging
Runoff from
construction and
agriculture
Threatened
•
•
•
•
Coastal development
Sewage
Dredging
Runoff from
construction and
agriculture
• Marine-based
threats
Threatened
• Overfishing especially
for herbivorous
species
Threatened
• Tourism pressure
Threatened
• Coral Bleaching
• Coral Disease
• Ocean Acidification
Threats to Coral Reefs from
Land-based Sources
• Sediment
Threats to Coral Reefs from
Land-based Sources
• Sediment
• Nutrients
Threats from Global Warming
Threats from Global Warming
• As sea temperatures rise corals, already weakened by
the loss of their zooxanthellae, become more prone to
attack by pathogenic bacteria;
• At the same time, the symbiotic bacteria that live in
the corals' guts and assist in their digestive process are
weakened, enabling the harmful bacteria to multiply and
wreak havoc on the corals.
• The corals' most effective defensive mechanism, a
surface mucus that acts as a shield against pathogens,
may also be inhibited by the combined impact of rising
temperatures and the bacterial assault.
Resilience of Coral Reefs
• Coral reefs appear to be much more
resilient to acute natural disturbances
than to chronic anthropogenic
disturbances.
– In the Florida Keys and in other reefs around
the world, corals reef ecosystems are being
impacted by a host of multiple stressors, and
are in very serious decline globally from
constellations of local anthropogenic impacts,
direct and indirect.
Eat coral
Parrot Fish
Poop sand