General

Little is known about the coral reefs that can be found in Scandinavian waters. They normally live at great depths but can at some places be found as shallow as 40 meters (the Trondheim fiord). Although several types of coral exists in cold waters, only three are able to build reefs: Lophelia pertusa, Madrepora oculata and Solenosmila variabilis where the most common is Lophelia pertusa.

History and overview

Scandinavian coral reefs are mentioned as early as 1752 but discovery was slow until the oil-age came along with its subsequent mapping of the sea floor of the North Sea which resulted in the discovery of several hundreds of reefs, although most of them are far from land at between 200-400 m depth. The structures can be very impressive with reef areas covering between 1300 and 12000 m2. The largest reefs are up to 140 m long, 120 m wide and 25 m high. Most reef explorations have been carried out with ROV’s (remote operated vehicles). 

Lophelia pertusa and sponges at 200 m, Nordleksa, West Norway

The diversity of life is impressive at Lophelia reefs and can in fact be compared to the diversity of tropical coral reefs.

Biology

There are several differences between tropical and cold water corals. The cold-water corals do not have zooxanthellae in their tissues which means that they need to rely upon their tentacles in order to feed themselves. This is not surprising, considering the depths where most reefs are located, which means that no light is present for zooxanthellae to utilise. Reproduction is similar to tropical corals. The larvae are free swimming, but it is unclear how far they can swim. The corals grow very slowly, only about 6 mm per year, which means that it takes thousands of years to build the massive reef structures.

Threats

The major threat to these reefs is deep-sea trawlers. Previously fishermen tried to avoid the reefs in order to save the trawls, which were damaged by hitting the structures. This, however, is not the case with modern trawlers and the impact can be severe.  The reefs in Sweden are badly damaged but agreements have been made to reduce the risk for further damage.
 

The arrow shows a trench from towed fishing gear. Smashed coral is littered around.

Tropical storms are the perhaps largest source of massive reef mortality. Hurricanes may mechanically destroy large areas of a reef, which is a major problem since most reefs are located in areas frequently traversed by typhoons or hurricanes.

Crown of thorns – Acanthaster planci

The sea star Acanthaster planci can, when the population increases, be another major source of mass morality. The sea star managed to devastate 90% of the coral reefs along 38 km of the Guam shoreline in 2,5 years. Little is known about the reason for the outbreak of these population explosions. Suggestions are that humans have removed an important predator of severe tropical storms opening space for juvenile settlement or adult aggregation.

These outbreaks are not new however. Acanthaster skeletal remains in sediment and local folklore of certain islands indicates earlier outbreaks as well, but it is impossible to tell whether they have increased or not. It is estimated that the recovery time for a reef devastated by Acanthaster is 7-40 years.

Crown of thorns on coral reef

El Niño

El Niño is a natural, recurring natural phenomenon manifested as a massive influx of warm and nutrient-poor water in usually cold and nutrient-rich waters. This is the effect of many variables, among them atmospheric pressure, winds, rainfall patterns, ocean currents etc. Thus, the effect varies greatly between occurrences. The 1982-83 El Niño has been called the worst during the 20th century and resulted in ocean temperatures 2-4C above normal for several months, which caused massive coral mortality.

Coral bleaching

Coral bleaching is the name for the phenomenon when corals expel the zooxanthellae from their tissues. The expulsion results in the corals getting white, which explains the name of the phenomenon. If the zooxanthellae are absent for longer periods, the coral will die. It is believed that stress induced by water temperatures higher than 30C is the main cause.

Coral bleaching

Human activities

Human activities affect coral reef to a large extent, mainly negative. Common such activities are dredging, pollution and over fishing. Over fishing may cause outbreaks of Acanthaster, which may devastate entire reefs. Dynamite and poison fishing in the Philippines is another major source of massive coral destruction. 

Dynamite fishing

The final section is about the Lophelia reefs in Scandinavia.

The number of fish species that can be found on coral reefs exceeds all other environments on earth and most of them are also brilliantly coloured. Reef areas in the Philippines have the greatest number of species, over 2000. As for coral, the numbers of species in Atlantic reefs are significantly lower, perhaps 500.

There are several explanations to why there is such diversity among reef fishes. The reef provides numerous habitats such as areas of sand, caves and crevices but this alone cannot explain this explosion of species.

Reef fish at Richelieu Rock

Generally speaking, there are four models and although none of them can be said to be the ultimate explanation, they do provide a solution that may be part of the explanation. More research must be made in order to fully understand the enormous diversity

The competition model (Smith and Tyler, 1972) is the perhaps most classical. This model suggests that all species are highly specialized and all species have a set of adaptations that give it a competitive advantage in at least one situation on the reef.

The lottery model (Sale, 1977, 1980) is based on the large number of larvae produced by most coral reef fishes. These larvae are dispersed as plankton. Also, this theory states that fish are not specialized which means that there is active competition among the species. If one species is successful, it is the result of chance more than anything else.

The predation-disturbance model (Tabot, Russel, Anderson, 1978) suggests that the fish populations do not reach equilibrium. Unpredictable events and predation ensure that populations never become large enough to undergo competitive exclusion, which could explain the high diversity.

The recruitment limitation model (Victor, 1983 and Doherty 1982) claims that larval supply is never sufficient for the adult population to reach carrying capacity. The adult population reflects variation in the larval recruitment.

The next section is about threats to coral reefs

This is part 4 of the essay about coral reefs.

Competition

The coral reefs are generally no open space at all. Coral or algae cover every single area. This results, of course, in fierce competition. Generally speaking there are two types of competition, exploitative competition and interference competition.

Exploitative competition is in general for fast growing corals such as branching corals. By growing faster and higher than the competitors, they are shutting off the light from underlying individuals which in the long run will kill them, freeing up space for the exploitative competitor.

Slow-growing massive and encrusting forms have developed another form of competition, interference competition. When the branching coral is closing in, the digestive filaments from the gastrovascular cavities are extended, killing the competing coral species. There are also other methods used, for example water-borne toxic chemical.

Competition is very complex and it is difficult to predict the winner in a coral battle. It is also reduced by the presence of grazing fish and invertebrates.

Predation

Crown of Thorns - Acanthaster Planci

There are several groups of predators on a coral reef. The first group, which consists of invertebrates such as nudibranches, polychaete crabs and molluscs, seems only to be able to cause minor effect on coral colonies. The two other groups however, the crown-of-thorns starfish Acanthaster planci and various fishes can severely modify the reef structure.

Acanthaster is only abundant in the Indo-Pacific where it feeds solely on living coral. When the starfish population is on a normal level, this is no major threat but large populations can devastate entire reefs. The corals can protect themselves by allying themselves with particular shrimps and in some cases by using their own nematocysts.

The Acanthaster itself is predated by the puffer and triggerfish, the shrimp Hymenocera and the polychaete Pherecardia.

The third group of fish can be divided in fish that consume the coral by biting off and ingesting portions of the coral skeleton (puffers (Tetradontidae), file fish (Monacanthidae), triggerfish (Balistidae) and butterfly fish (Chaetodontidae)) and a group of mulitvores such as surgeonfish (Acanthuridae) and parrotfish (Scaridae) that remove the coral polyps. These fish can, under certain conditions, largely modify the reef structure.

The next section is about the ecology of reef fishes.

This is part 3 of the essay about coral reefs.

Nutrition

Coral reefs require lots of nourishment, which means that the surrounding ocean is notoriously poor in plankton. Some corals use their ciliary mucous mechanism for capturing plankton but the most common is to snare them directly with the tentacles.

Plankton can, however, not support an entire reef. It has been estimated that only 5-10% of the total food requirement comes from plankton and the obvious question is: where does the rest come from?  There is in fact only one possible solution, the zooxanthellae in the coral tissues.

Corals in Thailand

Experiments have shown that coral can be kept in the dark for weeks, surviving by capturing zooplankton. It has also been shown that corals kept in light, but not being fed, still gained weight. This is considered to be an evidence for the importance of zooxanthellae and that corals are actually phototrophic (Porter (1976), Wellington (1982b). Species with large tentacles are supposed to be less dependant on zooxanthellae.

Sexual maturity and reproduction

Corals can reproduce both sexually and asexually.  As has been mentioned, asexual reproduction is generally accomplished by a new individual budding off the parent. This is generally used for expanding an existing colony. It appears that corals reach sexual maturity at seven to ten years of age. They are also primarily hermaphroditic.

Sexual production is mainly performed by broadcast spawning, releasing eggs and sperm into the seawater.  Some, however, retain fertilized eggs in the gastrovascular cavity until the planula larva is developed. Spawning is regulated by the lunar cycle and this can be quite spectacular in some regions where this event is performed synchronous. This is particularly true for the Great Barrier Reef where it occurs in the late spring to early summer.

The planulae are believed to swim in the open water for period of a few days up to a week or more. Since the adult corals are fixed in place, the planula is the only way for the coral to significantly disperse geographically.

The next section is about the ecology of tropical coral reefs.

Tropical coral reefs occupy about 0,17% of the total area of the planet. It has been estimated that 4-5% (91000) of all living species are found on coral reefs (Karlson, 1999). About half the calcium that enters the world’s oceans every year is taken up and bound to coral reefs as calcium carbonate. This process also requires a molecule of carbon dioxide, which means that the reefs remove some 700 billion kilograms of carbon dioxide every year. Thus, it is evident that coral reefs are very important for the health of both the marine environment and the planet as a whole.

Coral reefs are unique since they are the only marine structures built up entirely by biological activity. Essentially, the reefs are massive deposits of calcium carbonate, produced by corals with lesser additions from calcareous algae and other organisms.

Corals are divided in two groups, hermatypic and ahermatypic. Hermatypic corals are those who produce reef in contrast to ahermatypic corals. The distinguishing feature between the two is that hermatypic corals have in their tissues small symbiotic plant cells, zooxanthellae.

Reef distribution

Obviously tropical coral reefs are not distributed worldwide. Also, reefs in different locations are different. This is largely controlled by six physical factors that limit coral reef development:

1. Temperature
2. Depth
3. Light
4. Salinity
5. Sedimentation
6. Emergence into air

Temperature: Tropical coral reefs in general do not develop where the annual mean minimum temperature is below 18C. Optimal conditions are in waters where the annual mean minimum temperatures are 23-25C. Some reefs can tolerate temperatures up to 36-40C but this however, is rare. This explains the absence of coral reefs in may tropical areas such as the west coast of South and Central America and the west coast of Africa where strong upwelling of cold water in combination with north-flowing cold currents keep the temperature down.

Depth and light: Most tropical coral reefs grow in less than 25 meters. Generally speaking, reefs do not develop deeper than 50-70 meters. The limiting factor for depths is in fact the hermatypic corals need for light. Deeper water means less light. Absence of light reduced the photosynthetic rate and the ability to secrete calcium carbonate and form reefs is lost.

Salinity: Hermatypic corals are intolerant of salinities below 32-35 psu, which explains the absence of coral reefs close to major river outflows, such as the Amazon River. On the other hand, coral reefs can be found in the Persian Gulf where salinity is as high as 42 psu.

Sedimentation: Although hermatypic corals can remove limited amounts of sediment by ciliary action, heavy sedimentation means that their feeding structure are clogged which leads to coral mortality. Reduced visibility in the water caused by sedimentation also reduces the light intensity.

Emergence into air: Coral reefs are limited in an upward direction by a low tolerance to air exposure. Hermatypic corals may survive up to a few hours but longer periods will cause considerable mortality.
 

Global distribution of coral reefs

Structure of corals

Corals are members of the phylum Cnidaria and are thus related to jellyfish, hydroids and sea anemones. Also, corals as well as sea anemones belong to the same taxonomic class, Anthozoa.

Corals may be colonial or solitary but most hermatypic corals are colonial. Various individuals or polyps are occupying corallites on the massive skeleton. Each corallite has a number of bladelike sclerosepta rising from the base. Each polyp is a three-layered animal. Around the mouth is a series of tentacles that have stinging capsules that the animals use to capture their zooplankton food. The symbiotic zooxanthellae (a form of dinoflagellates) are found intracellularly.

Coral colonies grow by having the polyps bud off new polyps asexually in contrast to new colonies who are established through the settlement of a planktonic planula larva, a result of sexual reproduction.

Reef composition

Tropical coral reefs are not solely made up of corals but also from several other organisms. Members of practically all phyla and classes are represented all playing there role in the complex system. The role of many of these members is still unclear but it is understood that some groups are of more importance than other.

Coralline algae are one extremely important group in constructing and maintaining reefs. These red algae precipitate calcium carbonate and these algae are encrusting and spreading out in thin layers over the reef, welding together the pieces of the reef, protecting it from physical destruction.

Other common groups are various molluscs and among the most important are the giant clams, (Tridacna, Hippipus). Echinoderms (sea urchins, sea cucumbers, starfish and feather stars) are another common group but their role (except for the sea star Acanthaster) is incompletely understood, as is the role of various crustaceans and polychaete. Sponges are common but again, their role is unknown.

Finally, the most obvious group present is of course various fish species. The number of fish species can be numbered in hundreds or in some cases even thousands, making coral reefs the richest environment for fish on earth.

Atlantic vs. Indo-Pacific reefs

There are differences between the Atlantic and the Indo-Pacific reefs. These differences are not only in zoonation and structure but also in the flora and fauna. The number of coral species is much higher in the Indo-Pacific and this is not limited to corals. It is also true for most of the reef components including molluscs, fish and crustaceans. There are for example about 5000 species of molluscs in Indo-Pacific reefs opposed to only 1200 on Atlantic. The Atlantic reefs have a large number of gorgonians (sea fans and whips) whereas these are less common in Indo-Pacific reefs.

Indo-Pacific reefs hold some 2000 species of reef fish compared to 600 Atlantic species. Soft coral is much more common in the Indo-Pacific as well as giant clams which simply are absent in Atlantic reefs. The coral-eating starfish Acanthaster planci is only present in Indo-Pacific reefs, which is also true for crabs and shrimps guarding certain corals against predators.

The main reason for this difference is believed to be that the Atlantic reefs are much younger than the Indo-Pacific reefs. There are for example no atolls in the Atlantic region. Two explanations are at hand. The Atlantic is geologically more recent ocean, which means that there has not been time for the reefs to age. Second, it is believed that the last ice age made the Atlantic too cold for coral reefs, which means that the reefs that existed died and was replaced only when the seas were warm enough again.  Glynn (1973) states that Atlantic reefs do develop structural frameworks similar to those in the Indo-Pacific which can be an indication for the relevance for this theory.

The next section describes the biology of hermatypic corals.

Overview

Coral reefs demonstrate one of the most attractive marine environments for humans visiting sea while scuba diving. The diversity of life is truly impressive and the colours are dazzling. Huge schools of fish in different shapes and sizes are easily spotted and the experience of sharing environment with sharks is rarely beaten. However, most visitors never see beyond this explosion of impressions.

Dendronepthya

The general knowledge of what is actually seen is low and this essay is an effort to present an overview of the ecology of coral reefs, which will make the experience of scuba diving in such environments an even more rewarding activity. Hopefully, this will also increase the awareness of the urgent need for reef protection. Dynamite and cyanide fishing as well as tropical storms and human presence in the form of divers has severely damaged many areas. A deeper knowledge about the reefs importance can perhaps make some positive change.

This essay also includes a short part about coral reefs in Scandinavian waters. Little is known about the lophelia reefs, particularly among non-scientists. There are several reasons for this; the reefs are to a large extent out of range for recreational scuba divers, the deep sea floors outside Scandinavia has been unexplored until recently when the search for oil in the North Sea resulted in the discovery of many “new” lophelia reefs. The inclusion of these reefs in this essay gives an interesting perspective to the term “coral reefs”. Coral reefs exist not only in warm shallow tropical locations but also in deep, cold northern waters! There are several differences but they share the perhaps evident characteristics– the diversity of life.

The next section focuses on giving an overview of coral reefs characteristics. 

Oceanographic overview

The Mediterranean Basin was formed some 20 million years ago and there were several seas within it that evaporated occasionaly leaving behind gigantic salt marshes. At this time the western opening to the Atlantic were completely closed. An estimated 5 million years ago, a massive earthquake opened up the strait of Gibraltar and the Atlantic started to pour in and it took over a century to fill the basin to form the Mediterranean Sea. This connection is still today the only significant influx of water into the Med. A current of approx 4 km/h runs some 75 m below the surface, bringing in nutrients from the Atlantic. As the sun heats the water evaporation starts and the water becomes saltier and heavier. This water then flows out through the Strait of Gibraltar again – a process that supposedly takes about 80 years.

The Mediterranean

This means that the sea has a relatively low level of nutrients and it is also the reason behind the excellent visibility. Lots of plankton means low visilbility and although this is rare there are areas with a more dense plankton population. This is usually due to river flows or growth due to a reaised level of fertilizer in the water. The Med itself consists of several seas – the Alboran Sea, The Gulf of Lyons, The Ligurian Sea, The Adriatic, The Aegean, The Gulf of Trieste, The Ionian Sea, The Sea of Marmara and the Levant Sea. The deepest parties the Ionian Sea with a maximum depth of 5121 m. The average depth of the Med is as deep as 1500 meters! Thus, there is only a small area of the sea that has actually been explored by divers.

Biological overview

The water temperature never goes below 10 degrees and although this is not very cold it is still too low for coral such as cup corals and red coral. Other colourful species include various types of sponges and bryozoans. The diversity of the Med is lower than that of the neighbouring Atlantic and the Red Sea. This is due to the low plankton levels and the fact that the sea is relatively young. The older Med, now long since dried out, has traces of coral reefs but they disappeared with the water. Divers with keen eyes will see a lot of small marine life such as nudibranchs and flatworms – some very colourful. Those who wish to see bigger species have to depend more on luck. Large pelagic species do exist but are rarer than in other seas. The chance is always there however and this year both eagle rays, sunfish and dolphins have been spotted by divers.

Flying Gurnards

Seeing a shark diving in the Med is very unusal. Some say that we should be thankful for that given the size of the sharks in the Med. Although rare, there are recent recordings of Hammerheads off Wied-iz Zurrieq. Other shark species include the Blue Shark, The Great White, Mako and Thresher shark. All these have been caught and/or observer off Malta relatively recently.

Environmental issues

One cannot discuss the Mediterranean without mentioning the enormous historical relevance of the area. Here is the birthplace of the Greek, Roman and Arab cultures as well as three world religions – Judaism, Christianity and Islam. Needless to say, this extensive human activity has had a negative impact on the fragile environment. Many of the countries have highly developed fishing industries and the heavy traffic causes other types of pollution.

This has been recognised in several countries and Marine Reserves have been created with excellent results. More are needed however but the political challenge of managing a common resource shared by so many countries is enormous. There are talks about creating a marine reefs to thrive. There are other types of corals however reserve also in Malta but nothing is yet decided.

There opening of the Suez Channel has had a major impact with species migrating from the Red Sea into the Med. On average one species per year has established itself since then. Most of them are in the Eastern Mediterranean but some are moving west – some unusal ones have also been spotted in Malta. It seems that it is considerably easier for the Red Sea species to colonize the Med than the other way around. One reason for this is that the species in the Red Sea are much more specialised given the very tough competition faced by inhabitants of coral seas. In Malta we have, for example, spotted a trumpetfish in Cirkewwa during a nightdive recently.

The Mediterranean environment is in many aspects unique. Divers visiting get spoilt by the clear waters and there are excellent photo opportunities. It is an environment you cannot experience anywhere else on Earth, created by very special and unique circumstances. Next time you dive in the Med – take your time and look for the details that are unique and you be rewarded with great diving.