CORAL REEFS: CHARACTERISTICS, FORMATION, TYPES, FLORA, FAUNA - BIOLOGY - 2024

The coral reefs are elevations on the seabed formed by the biological action of organisms called coral polyps. These biological structures are found at shallow depths in tropical seas with temperatures between 20 and 30 ºC.

Coral polyps belong to the class Anthozoa (phylum Cnidaria) and have a simple anatomy. They have radial symmetry and a body with a cavity divided by partitions and made up of two layers of tissue.

Different types of corals. Source: I, Kzrulzuall

The body of the coral has a single opening to the outside or mouth that serves both to feed and to excrete. Around their mouths they have a series of stinging tentacles with which they capture their prey.

There are soft corals and hard corals, the latter are the ones that make up coral reefs. The hardness is given because they form a layer of calcite (crystallized calcium carbonate) on the body.

These polyps form extensive colonies combining sexual and asexual propagation and for their development they require salty, warm, clear and agitated waters. The development of these colonies creates a structure that establishes itself as a refuge against the currents and behaves as an attractor of life and nutrients.

Depending on the geological conditions and the ecological dynamics of the area, three basic types of coral reefs are formed. One is the littoral coral reef that forms along the shoreline.

The other types are the barrier coral reef located far from the coast and the atoll (island formed by a ring of coral reef and a central lagoon).

Various species of chlorophytic algae, macroalgae (brown, red and green) and coralline algae inhabit the reefs. The fauna has numerous species of corals, fish, invertebrates, reptiles (turtles) and even aquatic mammals such as the manatee.

Invertebrates include snails, octopus, squid, shrimp, starfish, sea urchins, and sea sponges.

The most important coral reefs in the world are the Coral Triangle of Southeast Asia and the Great Barrier Reef of Australia. Similarly, the Mesoamerican-Caribbean Coral Reef and the Red Sea Coral Reef.

Despite their importance to marine ecology and global biodiversity, coral reefs are threatened. Among the factors that endanger these ecosystems are global warming, pollution of the seas and the extraction of coral.

There are also biological threats such as excessive population growth of coral-eating species such as the crown-of-thorns starfish.

General characteristics

- Coral reef

A reef is any elevation on the seafloor that is 11 meters or less deep. It can be a sandbar or rocks, it can even be an artificial reef due to a sunken ship (wreck).

In the case of the coral reef, it is an elevation caused by a colony of organisms that produce a calcareous exoskeleton.

- Geographical distribution

Coral reefs develop in the tropical seas of the world and in America are the Gulf of Mexico, Florida and the Pacific coast from California to Colombia. They are also found on the Brazilian Atlantic coast and the Caribbean, including the continental and insular coast.

Littoral reef in Colombia. Source: luis barreto from MED, - COL

In Africa they extend along the tropical Atlantic coast while in Asia they are in the Red Sea, Indo-Malay archipelago, Australia, New Guinea, Micronesia, Fiji and Tonga.

Coral reefs are estimated to cover 284,300 to 920,000 km2, with 91% of this area being in the Indo-Pacific region. 44% of the coral reefs in the world are specifically between Indonesia, Australia and the Philippines.

- Taxonomy of corals

Class and phylum

Corals have been known since ancient times and their name comes from the classical Greek korallion which means "ornament of the sea".

Coral reefs are made up of millions of small organisms called polyps that belong to the class Anthozoa (phylum Cnidaria). This group is related to anemones and like these, they do not go through the state of jellyfish.

Groups

Corals have been classified into different informal groups depending on their structure. In this sense, there are so-called hard corals (hermatypic) that have a calcium carbonate skeleton. These are the corals that form the actual structure of the coral reef.

Then there are the so-called soft corals (ahermatypic), which do not form a hard skeleton and do not form reefs although they are part of the ecosystem.

Subclasses and Orders

Corals are grouped into two subclasses which are Octocorallia with polyps of 8 tentacles and Hexacorallia with tentacles in multiples of 6.

The octocorales include the orders Alcyonacea that groups soft corals (except for the genus Tubipora) and Helioporacea of ​​hard corals.

For their part, the hexacorals group 6 orders, where the Scleractinia order is the one that includes the so-called true corals or madrepores. These corals have a calcite skeleton and symbiosis with unicellular dinoflagellates (zooxanthellae).

- Morphology of coral polyps

Detail of encrusting pink coralline algae in the glass of aquarist Mike Giangrasso's coral reef aquarium. Source: FalsePerc

Polyps have radial symmetry and a body cavity divided into chambers by radial partitions, that is, they are like a sac (coelenterate). This sac called the gastrovascular cavity or enteron, includes a single opening to the outside (mouth).

The mouth serves both for the entry of food and for the expulsion of waste. Digestion takes place in the interior cavity or gastrovascular cavity.

Around the mouth there is a ring of tentacles with which they capture the prey and direct them to the mouth. These tentacles have stinging cells called nematoblasts or cnidoblasts.

Cnidoblasts

Cnidoblasts consist of a cavity filled with a stinging substance and a coiled filament. At its end it has a sensitive extension that, when excited by contact, triggers the wound filament.

The filament is impregnated with the stinging liquid and sticks into the tissue of the prey or the attacker.

Fabrics

The body of these animals is made up of two layers of cells; an external called ectoderm and an internal called endoderm. Between the two layers there is a gelatinous substance known as mesoglea.

- Breathing

Coral polyps do not have a specific respiratory organ and their cells take oxygen directly from the water.

- Association of polyps-zooxanthellae

Dinoflagellates (microscopic algae) inhabit the delicate translucent tissues of coral polyps. These algae are called zooxanthellae and maintain a symbiotic association with polyps.

This symbiosis is a mutualism (both organisms benefit from the relationship). Zooxanthellae supply polyps with carbonaceous and nitrogenous compounds, and polyps supply them with ammonia (nitrogen).

Although there are coral communities that do not have zooxanthellae, only those that present this association form coral reefs.

- Nutrition

Coral polyps, in addition to obtaining the nutrients supplied by zooxanthellae, hunt at night. For this they extend their tiny stinging tentacles and capture small marine animals.

These microscopic animals are part of the zooplankton that is carried away by ocean currents.

- Environmental conditions

Coral reefs require shallow, warm and rough seawater conditions.

Temperature

They do not develop in waters with temperatures below 20 ºC, but very high temperatures negatively affect them and their ideal temperature range is 20-30 ºC.

Some species can develop from 1 to 2,000 m deep in cold waters. As an example we have Madrepora oculata and Lophelia pertusa that are not associated with zooxanthellae and are white corals.

illumination

Corals cannot grow in deep areas, because zooxanthellae require sunlight to photosynthesize.

Balance in nutrient concentration

The waters where coral reefs develop are poor in nutrients. Thus, corals do not form in waters that receive periodic nutrient enrichment.

Therefore, the establishment of coral reefs requires a certain environmental stability.

- Darwin's paradox

Darwin was the first to draw attention to the paradox that the coral reef ecosystem represents. This consists in the contradiction of such a diverse ecosystem, developing in nutrient-poor waters.

Today this paradox is explained by the complex recycling of nutrients that occurs in the coral reef.

Here, intricate food webs are established between the various organisms that compose it. These networks allow the scarce nutrients to keep circulating in the ecosystem supporting the present biodiversity.

A life attractor

The key to the functioning of coral reefs is in the symbiotic association of polyps with zooxanthellae. These microscopic algae provide nutrients from sunlight through photosynthesis.

For this reason, the reef forms a platform that serves as a refuge and feeding site for many marine organisms. Among other things, the reef has a physical effect protecting from currents and attracting a greater concentration of nutrients.

In addition to the primary symbiotic association between coral and zooxanthellae, algae and cyanobacteria are present. These produce nutrients through photosynthesis and in the case of cyanobacteria fix environmental nitrogen.

Sponges also establish symbiotic relationships with photosynthetic organisms such as cyanobacteris, zooxanthellae, and diatoms. These organisms reproduce inside it, supplying it with nutrients and periodically the sponge expels quantities of them.

Other organisms such as fish come to feed on algae and corals and in turn others feed on these fish.

Coral reef-mangrove-seagrass relationship

This is another important relationship for the ecology of the coral reef, contributing to its high productivity.

The coastal mangroves provide nutrients to the waters that reach the reef and the reef protects the mangrove from the onslaught of the waves. This protection from the waves and currents also allows the development of underwater meadows of angiosperms.

In addition, many marine animals on the coral reef use the mangrove swamp and grasslands as breeding and feeding sites.

How are coral reefs formed?

- Colonial polyps

Although there are polyps that lead individual lives (actinias and anemones), there are others that form colonies. These colonies are created by the connection of the tissues of the individual polyps to each other, which in this case are called zooids.

All zooids are the same and perform the same functions. Colonies assume various shapes and can be hard or soft, due to the formation or not of a skeleton or polypere.

External skeleton

This polypero can be horny as in sea fans or calcareous as in corals. Corals secrete a matrix of organic molecules on which crystallized calcium carbonate (calcite) is deposited.

These plates of calcite are called sclerites generated by specialized cells and thus form the hard corals that form the base of the coral reef.

Shapes

The forms assumed by the colonies of each species of coral are very varied. Some are like antlers or branches, others like brains, church pipe organs, fans, and whips.

- Reproduction

Polyps produce eggs and sperm, and once fertilization has occurred, the eggs give rise to ciliated larvae or planules. The planulas, after a period of free life, settle to the bottom and form new polyps.

These polyps in turn will asexually produce (budding) other associated polyps and so on until they form a colony.

- Formation of the coral reef

In a solid substrate in the shallow benthic region, the planules that will give rise to polyps are deposited. These in turn will multiply generating larger and larger colonies.

If the conditions of light, temperature and agitation of the water are suitable, these colonies grow vertically and horizontally.

Old polyps die, but their calcareous skeletons remain and new colonies form on them. In this way, a biological reef is formed, which is called a coral reef.

Reef builders

Species of the genera Acropora and Montipora are pointed out as the main builders of coral reefs. Acropora is the genus with the largest number of species, reaching more than 130, while Montipora includes 85 species.

Acropora sarmentosa. Source: MDC Seamarc Maldives

Together they represent more than a third of the world's total coral reef building species.

In addition to their structural contribution to reef formation, they contribute to coral nutrition. This is because once they are dead, under the action of CO2, their skeleton becomes calcium bicarbonate assimilated by corals.

- Heterogeneity of the coral reef

There are marked differences between the slope of the coral reef that faces the coast and that facing the open sea swell. The face that faces the open sea grows faster and with greater profusion of coralline algae.

While the other side, being in calmer waters, allows the accumulation of sediments. Among these, the coral segments that give rise to the so-called coral sand that will become part of the beaches.

There are also variations in the horizontal and vertical dimension of the reef, determined by the dominant species. The latter in relation to the requirements of light and resistance to currents.

- Cold water “coral reefs”

Some species of the order Scleractinia, like the mentioned Madrepora oculata and Lophelia pertusa, form something similar to reefs at great depths. These are found in deep, cold waters of the North Sea, the Mediterranean, and the Gulf of Mexico.

Types

- Coastal reefs or strips

It is a coral reef that follows the line of the coast, extending it towards the sea as if it were a platform. For example, the reefs of the West Indies, Florida and the north coast of South America are of this type.

- Barrier reefs or coral reefs

In this case, the reef is far enough from the coast that a sea channel forms between it and the reef. Sometimes the channel is narrow, in others it becomes immense, as for example in the Great Australian Barrier.

- The coral islands or atolls

Atoll in the Pacific Ocean. Source: Atafu.jpg: NASA Johnson Space Centerderivative work: Talkstosocks

This third type of reef develops offshore in a ring shape, forming an island with a central lagoon. They are low islands with white sand beaches formed by coral remains, with the central lagoon abundant in marine life and many are distributed throughout the tropical zone of the Pacific Ocean.

- What determines one or another type of reef?

Darwinian hypothesis

Since Darwin's time, there are different hypotheses that try to explain these types of coral formations. Darwin considered that the basic mechanism that determined one type or another was geological.

It started from the fact that the basic type was the coastal reef, then if a subsidence of the coastal platform occurred, the reef was remote. In this case, a barrier-type coral reef would be established.

As for the atolls, Darwin explained them on the basis that a coastal reef formed around an island. Later if the island sank, the coral ring remained and an atoll was created.

Other hypothesis

Another approach to explain the formation of these types of coral reefs, part of the environmental requirements.

For example, if some factor muddies the water, the reef does not form or its rate of formation decreases and the basic form is considered to be the littoral reef.

In this way, if the water near the coast is cloudy due to contributions of terrestrial sediments or another cause, the coral forms further from the coast. In this case, a barrier-type coral reef will form.

For their part, the atolls are explained, according to this approach, as the result of the formation of a reef in a shallow area offshore. A coral mass is being formed that grows around the edges and attracts marine life.

The influx of life muddies the water in the center of the coral group complex and therefore the reef does not grow there or does so very slowly. As the reefs grow larger on the slope facing the open sea, they tend to form arches.

In this way a ring growth is generated, leaving a central lagoon full of marine life.

Flora

Making extensive use of the word flora, we must point out that various photosynthesizing organisms are found in the coral reef. Among these are cyanobacteria, diatoms, dinoflagellates such as zooxanthellae, and algae.

Coral reef algae

Various types of algae are found on coral reefs, from unicellular to multicellular. Chlorophyte algae (Chlorophyta), macroalgae (brown, red and green) and coralline algae (Corallinales) inhabit here.

Coralline algae

These algae have a hard body or thallus because they contain calcareous deposits on their cell walls, and they also have striking colors. They grow embedded on reefs and play an important ecological role as primary producers.

This is because they serve as food for various species of the coral reef, for example, urchins and parrotfish.

Fauna

Coral reefs are the most diverse ecosystems of all marine environments and are home to hundreds of species.

- Corals

One of the most appreciated corals is the red coral (Corallium rubrum), for its intense color. Other species that attract attention are the brain corals (family Mussidae), whose colony is shaped like this organ.

The most important groups in the construction of the coral reef are the genera Acropora and Montipora. There are corals that form foliaceous colonies (similar to large leaves), such as those of the Agariciidae family.

Other corals assume mushroom-like forms, such as species of the genus Podabacia.

- Fishes

Among the fish, the parrotfish (family Scaridae) stand out, which gnaws the corals, feeding on the algae and other organisms present. Other species present are the trumpet fish (Aulostomus strigosus) and the surgeonfish (Paracanthurus hepatus).

Surgeonfish (Paracanthurus hepatus) on the reef. Source: Tewy

Likewise, it is possible to find seahorses (Hippocampus spp.) And moray eels (family Muraenidae).

Various colors

Coral reefs are characterized by being inhabited by fish of various colors such as the clown fish (Amphiprion ocellaris). As well as the yellowtail (Ocyurus chrysurus), the emperor angelfish (Pomacanthus imperator) and the mandarin fish (Synchiropus splendidus).

Various shapes

Others have very peculiar forms, such as the ghost pipefish (Solenostomus spp.) Or the toadfish (family Antennariidae).

Sharks and rays

Due to the abundance of marine fauna, the coral reef is visited by various species of sharks and rays. For example, the white-fin reef shark (Triaenodon obesus), the Caribbean reef shark (Carcharhinus perezii), and the reef stingray (Taeniura lymma).

- Invertebrates

Invertebrates include mollusks, shrimp (Caridea), sea urchins (Echinoidea), starfish (Asteroidea), and sponges (Porifera).

Mollusks

The coral reef is home to octopus (Octopus vulgaris and other species), squid (Teuthida), giant clams (Tridacna gigas) and numerous species of snails. Among the latter, the predatory snail Conus geographus kills its prey by injecting it with insulin and can be fatal to humans.

Shrimp on the reef

Some species of shrimp have peculiar functions on the reef, for example the scarlet cleaner shrimp (Lysmata debelius). This small animal feeds on parasites and dead tissue, which is why reef fish come to it for cleaning.

Starfish and ophiuros

There are about 1,900 species of starfish, but not all of them inhabit coral reefs. Among the species found in this ecosystem, most feed on small mollusks, crustaceans and organic remains.

However, the crown-of-thorns species (Acanthaster planci), feeds on polyps of hard corals.

In equilibrium conditions the crown of thorns does not cause significant alterations, it is simply part of the trophic web. But when population explosions of this star occur, corals are threatened.

On the other hand, the so-called sea spiders (Ophiuroidea) are also found on the reefs. Although they look like starfish, they belong to another class of echinoderms, have flexible arms, and some are fast-moving.

The basket starfish (Gorgonocephalus spp.) Is a snake that has highly branched arms and is mistaken for a coral.

Sponges

These organisms play an important role in the coral reef due to their filtering capacity. Sponges filter seawater for food, thereby reducing the turbidity of the water.

They have the ability to retain pollutants very efficiently without being affected and excrete ammonia and mucilage. These excreted substances serve as food for other organisms on the reef.

- Turtles

Stupid turtle. Source: Mike Gonzalez (TheCoffee)

There are 8 known species of sea turtles and they all visit coral reefs. The largest is the leatherback turtle (Dermochelys coriacea) followed by the green turtle (Chelonia mydas).

There is also the hawksbill sea turtle (Eretmochelys imbricata), the olive sea turtle (Lepidochelys olivacea), the bastard turtle (Lepidochelys kempii) and the loggerhead turtle (Caretta caretta). In addition, the flat turtle (Natator depressus) is found as an endemic species in Australian and Malay coral reefs.

- Sea cow

sea ​​cow

The Caribbean manatee species (Trichechus manatus) inhabits the Mesoamerican coral reef.

Main reefs in the world

The Coral Triangle

Map of the Coral Triangle. Source: Benutzer: Devil_m25

It is located in Southeast Asia and has the greatest diversity of corals on the planet (500 species) and more than 2,000 species of fish. It spans the island complex of Indonesia, the Philippines, Papua New Guinea, the Solomon Islands, and East Timor.

A number of national parks have been established to protect its rich marine biodiversity.

The Great Barrier Reef of Australia

Aerial view of the Great Barrier Reef (Australia). Source: NASA, by MISR

It is a coral reef more than 2,000 km long and 150 km wide, being the largest in the world. Although this immense coral reef only occupies 0.1% of the oceanic surface, it is home to 8% of the world's fish species.

The Mesoamerican-Caribbean coral reef

It is the second largest coral reef in the world, extending for 1,000 km. It covers from the coast of Mexico to Honduras in the Caribbean Sea.

This coral reef is home to about 65 species of coral, 350 species of mollusks and 500 species of fish.

Caribbean

This continuous Mesoamerican extension is integrated with the entire coral reef system of the Caribbean Sea. This encompasses the rest of the Central American coast, the Colombian Caribbean coast and some areas of the Venezuelan coast and island.

Coral reefs are also found in the Greater Antilles and the Lesser Antilles.

Coral reefs of the Red Sea

These coral reefs, apart from having a high biological diversity, seem to resist conditions of high temperatures and acidification.

Specifically, investigations have been carried out with the species Stylophora pistillata. The specimens were taken from the Gulf of Aqaba in the northern part of the Red Sea, between Egypt and Saudi Arabia.

Threats

Global warming

The increase in the average global temperature due to the greenhouse effect increases the water temperature, especially in tropical areas. This mainly affects zooxanthellae and as their populations are reduced, the so-called bleaching of the coral occurs and its subsequent death.

The causes of global warming are fundamentally the greenhouse gases that human activity emits into the atmosphere.

Nutrient excess

An excessive supply of nutrients, especially nitrogen and phosphorus, affects the survival of the coral reef. The abundance of nutrients favors the growth of macroscopic algae that shade the coral and kill it.

The excess of nutrients can occur due to the contribution coming from the coast through runoff waters or rivers. These water flows carry waste from agricultural products such as fertilizers and others.

Coral extraction

Some species of coral are in demand commercially, because due to their calcareous layer they can be polished and used in jewelry. Due to this, they are intensively extracted for sale, irreparably affecting the coral reef.

Biological threat

The crown-of-thorns starfish (Acanthaster planci) can be a threat to the coral reef when its populations increase. This species destroys the hard coral and the effect on the reef becomes significant, like the Great Barrier Reef in Australia.

Possible cause

Possibly the cause of the population explosions of this starfish is the decrease of its natural predator, the barrel snail (Tonna galea). This is probably due to the fact that it is a rare species and highly sought after by collectors.

References

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3. Lesser, MP (2004). Experimental biology of coral reef ecosystems. Journal of Experimental Marine Biology and Ecology.
4. Purves, WK, Sadava, D., Orians, GH and Heller, HC (2001). Life. The science of biology.
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