ECOLOGY OF CORAL REEFS
I PhD, Dept. of FRM
Coral reefs are wave-
resistant structures notable
for their great species
ecological complexity and
They are unique in being
formed entirely by the
Often called rain forests of the sea, due to
their richest biodiversity. They occupy less
than 0.1% of the world’s ocean surface, but
provide home for 25% of marine species
ORIGIN OF REEFS
Subsidence theory – (Darwin-Dana)
Solution theory – (Semper-Murray)
Submerged bank theory
Glacial epoch theory - Daly
It involves understanding biotic
and abiotic factors influencing the
distribution and abundance of
living things of coral community
Corals occur throughout the oceans, but
colonial reef-building (hermatypic) corals are
confined to the tropics and sub-tropics where
calcification rates are greatly enhanced
Reefs grow best in warm, marine, shallow,
clear, sunny and agitated waters
Coral reefs, just like any other ecosystem on our
planet, rely on a variety of biotic and abiotic
factors to keep them healthy and functional.
Without stable temperature, pH, light intensity,
water flow, salinity, and chemical composition of
sea water, coral reefs could not exist, but
without a stable trophic cascade, coral reefs
could not survive.
TYPES OF CORALS
Hard corals, also known as scleractinian and
stony coral, produce a rigid skeleton made of
calcium carbonate (CaCO3) in crystal form
called aragonite. Anatomic structures such
as septa, tentacles, and mesenteries are
found in sets of six, so hard corals are also
often termed hexa corals.
One of the characteristic feature of hermatypic
corals is the presence of unicellular algae
(Symbiodinium microadriaticum) with in the
By their influence on coral growth and
calcification rates these symbiotic zooxanthellae
play a fundamental role in the reef-building
Soft coral, also known as Alcyonacea and ahermatypic coral,
do not produce a rigid calcium carbonate skeleton and do not
form reefs, though they may be present in a reef ecosystem.
Anatomic structures such as tentacles and mesenteries are
found in sets of eight, so soft corals are often called
DIVERSITY OF CORAL REEFS
Reefs are home to a large variety of organisms, including
fish, seabirds, sponges, cnidarians (which includes some types of
corals and jellyfish), worms, crustaceans (including shrimp, cleaner
shrimp, spinylobsters and crabs), mollusks (including
cephalopods), echinoderms (including starfish, sea
urchins and sea cucumbers), sea squirts, sea turtles and sea
Aside from humans, mammals are rare on coral reefs, with
visiting cetaceans such as dolphins being the main exception.
A few of these varied species fed directly on corals, while others
graze on algae on the reef.
Reef biomass is positively related to species diversity.
Corals have developed several unique ways
of feeding; they receive nutrients from
symbiotic algae, capture particles such as
plankton, and take up dissolved substances
from the water.
In order to fully appreciate the importance of a
balanced ecosystem such as a coral reef, we
must first understand its trophic structure; the
organisms that make up each trophic level, and
the functions of each level in the maintenance
of a healthy reef.
Coral reefs are complex ecosystems that require a balanced
trophic structure to function properly and efficiently.
Imbalances can occur in this intricate trophic cascade
from the top down or the bottom up. For an example of
bottom-up effects, nutrient-rich agricultural run-off can
cause a massive increase in primary productivity (e.g.
algal blooms), the effects of which often cannot be
buffered by consumers fast enough to prevent a coral
reef ecosystem from collapsing.
There are three categories of organisms in every
ecosystem: producers, consumers, and decomposers.
Primary consumers are herbivorous, whereas secondary
consumers prey on herbivores and tertiary consumers eat
Decomposers are responsible for breaking down dead
and decaying plant and animal matter into components
that are once again usable for growth by producers.
None of these three categories of organisms can exist
without the others in order to complete the cycle of
production, consumption, and decomposition.
PRIMARY PRODUCERS OF CORAL REEF
zooxanthellae in corals
filamentous algal scum (turf algae)
coralline (calcareous) algae
filamentous algae growing through the
upper layers of the porous reef rock
benthic and interstitial diatoms
- primary consumers
Phytoplankton and other single-celled primary
producers are eaten by primary consumers.
Due to the large community of primary consumers on
coral reefs, phytoplankton levels in coral reef waters
can be 15-65% lower than in adjacent open ocean
Benthic grazers and some coral species feed by
filtering phytoplankton out of the water while other
vertebrate and invertebrate grazers eat algae and
seaweed; many species of parrotfish, surgeonfish
and blennies have a diet that consists entirely of
coralline, filamentous, and calciferous algae.
The animals in this trophic level feed on primary
consumers and are consequently carnivorous.
Secondary consumers in a reef ecosystem can
be divided into four main groups: (1) zoo
plankton feeders, (2) corallivores - organisms
that feed on coral tissue, (3) feeders on other
benthic invertebrates, and (4) piscivores - fish
Plankton feeders can be small sessile
invertebrates like barnacles, corals like sun
polyps (Tubastrea sp.) and gorgonians, small
damselfish or 15-ton whale sharks.
Corallivores can be sub-divided into polyp
eaters, coral scrapers, mucus feeders etc.
Many species of butterfly fish, damselfish
specialize in eating coral polyps
Some common coral scrapers are specific
species of triggerfish, parrotfish, blennies,
puffers, and butterfly fish.
Some animals that feed on coral mucus are
coral guard crabs and shrimps
Benthic invertebrates like mollusks,
gastropods, worms, and crustaceans are
eaten by many kinds of fish (e.g. goatfish,
wrasses, triggerfish, etc.) and other larger
Piscivores are carnivores that eat fish; many
species of fish are piscivores as well as
some mollusks and arthropods.
Tertiary (top) consumers
Tertiary consumers are large reef fish at the
top of the food chain that eat many smaller
fish. Some examples of top consumers in a
coral reef ecosystem are sharks, barracudas,
and moray eels. Marine mammals such as
dolphins and seals, and sea birds, if present,
are considered tertiary consumers, too.
Decomposers (and Detritivores)
Decomposers serve an extremely important function in all
ecosystems; they break down dead biological matter and
waste products and convert them into usable energy
while returning important materials to the environment.
The main decomposers in coral reefs are bacteria; these
bacteria play an integral part in the nitrogen cycle
whereby ammonia (NH4) is converted into nitrite (NO2)
by bacteria in the genus Nitrosomonas, after which nitrite
is then converted into nitrate (NO3) by bacteria in the
The ultimate result is that levels of toxic wastes are kept
very low and that waste products are converted into
components that are available to producers in a readily-
Coral reefs are complex ecosystems that
require a balanced trophic structure to function
properly and efficiently.
Imbalances can occur in this intricate trophic
cascade from the top down or the bottom up.
For an example of bottom-up effects, nutrient-
rich agricultural run-off can cause a massive
increase in primary productivity (e.g. algal
blooms), the effects of which often cannot be
buffered by consumers fast enough to prevent a
coral reef ecosystem from collapsing.
PRODUCTIVITY OF CORAL REEFS
The symbiotic arrangement between the algae
and corals results in nutrients being tightly
recycled with in coral reefs. This internal
nutrient cycling is of primary importance in
maintaining the productivity of the reef
The gross primary productivity ranges from
about 1500 to 5000 g.C/m2/year, values much
higher than those of open tropical Oceans
IMPORTANCE OF CORAL REEFS
Protection from coastal erosion
Very high diversity = ecological stability
Source of important natural chemicals being researched as
cures for cancer, arthritis, human infections, viruses, etc
THREATS TO CORAL REEFS
There are two types of threats to coral reefs,
anthropogenic and natural
Destructive and non-sustainable fishery
Coral bleaching – socio economic impacts, reef
based tourism and fisheries
Coral mining – construction, lime industry,
Pollution – agriculture, coastal development
Sedimentation - deforestation
Diseases – Black band, white band and red
band diseases were observed in corals
especially in shallow areas
In addition to the direct human interferences,
global climate change poses serious threat –
increase in temperature, and a possible
increase in the incidence of storms
Reefs are subjected to physical-erosion and bio-
Physical erosion – intense wave action,
currents, tropical storms
Removal of large no. of animals from reefs may
alter the ecology
Sea urchins graze up on the coral frame work
( fish, molluscs are over fished)
Crown of thorns star fish – Acanthaster
( triton snail) Destruction of Great Barrier Reef
As building material – Maldive islands
For ornamental purposes
Destructive fishing methods – dynamite,
Anchoring – Reefs of Florida
Walking over the reef during low tide
Mangrove deforestation – Gulf of Kutch