Coral Reefs

Ever wonder are corals animal, plant or colourful rocks? My friends always get this question from me, and most of them are not quite right. The correct answer is animal. In fact, The Great Barrier Reef located in coral sea, off the coast of Queensland in Northeast Australia, is the biggest single living structure that can be seen from outer space!

The Great Barrier Reef from outer space

A coral polyp is a tubular saclike animal with a central mouth surrounded by a ring of tentacles. The end opposite the tentacles, called the base, is attached to the substrate.

Coral does not have a brain but it has a simple nervous system called a nerve net. The nerve net extends from the mouth to the tentacles. They can detect certain substances such as sugars and amino acids, this enables corals to detect prey. Tiny sensors in the ends of nematocysts in polyp's tentacles trigger the nematocyst to eject. This results in the tentacles to grab hold of its prey and transfer them into their mouth. The movements of the tentacles are delicate, requires detailed observation to notice them moving, they are not just swaying to the ocean current.
Reef-building corals have a mutualistic relationship with zooxanthellae, microscopic algae that live with coral polyp's tissues. Both the polyp and the zooanthellae benefit. For this reason, reef-building corals are found only in areas where symbiotic zooxanthellae can take in light for photosynthesis.

Through photosynthesis, zooxanthellae convert carbon dioxide and water into oxygen and carbohydrates. The coral polyp uses carbohydrates as a nutrient. The polyp also uses oxygen for respiration and in turns, returns carbon dioxide to the zooxanthella. Through this exchange, coral saves energy that would otherwise be used to eliminate the carbon dioxide.
Zooxanthellae also promote polyp calcification by removing carbon dioxide during photosynthesis. Under optimum conditions, this enhanced calcification builds the reef faster than it can be eroded by physical or physical or biological factors.

Heart Reef -Australia

Some corals eat zooplankton (tiny drifting animals) or small fishes. Others consume organic debris. Coral polyps are generally nocturnal feeders. At night, they extend their tentacles to capture food with the aid of nematocysts. That is the reason why divers go night diving, cause the coral reefs look very different from daytime as they are nocturnal and hunts for food at night, revealing their soft bodies and colourful and graceful tentacles.


Some corals secrete films or strands of mucus to collect fine organic particles. In reef-building corals, to mobile filaments originating from the stomach cavity can capture larger food particles. These filaments are also capable of digestion. The stomach cavities of colonial corals are interconnected. Food obtained by one polyp can be passed to other polyps in the colony (how cool is that). It excretes solid wastes through its mouth.

Corals can reproduce both sexually and asexually. An individual polyp may use both reproductive modes within its lifetime.

Corals reproduce sexually by either internal or external fertilization. The reproductive cells are borne on mesenteries (membranes) that radiate inward from the layer of tissue that lines the stomach cavity.

a. Internally fertilized eggs are brooded by the polyp for days to weeks. Free-swimming larvae are released into the water and settle within hours.

b. Externally fertilized eggs develop while adrift. After a few days, fertilized eggs develop into free-swimming larvae. Larvae settle within hours to days.

Synchronous spawning occurs in many corals. Polyps release eggs and sperm into the water at the same time. This spawning method disperses eggs over a larger area.

Sometimes, newly developing coral colonies split and form separate colonies. Budding occurs when a portion of the parent polyp pinches off to form a new individual. This is when asexual reproduction occurs.Budding enables the polyp to replicate itself several times and at the same time maintain tissue connections within the colony. Later, the same polyp may reproduce sexually.

Coral colonies growing in shallow water are often heavily branched. In contrast, deeper water corals often grow in sheets or plates. These flattened forms allow for more efficient use of lower light intensities in deeper waters. The growth rate of corals and coral reefs depends on factors such as light intensity, water temperature, salinity, turbidity, food availability, competition for space, and predation. Upward growth of coral colonies is generally between 0.5 to 4 in. (1-10 cm) a year.

Respiration (gas exchange) takes place through the body surface.

At one time it was mistakenly thought that coral grew at the bottom of deep tropical seas and succeeding generations grew on top of the dead calcium carbonate skeletons. This idea was dispelled by dredging operations that indicated that reef corals were able to grow only in shallow water.

Naturalist Charles Darwin's theory of coral formation is widely accepted. This theory recognizes three types of reefs: the fringing reef, the barrier reef, and the atoll.

a. The first type is a fringing reef. Fringing reefs border shorelines of continents and islands in tropical seas. Fringing reefs are commonly found in the South Pacific Hawaiian Islands, and parts of the Caribbean.

b. The next type is the barrier reef, which occurs farther offshore. Barrier reefs form when land masses sink, and fringing reefs become separated from shorelines by wide channels. Barrier reefs are common in the Caribbean and Indo-Pacific. The Great Barrier Reef off northern Australia in the Indo-Pacific is the largest barrier reef in the world. This reef stretches more than 1,240 miles (2,000 km).

c. If the land mass is a small island, it may eventually disappear below the ocean surface, and the reef becomes an atoll. Atolls are reefs that surround a central lagoon. The result is several low coral islands around a lagoon. Atolls commonly occur in the Indo- Pacific. The largest atoll, named Kwajalein, surrounds a lagoon over 60 miles (97 km) long.

The 3 types of coral reefs. (from left : fringing, barrier and atoll)

Little is known about the lifespan of corals. Generally, coral colonies may live for several decades to centuries.

Sadly, corals are in danger…

Ocean pollution poisons coral polyps. Pollution takes on many forms including oil slicks, pesticides and other chemicals, heavy metals, and garbage.

1. Ocean pollution poisons coral polyps. Pollution takes on many forms including oil slicks, pesticides and other chemicals, heavy metals, and garbage.

2. Fertilizer runoff and untreated sewage introduce added nutrients to coastal ecosystems. These elevated nutrient levels promote algae growth. Unfortunately, high concentrations of algae or solid sewage can overwhelm and smother the polyps.

3. Deforestation degrades more than just land habitats. When tropical forests are cut down to clear land for agriculture, pasture, or homes, topsoil washes down rivers into coastal ecosystems. Soil that settles on reefs smothers coral polyps and blocks out the sunlight needed for corals to live.

4. Coastal development and dredging ravages reefs. This development includes building seaside homes, hotels, and harbors.

5. Fishing with dynamite, cyanide, or bleach has killed coral reefs in the Indo-Pacific region. Between 1986 and 1991, half of the coral reefs in the Philippines have been demolished by these and other destructive fishing methods.

6. Besides fishes, fishermen harvest a variety of exotic seafood from the reef including conchs and lobsters. Overharvesting could lead to these species' demise. Careless handling of nets, lines, and lobster traps has led to some reef damage.

International seashell and aquarium trades have put a strain on coral reefs and reef inhabitants. Excessive collecting decimates reef species and upsets the balance of the reef ecosystems. a. The souvenir trade has created an international market for coral skeletons, shells, sponges, and other reef animals.b. Coral skeletons are also sold as "live rock" Live rock is popular in home saltwater aquariums because it is permeated with living bacteria and algae and acts a natural biological filter. The tropical fish trade has created a demand for reef fishes.

Careless water recreationist damage reefs. Divers and snorklers that stand on, sit on, or handle corals can injure the delicate polyps. Dropped boat anchors can gouge the reef and crush corals.
Divers should rest by floating or standing on the sandy bottom. They should be very careful not to grab on to any coral formations.)

A turtle chewing on a plastic bag which looks a lot like its favourite food-jellyfish.

NATURAL DISASTERS

a. A drop in sea level exposes corals.

b. A rise in sea level decreases the amount of available sunlight and may inhibit growth. Added emissions of carbon dioxide and other trace gases (called greenhouse gases) into our atmosphere may be causing a gradual warming of our planet. This warming could cause the polar ice caps to melt, thereby raising sea level.

c. Rises in sea level can also release nutrients trapped in soil.

d. Coral diseases can wipe out entire strands of coral reefs. Diseases may be connected to the sea level rise and nutrient level increase.

e. Coral bleaching occurs when coral expels its symbiotic zooxanthellae. As a result, the coral loses its coloration. Without zooxanthellae, the coral polyps have little energy available for growth or reproduction. Scientists aren't sure why bleaching occurs. Hypotheses include elevated water temperatures, ultraviolet radiation, and diseases or viruses affecting the zooxanthellae.

f. Major tropical storms can strip corals from miles of reef habitat.

That’s it about our beautiful coral reefs.

Here are videos on youtube perfect for introduction to coral reefs :