Lecture on coral reefs for The Evergreen State College program, Sea Change

1. Coral reefs
• What is a Reef?
• Types of Reefs
• Reef Formation
• Reef Distribution
• Coral Biology
• Coral Reef Ecology
• Reef Conservation
ML2007
S. Norton

2. What is a reef?
"...a rigid structure composed of calcareous
skeletons of various organisms, interlocked
or cemented together by growth, and of
detrital material derived from the break up
of such skeletons, and the structure
maintains its upper surface at or near the
level of the sea." (MacNiel, 1954)

3. Coral reefs
Limestone and calcareous sediments built by
living organisms.
Coral colonies are short-lived (years to decades).
They are the largest biogenic structures on the
face of the earth.
www.abc.net.au/science/news/
enviro/EnviroRepublish_1003558.htm
Coral reefs have been around for millions of years.

4. Reef Forming Organisms
• Worms (Polychaeta, etc.)
• Calcareous Algae
• Sponges (Porifera)
• Bivalves (and other members
of Mollusca)
• Other minor phyla (Bryozoa,
Brachiopoda, etc.)
• Corals (Cnidaria)

5. Hermatypic Corals

6. Corals
• Cnidaria
– Anthozoa
• Colonial polyps
• Photosynthetic zooxanthellae
– Symbiodinium spp.
• Hermatypic (Scleractinia) corals form reefs

7. Symbiodinium spp.
Virtual look-alikes;
distinct species.

8. Reef Formation
“Evolutionary Sequence”
Fringing > Barrier > Atoll

9. Three major types of coral reefs
• Fringing
• Barrier
• Atoll
• Proposed by Darwin (1842)
“On the Structure and
Distribution of Coral Reefs”

10. Three major types of coral reefs
• Fringing
• Barrier
• Atoll
• ...cuspate,
mesh,
patch,
platform...
• Proposed by Darwin (1842)
“On the Structure and
Distribution of Coral Reefs”
• Morphology based on “size,
shape and nearby land if
any”

11. Fringing Reef
Nearshore, well-lighted marine waters
Narrow zone if submarine slope is steep
Wide zone if submarine slope is gentle
Young volcanic islands in tropics ideal:
•Lots of oxygen and nutrients
•Very low FW input, so high S‰ & low silt

12. Fringing Reefs
Coral Rock of Fringing Reefs is relatively
“thin” (usually 25-50 m), except where
landmass is subsiding
Longest reef in world is Red Sea = 4000 km if
straightened out.

13. Fringe Reefs
Castro & Huber Fig. 13.14
Reef regions include the flat, crest, and slope.
Mangroves, seagrass or sandy beaches may line the shore;
fringing reefs typically lack a lagoon.

14. Fringing Reefs grade into Barrier Reefs
Castro & Huber Fig. 13.17
www.tropichorizon.com/belize.html
Barrier reefs typically have a deeper water lagoon
or channel separating the reef crest from the shore.

15. Barrier Reefs
Linear structures separated by a lagoon (up to 100 km)
100-200 m thickness of coral rock
Annular around island
Reefs rise from terrace or platform

16. Great Barrier Reef
• 2000 km long
• 5-75 km wide
• Long, complex geographic
history

17. Atolls
Name originally from Darwin (1842).
Maldives

18. Atolls
Castro & Huber Fig. 13.22
Found primarily in the Pacific, atolls develop on the tops of seamounts.

19. Formation of Atolls
Raven et al. 6.21
Subsidence Theory:
Atolls begin as fringing
reefs around volcanic
islands that have emerged
above sea level.

20. Formation of Atolls
Raven et al. 6.21
Subsidence Theory:
As magma sources drop, the
seamount subsides and the
reef becomes a barrier reef.

21. Formation of Atolls
Raven et al. 6.21
Subsidence Theory:
As the seamount continues
its subsidence below the
surface, corals grow on top
of the seamount, keeping
corals in the photic zone.

22. Atoll Examples
Deep sea atolls from sea floor (e.g.,
Palau).
Shelf atolls from continental
shelf (e.g., Belize).
Eniwetok Atoll is 1.25 km of
shallow-water coral limestones on
top of a 3.25 km high volcano

23. A Map of Earth's Coral Reefs
• The pink regions represent the primary reef areas.
• The dark gray lines are major ocean currents.
• The lighter gray lines represent the 20 degree isotherm latitude. It is above or
below this location that the water is too cold to sustain coral polyps.

24. Global distribution of coral reefs
Purves et al. Fig. 54.28
Reefs are absent where coastal waters are too cold or rivers bring in
low-salinity water and high sediment loads.

25. Global diversity of corals
At the species level (??), there are over 700 species of corals in the
Indo-Pacific versus approximately 60 in the Atlantic.
Purves et al. Fig. 54.28
The center of biodiversity of corals is the Indo-Pacific: Australia,
Micronesia, Indonesia, Philippines, etc.
Genera of corals

26. Biodiversity of coral reefs
Coral reefs harbor more examples of evolutionary diversity (phyla,
classes, etc.) than any place on Earth.
The Great Barrier Reef
~350 spp. of hermatypic corals
>4000 spp. of molluscs
1500 spp. of fish
240 spp. of seabirds.
Nybakken & Bertness Table 9.2

27. Clines in diversity
Clines in species richness have been recorded for marine organisms
as well, including:
fish
corals
snails
lobsters
Roberts et al. 2002. Science 295:1280-1284

28. General Conditions for Coral Reefs
• 20-30°C
• 32‰ S or above
• Adequate Surface Substrate
• High Light Levels
• 25 m or shallower (light)
• Low Turbidity (feeding, light, substrate)

29. Necessary conditions for coral reefs: Light
The symbionts require light for
photosynthesis.
Karleskint Fig. 15.10

30. Necessary conditions for coral reefs: Light
Most coral growth occurs shallower
than 25 m.
Karleskint Fig. 15.10

31. Necessary conditions for coral reefs: Light
The compensation point is 1-2% of
surface intensity.
Karleskint Fig. 15.10

32. Necessary conditions for coral reefs: Light
Shallow corals synthesize UV
absorbing compounds.
Karleskint Fig. 15.10

33. Necessary conditions for coral reefs:
Low turbidity and sedimentation
Sedimentation and turbidity harm
coral nutrition.
www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.html

34. Suspended particles absorb/scatter
sunlight.
www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.html
Necessary conditions for coral reefs:
Low turbidity and sedimentation

35. Sediments smother small corals
and/or inhibit feeding.
www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.html
Necessary conditions for coral reefs:
Low turbidity and sedimentation

36. Some corals produce large
quantities of mucus to trap and
remove sediment.
www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.html
Necessary conditions for coral reefs:
Low turbidity and sedimentation

37. Necessary conditions for coral reefs: Full salinity waters
Corals are stenohaline
photography.nationalgeographic.com

38. Necessary conditions for coral reefs: Subtidal waters
The upper limit of coral
colonies is the lower
part of the intertidal.
Castro&HuberFig.13.16

39. Necessary conditions for coral reefs: Low [Nutrients]
Grow well under low
[nutrient].
Under higher
[nutrient], fleshy
algae flourish.
knowledge.allianz.com

40. Necessary conditions for coral reefs: High wave energy
At least moderate amounts of wave or current

41. Necessary conditions for coral reefs: High wave energy
Waves and longshore currents
bring nutrients and
phytoplankton.
dnr.louisiana.gov

42. Necessary conditions for coral reefs: High wave energy
Storms destroy coral calcium
carbonate skeletons.

43. Castro & Huber 13.7
Branching corals:
fast growth rates
vulnerable to storm damage
Morphological diversity among corals

44. Castro & Huber 13.7
Massive corals:
grow more slowly
but are very stable
Morphological diversity among corals

45. Castro & Huber 13.7
Plate-like corals:
enhance light collection
Morphological diversity among corals

46. Castro & Huber 13.7
Encrusting corals:
high wave energy.
Morphological diversity among corals

47. Castro & Huber 13.7
Free-living corals:
single polyp
not cemented to the surface
Morphological diversity among corals

48. Mechanisms for reef growth
Castro & Huber Fig. 13.8
Corals provide a large framework for
reef development.
Encrusting coralline algae, sponges, and
bryozoans bind the CaCO3 sediments
together.
They build a 3-D framework.
Calcareous sediments from algae and
algal rubble collect in gaps.
Further geological processes turn this
complex into limestone.

49. Importance of calcareous algae
On the Great Barrier Reef, 17-40% of surface sediment are derived
from calcareous red algae and 10-30% from Halimeda.
coexploration.org/bbsr/coral/
assets/images/halimeda.jpg
Halimeda and Porolithon, have 95% or more
of their body mass as calcium carbonate.
www.coralreefnetwork.com/
marlife/stepath/rhodophyta2.html

50. Distribution of corals along a reef
Nybakken & Bertness Fig. 9.23
The inner lagoon/reef flat.

51. Distribution of corals along a reef
Nybakken & Bertness Fig. 9.23
At the reef crest:
-Pacific: crustose coraline algae, heavy
branching corals
-Atlantic: just branching corals.

52. Distribution of corals along a reef
Massive corals dominate the
middle depths of the reef
slope.

53. Patterns of coral recruitment
Both sexual reproduction (brooding and broadcast) and asexual
reproduction (fragmentation) contribute to the recruitment of new corals.

54. Patterns of coral recruitment
Levinton 2001 Fig. 5.12
Planulae settle close to their
parents.
Both sexual reproduction (brooding and broadcast) and asexual
reproduction (fragmentation) contribute to the recruitment of new corals.

55. Interspecific interactions on coral reefs
Among primary space occupiers,
competition is a critically important
phenomenon.

56. Interspecific interactions on coral reefs
Exploitative Competition
Heron Island, Joe Connell in Nybakken & Bertness Fig. 9.27
1963
1965
Faster growers may extend over
and shade slower-growing forms.

57. Interference Competition
-Mesenterial filaments
Interspecific interactions on coral reefs

58. Importance of Invertebrate-Alga Symbiosis
Coral Reefs
High productivity in low [nutrient] water (over 2000
g C m-2
y-1
)
Some octocorals have no nematocysts, reduced
digestive tracts and show no behavioural changes in
the presence of µzooplankton prey
Some die without light, but can live and grow in FSW
Insufficient zooplankton populations to sustain reef
growth (~10%)

59. Autotrophy vs. heterotrophy in corals
In general, shallow water corals rely more
on their symbionts for nutrition.
Nybakken & Bertness Fig. 9.8
Deeper water corals are more predaceous.
Nybakken & Bertness Fig. 9.26
This pattern can even be seen
intraspecifically in these skeletons
of Montastrea cavernosa from
shallow water (left) vs. deep water
(right) in the Caribbean Sea .

60. Importance of Reefs
While they comprise only 0.2% of the ocean’s surface, coral reefs are
home to a third of the ocean’s fish species.

61. Importance of Reefs
Coral reefs provide food for up to
one billion people in Asia alone.
wwf.org

62. Importance of Reefs
Tourism in the Caribbean is worth over $10 billion.
noaa.gov

63. Threats
• Coral bleaching
• Climate Change
• Sea Level Rise
• Ocean Acidification
• Temperature Increases
• Crown of Thorns Starfish
• Diadema antillarum losses
• Harvesting for coral and for
aquarium trade
• Tourism

64. a-sul.blogspot.com
Importance of Reefs
Tourism in the Caribbean is worth over $10 billion.

65. Coral Bleaching
Exposed reef flat in Hawai’i with white patches of coral
that have expulsed their algal symbionts.

66. • Archipelago is 823 km long and 130 km wide at its greatest width.
• ~90,000 square km.
• 26 natural Atolls
• About 200 islands are inhabited.
Sea level rise and the Maldives

67. • Maldives islands are on average
1.5 meters above sea level
• Flooding will be a problem
• Sea level may rise so quickly that
it will erode the coral islands.
Sea level rise and the Maldives

68. Ocean Acidification
Increased Atmospheric
[CO2] results in
decrease in pH of the
oceans.
Corals and other
organisms with calcium
carbonate skeletons
can’t lay down CaCO3
and skeletons dissolve.

69. Tales of two echinoderms
Diadema antillarum -
long-spined urchin
While not described as such, both species play major roles in the
health of coral reefs - keystone species??
Acanthaster planci -
Crown of thorns starfish

70. Diadema’s story
sv.wikipedia.org/wiki/Tagghudingar
Before 1983, D. antillarum was a
large, abundant herbivore found on
Caribbean coral reefs, eelgrass beds,
mangrove roots, and sand flats.
Noctural foraging by D. antillarum
in seagrass beds brought nutrients
into the patch reefs that it used for
shelter.
Levinton 2001, CD

71. Lessios 1988. Fig. 1
In January 1983, sick D. antillarum appeared at Punta Galeta,
Panama.
Spatial and temporal patterns of Diadema mortality
Within a year, the
disease spread
2000 km east.
At a wide range of sites, D. antillarum populations plummeted by
more than 93% from pre-outbreak levels, up to 98% reductions.

72. What was responsible?
The fast spread of the disease and
spread of disease to lab animals
living in running seawater indicated
a water-borne agent.
www.health.qld.gov.au/EndoscopeReprocessing
/images/page_images/137_clostridium.jpg
Two species of Clostridium bacteria,
collected from infected animals,
were lethal when injected into
healthy animals.

73. The delayed recovery
Recruitment of D. antillarum failed across the Caribbean within 5-7 months after the
epidemic.
Over a decade later, recruitment of D. antillarum was still sparse.
Hughes (1994. Science 265:1547-1551)
documented changes in Diadema
populations and community impacts in
Jamaica.

74. In Jamaica, coral
% cover declined
from an average of
52% in 1980 to
3% in the 1990’s.
Hughes 1984. Fig. 5
Within months of the mass mortality of Diadema, populations of fleshy and
filamentous algae exploded across the Caribbean.
Consequences of Diadema’s absence

75. The bottom line
In Jamaica, overfishing of herbivorous fishes increased the
vulnerability of the coral reef to algal competition, but the fleshy algae
were kept in check by Diadema.
Hughes 1984. Fig. 6
Reefs dominated by corals
in the 1970’s were
dominated by algae in the
1990’s.

76. The NEW bottom line
Idjadi1, Haring, Precht, 2011 MEPS

77. The story of Acanthaster planci
Acanthaster planci ranges from the
Indian Ocean, through the Indo-Pacific
to the Pacific Coast of central America.
In general, this species prefers
feeding on hermatypic corals,
especially the dominant coral in
the Indo-Pacific, Acropora spp.

78. The story of A. planci
Acanthaster planci feeds on corals by
everting its stomach over the coral
surface.
www.reef.crc.org.au/discover/plantsanimals/cots/
On the Great Barrier Reef (GBR), a healthy coral reef (40-50% coral
cover) can support 20-30 individuals per hectare.

79. The story of Acanthaster
Outbreaks have been noted since
the 1960’s.

80. Recovery after Acanthaster outbreaks
Even on heavily
damaged reefs, some
corals remain alive
and begin recovery.
Reefs with poorer larval supply may not recover after 15 years or more.
Levinton 2001. CD

81. What causes outbreaks?
2) They are the result of natural fluctuations caused by variation in the number of
new recruits.
1) Under conditions of food stress (e.g, after a storm), adult Acanthaster living in
deep water will use chemical cues to form aggregations in shallow water.

82. Milne 1995. Fig. 15.8
Examining rubble on
reefs does reveal periods
with high concentrations
of Acanthaster spines as
far back as 3,355 years
ago.
What causes
outbreaks?

83. www.culture-bx1.u-bordeaux.fr/Ateliers
/Internet/Etudiants/recifs/stress.html
The giant triton, Charonia tritonus, cuts
open an adult Acanthaster with its radula
and scapes up gonads and viscera.
What causes outbreaks?
3) Overharvesting of predators leads to higher
survival of Acanthaster juveniles and adults.
This species has been collected for its shell and numbers are
severely reduced.
Its effectiveness, even before its overharvest, is questioned because
each triton typically eats only one adult starfish per week.

84. What causes outbreaks?
If predation is important, why would one see sudden, simultaneous
outbreaks in multiple places in the same years?
In the Red Sea, pufferfishes and triggerfishes kill up to 4,000
individuals/ha/yr, leading to low abundance of A. planci there.
www.bio.davidson.edu/people
/midorcas/animalphysiology/
websites/1999/Rice/Aaron.htm
In the Indo-Pacific, the humphead
wrasse Cheilinus undulatus) is an active
predator on Acanthaster.
While adults are large (up to 200 kg),
they are not known to occur at high
densities.

85. What causes outbreaks?
4) Anthopogenic changes in water quality, especially after high rainfall
on land, have been proposed (Birkeland 1982) to drive outbreaks.
A) Low salinities and high temperatures from terrestrial runoff may
improve larval survival.
B) Increases in nutrients in runoff stimulates phytoplankton
production and this improves larval survival.

86. Strategies for Acanthaster control
Acanthaster outbreaks at economically-valuable sites (e.g.,
tourist sites) may be controlled by injecting sodium
bisulfate into each starfish.
Efforts of control the latest outbreak at just a few sites cost
the Australian government $1 million/yr.

87. Acknowledgements (Incomplete)
• Reef map courtesy of Jeremy Staford-Deitsch from his book entitled Reef.
• University of Hawaii, http://www.hawaii.edu/HIMB
• http://earthobservatory.nasa.gov
• Chuck Fisher
• Steve Norton