Coral ReefsTHE MARINE SCOOP GUIDE TO…

Contents

Introduction

What is a coral reef?

Polyps and zooxanthellae

Reefs at risk

Coral biology

Summary of reef formations

New oceanic islands

Fringing reefs

Barrier reefs

Atolls and seamounts

Reef growth

Coral reproduction

Examples of coral species

Coral reefs, the “rain forests of the sea,” are

among the most biologically rich and productive

ecosystems on earth; some 4,000 species of fish

are found here (approximately one-quarter of all

marine fish species), along with a vast array of

other life forms—molluscs, crustaceans, sea

urchins, starfish, sponges, tube-worms and many

more. There are perhaps 1 million species found

in a habitat that covers a total of about 250,000

sq km (roughly the area of the United Kingdom).

They also provide valuable ecosystem benefits

to millions of coastal people. They are important

sources of food and income, serve as nurseries

for commercial fish species, attract divers and

snorkelers from around the world, generate the

sand on tourist beaches, and protect shorelines

from the ravages of storms. Latest estimates

suggest coral reefs provide close to US$30 billion each year in goods and services.

"Coral reefs are the

most biodiverse of all

known marine

ecosystems, and

maintain much higher

genetic diversity than

tropical rainforests.”

John McManus, The International Coral

Reef Initiative

Introduction

Coral reefs are physical structures built by the actions of many tiny coral

animals that live in large colonies and lay down communal limestone

skeletons. Over millennia, the combined mass of skeletons build up into huge

reefs, some of which are visible from space. There are some 800 species of

reef-building or hermatypic corals and they have exacting requirements,

needing bright, clear, and warm waters.

What is a coral reef?

“Coral reefs are diverse underwater ecosystems

held together by calcium carbonate structures

secreted by corals.”

The individual coral animals, known as

polyps, have a tubular body and central

mouth ringed by stinging tentacles, which

can capture food. Living within their body

tissues are microscopic single-celled

dinoflagellate algae (zooxanthellae) that need sunlight to survive. These algae

convert sunlight into sugars, which

produces energy to help sustain their coral

hosts. These same algae also provide the

corals with their vibrant colours.

Polyps and zooxanthellae

Kingdom: Animalia Phylum: Cnidaria Class: Anthozoa Order: Scleractinia

Coral reefs face a wide and intensifying array of threats—including impacts from overfishing, coastal

development, agricultural runoff, and shipping. In addition, the global threat of climate change has

begun to compound these more local threats to coral reefs in multiple ways. Warming seas have

already caused widespread damage to reefs, with high temperatures driving a stress response called

coral bleaching, where corals lose their colourful symbiotic algae, exposing their white skeletons. This

is projected to intensify in coming decades. In addition, increasing carbon dioxide (CO2) emissions

are slowly causing the world’s oceans to become more acidic. Ocean acidification reduces coral growth rates and, if unchecked, could reduce their ability to maintain their physical structure.

Reefs at risk “Over 60% of the world’s coral reefs are under immediate threat”Reefs at risk revisited 2011

Coral biology

If you have ever seen branching corals spreading their arms out like tree limbs, you can see why early

scientists thought corals were plants. But these tiny, soft-bodied creatures are carnivores, despite being

sessile, or fixed to one spot. Like their relatives, the jellyfish and the sea anemone in the Cnidaria

phylum, each individual coral, or polyp, has barbed, stinging cells called nematocysts it can extend to

capture prey like zooplankton or small fish.

Although those early scientists got it wrong, it's easy to understand their mistake. Coral almost could be

considered half-plant because of the zooxanthellae (pronounced zoo-zan-thelly) algae that live just

inside each polyp's cell walls. The zooxanthellae supplies the polyp with the by-products of

photosynthesis, which the polyp turns into proteins, fats and carbohydrates. In turn, the polyp shelters

the zooxanthellae and provides the carbon, nitrates and phosphates the algae need for photosynthesis.

Up to 90 percent of the energy produced by zooxanthellae's photosynthesis is transferred to the coral

host. This mutually beneficial arrangement is called symbiosis.

Coral polyps also use the energy supplied by their symbiotic algae to produce calcium carbonate, or

limestone. They secrete the limestone from their base, creating a protective skeleton and a hollow

chamber called a cup. The polyps retreat into their cup to hide when predators come looking for them.

Polyps rarely exist alone. They usually join other polyps to form a larger colony that acts as a single

organism. While the individual minuscule polyps grow to 1-3 mm on average, colonies can weigh tons.

Even a single branching coral comprises thousands of individual polyps. Over hundreds or thousands of

years, these colonies may connect to form a reef.

Summary of reef formations

Depending on their structural characteristics, coral reefs are generally described using three major reef

types:

1) Fringing reefs, the most common type of the reef forms, extend seaward from the shore, sometimes

separated by a narrow stretch of water or lagoon. They are generally found in shallow waters with the

reef flats becoming exposed during low water.

2) Barrier reefs, like the Great Barrier Reef in Australia, border the shoreline but are separated from the

land by a large lagoon that may be of considerable depth. Barrier reefs are frequently formed by a

network of individual reef systems, often encompassing fringing reefs extending from offshore islands.

3) Atolls are circular or horseshoe shaped coral islands that surround a central lagoon. Darwin

suggested that atolls were created after periods of transition caused by the subsidence of oceanic

volcanoes. This notion of transition from fringing reefs to an atoll remains widely supported by scientists

today.

New oceanic islandsClassic coral reef formations change through time. They begin with a brand new tropical island

(produced by an oceanic hot spot or at a plate boundary) and gradually change through thousands of

years from a fringing reef, to a barrier reef, to an atoll, and finally to an extinct reef as a seamount or

guyot.

A new oceanic island in the tropics will have no terrestrial or marine life when it is first formed. This new

island will be made entirely of crustal material (rock) with little or no evidence of life forms. If there is no

more volcanic activity and the island cools there will be a succession of life forms that may arrive by

flying, floating or swimming. These forms may stay and establish themselves if conditions there are

good for them. Over time a thriving community will be found on land as well as in the ocean.

Fringing reefsA fringing reef forms along the shoreline of most new tropical

islands. This is because the reef building coral animal is one

of the few marine organisms that can survive in the warm,

nutrient-poor surface tropical waters. In fact, the reef

building coral (hermatypic) can only survive in warm, clear

ocean waters (such as would be common around a new

oceanic island). The coral animal reproduces sexually with a

microscopic planktonic larval form that is abundant in

tropical waters during coral reproduction. Most of these

larvae never grow to adults because there is no place for

them to exist in the warm surface ocean waters on a solid

surface. They need to attach to a solid surface that will stay

in the warm, sunlit, clear tropical water and most places are

already taken … but not on a new island. So a coral reef

forms along the edge of this new island, right next to the

island and is called a fringing reef - the first step in 'classical'

coral reef formation.

Barrier reefsA barrier reef forms as the oceanic island begins to sink into

Earth's crust due to the absence of volcanic island building

forces, the added weight of the coral reef, and erosion at the

surface of the island. As the island sinks, the coral reef

continues to grow upward. The coral animals lay down

tremendous amounts of calcium carbonate (in part due to

their symbiotic zooxanthellae) in the form of their skeletons

(called corallites). If conditions are good they can usually

keep pace with the sinking island. Their living tissue remains in

the upper part of the reef in the warm, clear, tropical water.

The lower part of the reef is composed of the calcium

carbonate skeletons left by the reef building coral. This

upward growth creates a lagoon of water between the top of

the reef (that started as a fringing reef) and the sinking crustal

island. The lagoon fills in with eroded material from both the

reef and the island and is a haven for marine forms that

require protection from waves and storms. This barrier reef is

generally not a complete circle as it may be broken, here and

there, by storms.

Atolls and seamountsAn atoll forms when the oceanic island sinks below the

surface of the ocean but the coral reef continues to grow

upward. The atoll is usually circular in shape but a broken

circle (due to the history of storms). Eroded reef material may

pile up on parts of the reef, creating an area above sea level,

an island called a 'cay.' Cays may become stabilized enough

(often through plants) to provide a permanent island for

buildings. Cays may also be unstable and move across the

reef or disappear.

A seamount or guyot forms when the coral reef cannot keep

up with the sinking of the island. Seamounts and guyots are

below the surface of the ocean and may be home to a large

number of species depending on their location and depth.

Seamounts are rounded on the top and guyots are flat-

topped. The flat top of the guyot is from its existence at the

surface for some time and being planed down by waves (thus

the flat top).

Reef growth

Reefs grow in one of two ways. One way is to periodically add on to their limestone base. They simply

secrete more calcium carbonate under and around their current cup, creating the framework of the

reef and causing it to grow both upwards and outwards. They also grow by reproducing. Corals may

reproduce either asexually, by dividing and producing identical clones, or sexually, by sending out

eggs or sperm.

Either way, new coral polyps settle towards the ocean bottom until they find a hard substrate to call

home, either combining with a pre-existing coral colony or starting one of their own. In addition to

being connected at their bases, coral polyps link to one another laterally by a thin tissue called the

coenosarc. The coenosarcs and the polyps form the visible living part of the reef, while the limestone

base forms the non-living part.

Coral reproductionCorals can reproduce asexually and sexually. In asexual reproduction, new clonal polyps bud off from

parent polyps to expand or begin new colonies. This occurs when the parent polyp reaches a certain

size and divides. This process continues throughout the animal’s life.

About three-quarters of all stony corals produce male and/or female gametes. Most of these species

are broadcast spawners, releasing massive numbers of eggs and sperm into the water to distribute their

offspring over a broad geographic area. The eggs and sperm join to form free-floating, or planktonic,

larvae called planulae. Large numbers of planulae are produced to compensate for the many hazards,

such as predators, that they encounter as they are carried by water currents. The time between

planulae formation and settlement is a period of exceptionally high mortality among corals

Along many reefs, spawning occurs as a mass synchronized event, when all the coral species in an

area release their eggs and sperm at about the same time. The timing of a broadcast spawning event is

very important because males and female corals cannot move into reproductive contact with each

other. Because colonies may be separated by wide distances, this release must be both precisely and

broadly timed, and usually occurs in response to multiple environmental cues.

Coral reproduction continuedThe long-term control of spawning may be related to temperature, day length and/or rate of

temperature change (either increasing or decreasing). The short-term (getting ready to spawn) control

is usually based on lunar cues. The final release, or spawn, is usually based on the time of sunset.

Planulae swim upward toward the light (exhibiting positive photo taxis), entering the surface waters and

being transported by the current. After floating at the surface, the planulae swim back down to the

bottom, where, if conditions are favourable, they will settle. Once the planulae settle, they

metamorphose into polyps and form colonies that increase in size. In most species, the larvae settle

within two days, although some will swim for up to three weeks, and in one known instance, two

months.

Examples of coral species