Reef building corals are a formation of many coral polyps that are interconnected by living tissue, allowing them
to function as one single organism. Coral polyps are animals belonging to the Phylum Cnidaria, which also include anemones,
jellyfish, box jellies and hydroids. All members of this particular type of Phylum, are made up of more than 95% water with
soft, sack-shaped bodies that are radially symmetrical.
Each polyp is made up of two layers of tissue, each only one cell thick. The outer layer is called the epidermis
and acts as a protective layer and absorbs nutrients. The inner layer, called the gastrodermis, lines the inner body cavity
which carries out digestion and produces reproductive cells. The epithelial cells that make up these layers are surrounded
by cell membranes, which are composed of a phospholipid bilayer which is selectively permeable and therefore regulates the
diffusion of molecules into and out of the polyps’ cells. These layers are separated and connected by a gelatinous material
called the mesoglea.
Coral polyps are sessile animals. They extract calcium ions and carbon dioxide from the sea to produce calcium carbonate,
which is then deposited at their blind end. These deposits extend vertically from their base providing them with an area for
refuge.
Cilia and tentacles armed with nematocysts (stinging cells) surround the polyps open end and aid the coral in grasping
prey and removing waste. Coral polyps also continually produce an exterior coating of sticky mucous that catches any free-swimming
organisms in its immediate vicinity. This sticky coat also acts as a protective shield. Although all corals can feed themselves,
shallow water corals receive 90% of their essential nutrients from symbiotic algae called Zooxanthellae.
Zooxanthellae require sunlight in order to carry out photosynthesis within polyp cells. They extract carbon and phosphates
from the ocean in order to provide the polyp with oxygen and organic compounds such as glucose and amino acids. They also
promote coral growth by speeding up the manufacturing process of calcium carbonate. In return the algae are provided with
a safe home and a constant supply of nutrients. Without Zooxanthellae coral growth would be extremely slow and reef structures
would take thousands of years to form. The limestone exoskeleton produced by the algae is also a lot stronger and more resistant
than that produced by the coral itself.
Deep sea corals grow from depths of 200m to more than 6000m. Their composition is very similar to that of shallow
water corals except they lack Zooxanthellae. Until recently, no-one (even marine scientists) knew that is was possible for
corals without Zooxanthellae to form reefs.
Deep sea reefs, up to 13,000m long, 400m wide and 35m tall, have recently been found all over the world. They grow
at a rate of 1.74cms per year and scientists have determined that the oldest is more than 8000 years old.
Because coral polyps are only two cells thick it is the simplicity of their composition that makes them extremely
sensitive and fragile. Both, shallow and deep sea reefs are very vulnerable and are extremely sensitive to changes in temperature,
salinity, nutrients, turbidity, PH and light.
Coral reefs are a natural habitat for many fish, invertebrates and algae. They have been described by scientists
as “the rain forest of the sea”, and are potentially the most diverse ecosystems on Earth.
These coral reef ecosystems are being destroyed by over-fishing, destructive fishing methods, interference from recreational
divers, coastal development and global warming.
Due to advances in technology, fishing has become very efficient, over one hundred million tons of fish are caught
each year. Sadly this has led to the downfall of many species of fish causing disruptions to the oceans food web. As the numbers
of larger fish decline, smaller fish are caught to continue to meet demand. The declining numbers of smaller fish allow accelerated
algal growth which starves the corals of sunlight preventing Zooxanthellae from photosynthesising and slows growth. This reduced
growth makes corals vulnerable to boring animals, such as sponges, polychaete worms and molluscs which weaken the corals structure
and cause them to collapse.
Many fishing techniques are incredibly damaging. Trawl nets used for deep sea fishing are armed with steel weights
that are bulldozing entire reef ecosystems. Reefs that have taken thousands of years to develop are being reduced to rubble
in an instant. These reefs will not grow back in our lifetime.
In Indonesia, the use of cyanide to catch fish for the aquarium trade is also destroying
reefs. Cyanide is a non-competitive inhibitor that attaches itself to enzymes within polyp cells. Once attached, cyanide changes
the shape of the enzyme’s active site, preventing them from being able to carry out the reactions necessary to sustain
polyp life.
Forms of fishing that appear to be ecological, such as farming are also affecting corals adversely. Scientific studies
have shown that waste from fish farms increases nutrient levels in surrounding waters, resulting in excess algae.
Tourists who visit areas for their beautiful coral reefs are inadvertently ruining them. Large scale coastal developments
in tourist destinations have resulted in deforestation, the removal of coastal mangroves and dredging which has caused an
increase in sediment in surrounding waters. The accumulation of sediment also prevents Zooxanthellae from photosynthesising,
reduces the permeability of the polyps’ cell membranes and prevents the polyps from being able to absorb nutrients.
As a result of coastal development untreated sewage is also making its way into reef ecosystems. The increase in nutrients
in the ocean from sewage has caused an influx in algal growth which again prevents sunlight from reaching coral’s Zooxanthellae.
Global warming is one of the biggest factors affecting coral reefs. Due to the burning of fossil fuels, carbon dioxide
build up in the atmosphere has caused a rise in the earths’ temperature. An increase of just one or two degrees Celsius
stresses corals enough to make them expel their Zooxanthellae, subsequently making them bright white, known as coral bleaching.
Corals are able to live without their symbiotic algae for a short time, however longer periods without Zooxanthellae have
lead to large scale coral demise all over the world. As a result of bleaching, two known species of coral in Panama have become extinct and possibly many more throughout
the world.
Global warming has also led to a change in ocean salinity and as a result of increasing temperatures and evaporation
tropical ocean waters are becoming saltier. Waters near the poles are becoming fresher as more fresh water vapour is transported
to the area via the atmosphere. Melting glaciers and Arctic sea ice are also contributing to an increase in fresh water. If
this continues, tropical surface waters may become salty enough to affect osmosis within polyp cells. Saltier water in the
tropics will cause water to move through osmosis from a region of higher net water potential in polyp cells to a region of
lower net water potential into the ocean. This would result in the polyp cells plasmolysing and eventually collapsing.
The Intergovernmental Oceanographic Commission reports that oceans are now absorbing “25 million tonnes of
carbon dioxide each day”. This has caused an increase in ocean acidity and therefore a decrease in PH, causing coral
exoskeletons to dissolve leaving them vulnerable to predators and disease. In shallow waters a prolonged decline in PH will
upset the charge balance in the ocean, which may stop enzymes in polyp cells from being able to attract substrates optimally.
Consequently the enzymes will not be able to carry out the functions necessary to sustain coral polyps. Scientists in Seattle have reported that ocean acidity is a temporary state that naturally
corrects itself over time, this however is only applicable if we reduce the amount of carbon dioxide we are pumping into the
atmosphere.
Coral reefs are an integral element of marine life, without them habitats and species will be lost forever, forming
an irreplaceable missing link in the food chain. They are under extreme pressure from anthropogenic activities and scientists
believe that 10% of corals, worldwide, have died and a further 70% will be dead within twenty to forty years. The combination
of a multitude of anthropogenic activities is a recipe for complete coral reef eradication.
Scientists have found that countries protecting their coral reefs with marine parks have had a positive impact on
coral re-growth. This suggests that by finding solutions to minimise the affects of anthropogenic activities, coral reefs
could recover.
Governments throughout the world need to be more stringent with coral reef protection. Many more marine parks and
no-take zones should be introduced. Already existing marine parks need to be better managed and governments need to enforce
better fishing techniques. More sustainable development and a means of reducing global warming need to be executed. There
also needs to be an increase in awareness of our impact on coral reefs.
With global compliance and cooperation I believe that we can prevent “the rainforest of the sea” becoming
an “ocean desert”.
