I. Importance of coral reefs
A. High biodiversity –
B. Store CO2 (to produce CaCO3)
C. High productivity
D. Indicator of stresses on marine ecosystems
E. Human economy
1. tourism
2. food
3. medicine
F. Basic biological and paleontological questions
II. Reef distribution and limiting factors
A. Structure of corals
1. reef is built entirely
by biological activity
a. reef-building corals – Phylum Cnidaria, Class Anthozoa, Order
Scleractinia
b. calcareous algae and a few other organisms also contribute
2. hermatypic corals
a. reef-producing corals
b. found only in the tropics
c. possess zooxanthellae in their tissues (symbiotic algae –
dinoflagellate)
d. most are colonial
(1) individual coral animals (polyps) sit in corallites
(2) each polyp is a 3 layered animal --
outer
epidermis, middle mesoglea,
inner gastrodermis – contains the zooxanthellae
(3) tentacles have nematocysts (stinging cells) for capturing
zooplankton
e. reproduction
(1) polyps can bud off new polyps asexually
(2) new colonies can form asexually from fragmented colonies
(3) sexual reproduction produces a planula larvae that
disperses
and settles
3. ahermatypic corals
a. do not form reefs
b. distributed worldwide
B. Distribution of reefs –limiting factors
1. temperature
a. Minimum
b.
Optimum
c. bleaching
2. depth/light
a. do not grow deeper than about 50-70 m; generally <25 m
b. requirement for light
3. salinity
a. hermatypic corals are intolerant of variations in salinity
b. areas of freshwater input
c. areas of high
salinity
4. sedimentation rate
5. wave action
6. exposure to air --
tidal
extension
III. Types and composition of reefs
A. Three general categories of reefs
1. atolls – ring of reef
surrounding a lagoon, in deep water far from land
2. fringing reefs
a. simplest and most common - grow in band close to shore
b.
is a continuum between fringing and barrier reefs
3. barrier reefs
a. adjacent to, but separated from a landmass
b. lagoon lies between reef and shore – more protected area with soft
bottom
B. How reefs form
1. Atolls –
How could reefs develop in deep water,
miles from shore, when reef-building
corals can't live deeper than 50-70 m?
a. subsidence
(compensation) hypothesis
(proposed by Darwin)
(1) atolls are created as fringing reefs on the
shores of new volcanic islands;
as the islands subside slowly,
reef growth keeps up with the subsidence; in the center there is quiet
water with high sedimentation that prevents
vigorous coral growth -- results in a lagoon
(2) tested >100 years later
2. Fringing and barrier
reefs
C. Common Biota on Reefs
1. stony corals (Phylum
Cnidaria, Class Anthozoa, Order Scleractinia)
2. Gorgonians (Phylum
Cnidaria, Class Anthozoa, Order Gorgonacea)
a. sea fans and sea whips
b.
common in Atlantic reefs
3. soft corals (6 different
orders of Cnidarian; subclass Octocorallia)
a. common in Indo-Pacific and rare in Atlantic
b. don’t contribute to reef formation
4. hydrocorals (Phyllum Cnidaria, Class
Hydrozoa, Order Hydrocorallina)
a. include fire coral, Millepora
b.
can build reefs
c.
do have a medusa stage
5. coralline algae
a. red algae
(Rhodophyta)
b. precipitate CaCO3
c. cements the reef together
d. form the algal ridge
6. calcareous green algae
7. mollusks
a. giant clams
b. gastropods abundant but inconspicuous
8. echinoderms (e.g.,
urchins,
sea cucumbers, starfish, feather stars)
9. sponges --
a. common
b. minor role
in modern reef formation
c.
sclerosponges on deep slopes
d. can have cyanobacterial symbionts that increase productivity
10. fishes -- can be
hundreds
or thousands of species
D. Have been reefs throughout geologic history
1. modern scleractinian
reefs developed in the mid-Triassic (235 mybp)
2. earlier reefs dominated
by algae, sponges, clams (rudists), extinct corals
3. geological mass
extinctions,
followed by new reef communities
IV. Reef Zonation and Biogeography
A. Indo-Pacific has ~10X more species of coral than
does the Atlantic Ocean
B. Few species at the edges of the range
C. Physical structure is complex
D. Zonation patterns generally the result
of physical factors; atoll example
1. windward side
a. outer seaward slope
b. ~15 m - is often a terrace; area of most severe wave action;
poorly studied
c. windward reef margin -- at surface
d. buttress zone (spur and grove zone) coral alternates with
deep
surge channels;
optimal conditions here - fastest growth
e. algal ridge (lithothamnion ridge) -- coral free area with
coralline
algae
f. reef flat – shallow, strong gradients, good conditions for
growth
g. seaward beach of the island -- corals become less abundant
2. lagoon side of the island
a. narrow beach
b. lagoon (generally <50 m)
(1) lagoon reefs
(2) lagoon floor
3. leeward side
a. boulder zone -- area of poor coral growth
b. leeward reef flat -- narrower than windward side
c. leeward reef margin --
(1) algal ridge is only poorly developed;
(2) no surge channels or buttresses;
(3) down to 15-20 m coral is abundant and diverse
d. leeward reef slope -- similar to windward slope; sparse corals
4. fringing and barrier
reefs have similar zonations
E. Small scale patchiness
F. Differences between Atlantic (Atl) and
Indo-Pacific (IP) Reefs
1. Differences in coral species
2. IP reefs also have more
sponges, mollusks,
crustaceans,
cnidarians, and fishes
3. Some taxa are more common in
Atlantic
a.
gorgonians (sea fans and
whips)
b. sponge
biomass 2-10X
higher (but few have photosynthetic symbionts)
4. Taxa less common or absent in
Atlantic
a. no
true anemone fishes,
although
some fish do associate with anemones
b. no
giant clams; few
octocorals other than gorgonians
c. no
crab and shrimp
guarders
of corals
d. no coral
eating starfish, like Acanthaster
planci
5. Dominance
a.
Atlantic dominated by
Acropora palmata (elkhorn coral) and Millepora complanata
(a hydrocoral)
b. IP not
have any reefs
dominated by Millepora
c.
Dominant Atlantic grazers
are echinoids; dominant IP grazers are fishes
6. Diurnal rhythms
a.
Atlantic corals mostly
nocturnal
b. IP
corals also can be
diurnal
7. IP and Atl also differ in reef
age and shape
–
a.
Atlantic reefs are
usually
on shallow banks or platforms
b. In
Atlantic corals often
grow deeper
c. In
Atlantic corals
usually
cover 60% of the reef area; IP they cover 80-90%
d. IP
reefs are older
V. Reef Productivity ('oases in a watery desert')
A. 1,500-5,000
g C/m2/yr – about 100X more productive than tropical open
ocean systems (18-50
g C/m2/yr)
B. Production by zooxanthellae, algae, and
phytoplankton
C. Some nitrogen fixation from cyanobacteria –
some free-living and some symbiotic
D. Reef systems hold and recycle nutrients
efficiently --
1. filter oceanic
productivity
2. many primary producers
3. symbioses
4. large bacterial
populations
-- efficient recycling
VII. Biology of Hermatypic Corals
A. Nutrition
1. Corals are predatory;
nematocysts to capture plankton; 10% of requirements
2. Corals also secrete mucus
that entraps small organisms
3. Zooxanthellae live in
coral polyps -- most
of the energy for the corals
4. DOC from the water can
be utilized by coral directly
5. Corals have an
interconnected
gastrovascular cavity – excess food transported from one region of the
colony to
another region that is receiving less food
B. Growth and Calcification
1. zooxanthellae are
important
in calcification
2. Hard to measure growth
rates
C. Reproduction
1. asexual – budding (binary
fission); fragmentation
2. sexual – planula larvae
that establish new colonies when they settle
3. some corals can produce
planulae asexually
4. mature at 7-10 years
of age
5. most are broadcast
spawners
(some simultaneous), but some brood;
D. Recruitment
1. virtually all reef fishes
and 85% of corals have a planktonic larval stage
2. recruitment lower in
the Caribbean than in the Great Barrier Reef
3. may affect recovery from
catastrophic events
4. mortality of new colonies
appears to be high
VIII. Species interactions and ecology of reefs
A. Competition
1. space in short supply
2. corals and algae compete
for space
3. exploitative
competition
4. How can slower growing
corals exist?
a. interference competition (direct interactions)
b. shade-tolerance; depth zonation
c. some soft corals have water-borne chemicals -- cause tissue damage
and growth inhibition in hard corals
5. corals must also compete for
space with other encrusting invertebrates
a. sponges
b. fast-growing algae
B. Predation
1. cryptofauna –
(hidden animals) if exposed, they are consumed by fishes
2. predators grazing on
the reef itself – gastropods, polychaetes, starfish (Acanthaster
planci),
fish
a. predation on coral often resembles grazing -- whole colony
not killed
b. corallivorous fish consume pieces of the coral skeleton to eat the polyps
c. physical disturbance (hurricanes; temperature
change) may kill corals and concentrate predators on the few remaining
corals
e. Acanthaster planci, crown of thorns starfish
C. Grazing – without grazing, many corals would
be overgrown with algae
1. herbivorous fish and
urchins
a. can remove 50-100% of total algal production
b.
prefer fleshy filamentous algae
c. cause
increased growth of coralline
algae
2. algal defenses
3. die-off of the grazing
urchin, Diadema, in the Caribbean in the early 1980's
(effects compounded by overfishing)
4. urchins are unselective
and can also destroy young coral colonies while grazing
5. damselfish
(Pomacentridae)
are territorial and graze and ‘farm’ the algae in their territory
D. Bacteria in Reef Systems
1. are abundant
2. some cyanobacteria may
erode reefs and cause reef diseases
E. Algae in Reef Systems
1. Can have high productivity --
eaten by herbivores
2. Red coralline algae
cement
the reef
3. green calcareous algae
(Halimeda) contribute to sand
in reefs and lagoons
4. Some algae bore into
coral skeletons