Chapter 14
Coral Reefs
Corals
• Corals are _____, a
•
•
phylum of animals
characterized by the
presence of _____ on
their tentacles
These _____ are used
to capture prey
primarily, but can also
be defensive
The soft _____ is
enclosed in a hard
shell of calcium
carbonate created by
the coral.
Anatomy of a Coral Reef
• Coral reef polyps
•
are interconnected
by thin layers of
tissue
They share a
nervous system and
digestive connection
– this is due to the
fact that the entire
colony came from
one original polyp
that reproduced
asexually by
_____to produce
the colony
Anatomy of a Coral Reef
• Each polyp continues
•
•
to lay down new
CaCO3 beneath the
polyp body so the
entire coral colony
continues to grow
upward
Each polyp is
normally 1 to 3 mm
in width
However, the
collection of polyps
together can
collectively grow to
extremely large sizes
and weigh several
tons
Coral Nutrition
• The zooxanthellae housed inside the coral
produce _____ through photosynthesis
• Some of this _____ is passed to the coral
• This mutualism aids the coral and helps
the entire reef grow faster
• In fact, if zooxanthellae are supplied with
enough light, they can feed the coral
completely
Coral Nutrition
• Of course, coral also possess ____ that
contain nematocysts to sting potential
prey that come within reach of the
tentacles
• These cells allow the coral to feed on
small organisms in the water, mostly
zooplankton
• Some corals also produce sheets of
mucous to capture zooxanthellae or
detritus
Coral Nutrition
• Coral can also feed
•
•
using extensions of
the gut wall called
mesenterial filaments
These filaments
secrete digestive
enzymes
The coral can extend
the filaments with its
enzymes allowing the
coral to feed and
digest food outside
the body
Conditions Required for Reef
Growth
• Hard substrate
• Light so in relatively shallow water of
photic zone
• Narrow temperature range= 68-86
degrees F
• Stenohaline staying around 35 ppt
• Low sediment load in water
• Low pollution
Reproduction and Growth of
New Coral
• Corals reproduce both sexually and asexually
• Asexual reproduction allows for the growth of
•
•
buds from a single polyp
Sexual reproduction results in the creation of a
planula larvae that is able to ride on the water
currents to a new environment
These larvae will settle out of the currents to
begin growth on existing reefs or other hard
substrate
Coral Stress and Bleaching
• The expulsion of
•
zooxanthellae is
termed bleaching
because the normally
colored coral is bleach
white after this
expulsion
The zooxanthellae
give the corals their
distinctive color
Coral Stress and Bleaching
• High temperatures (> 86 degrees F) can
cause coral bleaching
• Poor water quality, increased sediment in
water column, wave stress or disease can
also cause bleaching
• If corals remain bleached for too long, it
can result in death of the coral
• Widespread bleaching often occurs as a
result of extreme weather events such as
hurricanes
Coral Sensitivity to Pollution
• Even low levels of pollution can kill coral
polyps
• High nutrient levels (eutrophication) can
also allow algae levels to rise
• Increased algal growth can shade lightsensitive corals and their zooxanthellae
Types of Coral Growth Forms
• Coral colonies can come in a variety of
shapes and sizes
• Taller, more branching tend to be more
common in shallower areas of the reef
due to intense competition for space and
light
• Flatter forms tend to be more common in
deeper areas of the reef; this probably
assists with capturing all possible light
available.
Types of Coral Growth Forms
Branched and massive respectively. (Photographs by Dr. Donald Keith)
Foliaceous. Photograph by Dr. Donald Keith
Brain, lettuce leaf, and staghorn would be a few of the corals seen in the Flower
Gardens Coral Reef off the Galveston, Texas Coast
Other Organisms that Contribute to
Reef Structure Besides Stony Corals
• Coralline algae (also produce calcium carbonate)
• Soft corals such as sea whips and sea fans
•
•
(gorgonians) (produce a soft protein)
Other cnidarians such as hydrozoans or
anemones
Sponges
Types of Coral Reefs
1. Fringing Reefs
2. Barrier reefs
3. Atolls
Fringing Reefs
• Simplest and most common form of reefs
• These reefs develop near the shore in tropical
•
•
waters
These reefs develop as narrow strips along the
shore
The fringing reef shown in the next photograph
encircles the island it has formed near
Fringing Reefs
• Fringing reefs
consist of an inner
reef flat and an
outer reef slope
• The reef flat is wide,
gently sloping and
may be exposed in
places at low tide
• The reef slope is
much more steep
and is not exposed
to air
• Growth is normally
high in this area
Barrier Reefs
• Occur along shore also
but separated by a
lagoon.
• Typical construction of a
barrier reef: a back-reef
slope, a reef flat and crest
and a fore-reef slope
Barrier Reefs
• Waves often wash sediment onto the back reef
•
•
slope or reef flat which causes reduced coral
growth in this area
Enough sediment may accrue to form small
islands on the reef
These islands are called keys in the US or cayes
elsewhere. Florida has many keys.
A rich back-reef slope of a Pacific barrier reef not covered by sand
Great Barrier Reef, Australia
• The Great Barrier Reef is considered the largest
•
•
•
reef structure in the world with regards to total
coral area
The length of the reef systems exceeds 1200
miles
The width varies from 10 – 200 miles
Many cayes exist along the reef system
Atolls
• An atoll is a circular
•
•
reef structure
surrounding a central
lagoon
Sand cayes may be a
part of the atoll
structure
Width varies from less
than one mile to over
20 miles
Atoll Formation
• Atolls form when a volcanic
island is formed
• The coral begins as a
fringing reef around the
newly formed island
• Over time, weathering and
geologic activity lowers the
level of the island
• Eventually, the island
disappears below the
water’s surface, leaving only
the outer band of coral
surrounding a lagoon
Atolls
• Atolls possess a reef flat and inner and
outer reef slopes
• The inner slope has a more gradual slope
than the outer slope and the lagoon
normally has a depth of 200 feet or less
• The two largest atolls are in the Maldives
(Indian Ocean) and the Marshall Islands
(Pacific)
Coral Reef Ecology
• Like any marine environment, the coral reef
•
•
community is structured due to limiting
resources, competition, and predation
The water column surrounding the reef is
normally very low in nutrients
This is due to the position of reefs away from
rivers and other sources of nutrients (remember
corals form best away from possible
sedimentation sources)
Coral Reef Ecology
• Due to this low nutrient
content, the water
surrounding the reef does
not support the degree of
primary production by
phytoplankton seen in other
marine communities
• The food chain here is
based around the primary
production of zooxanthellae
located in the body of the
corals and other organisms
• The coral benefits the
zooxanthellae by providing
carbon dioxide, nitrogen
and phosphorus (byproducts), and shelter
• The zooxanthellae provide
sugars to the corals (and
any organism that feeds on
the corals or their parts
Coral Reef Ecology
• Cyanobacteria present on the reef are able
to fix nitrogen which is then available to
other reef inhabitants who feed on the
cyanobacteria
• Seagrasses and algae can also feed reef
inhabitants
• Water currents may bring some phytoand zooplankton to the reef community
Typical Coral Reef Food Web
Many Organisms Feed Directly on Corals
or their mucus, eggs, & larvae
Deterring Predation
• Many coral reef inhabitants can deter predation by
producing toxic or foul tasting chemicals – this is
common in soft corals and sponges
• These chemicals can be released into the water to kill
organisms or limit growth surrounding these organisms
• Other reef dwellers may produce hard structures to
deter predation – sponges produce spicules of calcium
carbonate or silica while soft corals produce calcium
carbonate needles known as sclerites
• Coralline algaes as well as other organisms grow in an
encrusting form that is difficult to feed upon because of
its thinness and hardness
Competition on the Reef
• There is high competition for space
•
•
•
•
and light on the reef
To limit competition, some corals
grow fast and upright to maximize
their ability to reproduce quickly
Others grow slow but massive to outcompete other organisms in the long
run
Corals may feed directly or sting one
another if they contact each other
Shown at right is interspecific coral
competition (and some coral death in
the pink area)
Competition on the Reef
• Other organisms
•
•
such as fish and
crustaceans must
also compete for
limited resources
They normally do
this by feeding in
slightly different
areas or in
slightly different
ways
Each has its own
ecological niche