What do you think?
Rate this book
Very Short Introductions #391
Coral Reefs: A Very Short Introduction
Coral reefs have been long regarded with awe by the millions of people who have encountered them over the centuries. Early seafarers were wary of them, naturalists were confused by them, yet many coastal people benefited greatly from these mysterious rocky structures that grew up to the surface of the sea. They have been rich in their supply of food, and they provided a breakwater from storms and high waves to countless coastal communities that developed from their protection. Their scale is enormous and their value high. Found in countless locations around the world, from the Indo-Pacific coral reef province to the Caribbean and Australia, they support both marine and human life. In this Very Short Introduction, Charles Sheppard provides an account of what coral reefs are, how they are formed, how they have evolved, and the biological lessons we can learn from them. Today, the vibrancy and diversity of these fascinating ecosystems are under threat from over exploitation and could face future extinction, unless our conservation efforts are stepped up in order to save them. ABOUT THE SERIES: The Very Short Introductions series from Oxford University Press contains hundreds of titles in almost every subject area. These pocket-sized books are the perfect way to get ahead in a new subject quickly. Our expert authors combine facts, analysis, perspective, new ideas, and enthusiasm to make interesting and challenging topics highly readable.
144 pages, Paperback
First published May 26, 2014
31 people are currently reading
335 people want to read
Ratings & Reviews
Friends & Following
Create a free account to discover what your friends think of this book!
Community Reviews
Displaying 1 - 8 of 8 reviews
May 19, 2021
kompakt korallkunskap
June 20, 2023
Coral Reefs: A Very Short Introduction (2014) by Charles Sheppard describes some of the ocean's most biodiverse, productive, commercially valuable, and vulnerable ecosystems. Sheppard introduces the reader to corals, the mighty reefs that some coral species build, and the many factors (natural and human-caused) that damage or destroy them.
The book is mostly text, with some illustrations, but not nearly enough to do justice to the topic. Coral reefs, at least the relatively undamaged ones, are some of the most visually dazzling environments on Earth. The limited space of a VSI permits a few photos and diagrams but not nearly enough to do justice to the topic. That might be a problem if you were stuck somewhere with only a paper copy of this book and no Internet access, but in the modern era there are Wikipedia articles and YouTube videos for seemingly every topic in the book.
An interesting and valuable exercise might be to create a mashup video of excerpts depicting every topic in the book.
Humans are destroying much of Earth's biodiversity, one of the reasons many scientists call our current epoch the Anthropocene. Coral reefs are sadly one of the ecosystems being most rapidly destroyed. Sheppard details the destruction. It was refreshing to see that he doesn't shy away from the population taboo. He points out that it isn't just the usual mega-corporations doing all the damage. Even the normally-sainted indigenous human populations, usually regarded as environmentally sinless, are anything but when it comes to coral reefs. There doesn't seem to be any level of human exploitation of a reef that doesn't have a profound impact. Sheppard makes the point that the largest fish are the most prized by people who hunt for fish, but killing these fish has profound impacts, since the largest female fish of a species account for vastly more of the egg production than their smaller conspecifics. Since fish have to lay a lot of eggs to get some of them through to maturity, the loss of the largest and most prolific egg-producers makes a big difference.
Sheppard also describes the damaging impacts of agricultural, industrial, and commercial development near coral reefs. The shallow waters where coral reefs grow often attract human populations to the adjoining land areas, given that coral reefs at least initially provide a bounty of easily exploitable food for humans. The result is that today many millions of people depend on reefs for a substantial portion of their food, and these populations are growing. It's nice that Sheppard confronts the population problem honestly - a topic most environmental campaigners struggle to avoid, with some even mischaracterizing it as racism. Given that many environmentalists tend to be white, and the rapidly increasing indigenous populations tend to be browner in hue, it dredges up too much uncomfortable and recent history of colonization, etc. But the reefs don't care about what makes people uncomfortable - the simple fact remains that as more people depend on reefs for food, they cause more harm to them.
Then there's the ultimate coral-killer: human-caused global warming, which threatens to render all traditional conservation efforts moot. Sheppard describes some available tools for conserving reefs which have worked, but at some point they'll be little more than rearranging the deck chairs on the Titanic as global warming ticks up. There is some level of carbon dioxide in the atmosphere that probably spells doom for virtually all coral reefs, and there is nothing to indicate that humans aren't going to blow past that level, whatever it is. There is no binding global agreement to leave fossil fuels in the ground, nor any hint that we'll create and enforce such an agreement. As long as humans are still digging coal and pumping oil and gas and burning them, coral reefs have no future. Sheppard as much as acknowledges that this is the case, but then says he'll carry on with traditional conservation efforts anyway.
Sheppard exhibits a bit of special pleading where he tells about refusing to eat the shark-fin soup he was served at a science conference in China. Now, eating sharks is bad for the environment, so Sheppard took a stand. But flying to China is also bad for the environment, since aviation is overwhelmingly dependent on burning jet fuel made from petroleum. Whatever excuses a scientist might offer for jetting around the world would apply equally to eating a shark. Namely, that the individual's behavior is a drop in the bucket. But apparently Sheppard views one of those drops as more important than the others, despite the fact that climate change remains solidly on pace to end the whole show.
Another odd bit was where Sheppard describes human behavior as somehow being outside the wheelhouse of science, in that larger quote about the population problem I cited above. Has Sheppard never heard of psychology, in particular environmental psychology, or does he not consider psychology to be part of science? Granted, psychology as a science is far less mature than the physical sciences, given the vastly greater complexity of the human brain, but there is no question of human behavior that is not ultimately a question of science - unless Sheppard believes in some flavor of dualism (i.e. that there is some aspect of human behavior which is not ultimately reducible to physics and chemistry). Studying people is no different than studying coral reefs except in terms of complexity and thus difficulty.
All environmental problems caused by humans ultimately result from human behavioral choices, such as the choices of humans to eat shark-fin soup, fly to China for a conference, to produce a certain number of children, and what policies and politicians to vote for. Behavioral scientists have not yet figured out why people make all the choices they make, much less how to persuade people to make less destructive choices, but if coral reefs and the rest of the biosphere are to have any chance, we need behavioral scientists to get cracking.
Given that civilization might have only a few decades left, why are behavioral scientists studying anything else? When your house is on fire, that's the problem to solve first, and then you can get back to whatever else you like doing.
The book is mostly text, with some illustrations, but not nearly enough to do justice to the topic. Coral reefs, at least the relatively undamaged ones, are some of the most visually dazzling environments on Earth. The limited space of a VSI permits a few photos and diagrams but not nearly enough to do justice to the topic. That might be a problem if you were stuck somewhere with only a paper copy of this book and no Internet access, but in the modern era there are Wikipedia articles and YouTube videos for seemingly every topic in the book.
An interesting and valuable exercise might be to create a mashup video of excerpts depicting every topic in the book.
Humans are destroying much of Earth's biodiversity, one of the reasons many scientists call our current epoch the Anthropocene. Coral reefs are sadly one of the ecosystems being most rapidly destroyed. Sheppard details the destruction. It was refreshing to see that he doesn't shy away from the population taboo. He points out that it isn't just the usual mega-corporations doing all the damage. Even the normally-sainted indigenous human populations, usually regarded as environmentally sinless, are anything but when it comes to coral reefs. There doesn't seem to be any level of human exploitation of a reef that doesn't have a profound impact. Sheppard makes the point that the largest fish are the most prized by people who hunt for fish, but killing these fish has profound impacts, since the largest female fish of a species account for vastly more of the egg production than their smaller conspecifics. Since fish have to lay a lot of eggs to get some of them through to maturity, the loss of the largest and most prolific egg-producers makes a big difference.
Sheppard also describes the damaging impacts of agricultural, industrial, and commercial development near coral reefs. The shallow waters where coral reefs grow often attract human populations to the adjoining land areas, given that coral reefs at least initially provide a bounty of easily exploitable food for humans. The result is that today many millions of people depend on reefs for a substantial portion of their food, and these populations are growing. It's nice that Sheppard confronts the population problem honestly - a topic most environmental campaigners struggle to avoid, with some even mischaracterizing it as racism. Given that many environmentalists tend to be white, and the rapidly increasing indigenous populations tend to be browner in hue, it dredges up too much uncomfortable and recent history of colonization, etc. But the reefs don't care about what makes people uncomfortable - the simple fact remains that as more people depend on reefs for food, they cause more harm to them.
Then there's the ultimate coral-killer: human-caused global warming, which threatens to render all traditional conservation efforts moot. Sheppard describes some available tools for conserving reefs which have worked, but at some point they'll be little more than rearranging the deck chairs on the Titanic as global warming ticks up. There is some level of carbon dioxide in the atmosphere that probably spells doom for virtually all coral reefs, and there is nothing to indicate that humans aren't going to blow past that level, whatever it is. There is no binding global agreement to leave fossil fuels in the ground, nor any hint that we'll create and enforce such an agreement. As long as humans are still digging coal and pumping oil and gas and burning them, coral reefs have no future. Sheppard as much as acknowledges that this is the case, but then says he'll carry on with traditional conservation efforts anyway.
Sheppard exhibits a bit of special pleading where he tells about refusing to eat the shark-fin soup he was served at a science conference in China. Now, eating sharks is bad for the environment, so Sheppard took a stand. But flying to China is also bad for the environment, since aviation is overwhelmingly dependent on burning jet fuel made from petroleum. Whatever excuses a scientist might offer for jetting around the world would apply equally to eating a shark. Namely, that the individual's behavior is a drop in the bucket. But apparently Sheppard views one of those drops as more important than the others, despite the fact that climate change remains solidly on pace to end the whole show.
Another odd bit was where Sheppard describes human behavior as somehow being outside the wheelhouse of science, in that larger quote about the population problem I cited above. Has Sheppard never heard of psychology, in particular environmental psychology, or does he not consider psychology to be part of science? Granted, psychology as a science is far less mature than the physical sciences, given the vastly greater complexity of the human brain, but there is no question of human behavior that is not ultimately a question of science - unless Sheppard believes in some flavor of dualism (i.e. that there is some aspect of human behavior which is not ultimately reducible to physics and chemistry). Studying people is no different than studying coral reefs except in terms of complexity and thus difficulty.
All environmental problems caused by humans ultimately result from human behavioral choices, such as the choices of humans to eat shark-fin soup, fly to China for a conference, to produce a certain number of children, and what policies and politicians to vote for. Behavioral scientists have not yet figured out why people make all the choices they make, much less how to persuade people to make less destructive choices, but if coral reefs and the rest of the biosphere are to have any chance, we need behavioral scientists to get cracking.
Given that civilization might have only a few decades left, why are behavioral scientists studying anything else? When your house is on fire, that's the problem to solve first, and then you can get back to whatever else you like doing.
April 2, 2022
Clippings
In Precambrian times, assemblages of simple marine organisms laid down reefs over aeons of time. These were stromatolites, built by photosynthetic cyanobacteria, depositing limestone but also trapping sediment to form layered domes. In that Archaean Eon, long before the advent of complex life, atmospheric oxygen was low.
Sponges never went away, and are still abundant on reefs. These were joined for many millions of years by stromatoporoids, which appear to be related to sponges, and which dominated from the Ordovician Period but began to decline in the Carboniferous—the time, some 350–300 million years ago, during which thrived the lush vegetation whose remains eventually became transformed into today’s great seams of coal.
coral reefs as a whole have been described as being the canaries in the coal mine—they are the part of the ecosystem that will be the first to succumb to overexploitation and abuse.
In the 1970s the shallowest 3 metres of just about all reefs there were covered with the world’s largest species of coral, Acropora palmata, known as elkhorn coral.
by the 1980s something called White Band disease had killed almost all of them, a disease linked to human sewage.
‘local factors’, which include impacts from sewage and industrial pollution, along with shoreline dredging, construction, and overfishing. Others are ‘global factors’ which are all related to the rising amount of CO2 in the atmosphere, which causes temperatures to rise and the seawater to become less alkaline (acidification).
southwards along the Great Barrier Reef, from well over 300 species in the northern and central regions to 244 in the south, and just 87 at Lord Howe Island at 32° South. Similarly, over 50 species of corals can be found in the Caribbean, but only 21 are known from Bermuda which, at 32° North, is the most northerly reef system. Importantly however, the vibrancy and productivity of a reef appears to be more or less unrelated to its diversity.
In the tentacles, the outer layer, or ectoderm, contains the stinging cells (cnidocytes) that define the entire Phylum, Cnidaria. These are used for catching food.
On the underside of the polyp, the ectoderm contains no stinging cells but instead structures needed for depositing limestone in a crystalline form called aragonite (Figure 7, bottom). These structures secrete an acidic organic matrix into the minute space between the polyp and the existing skeleton. This mixture forms a kind of ‘scaffold’. Proteins in the matrix bind on to dissolved calcium taken from the seawater. Carbonate ions in the seawater are then attracted to the calcium ions and combine to form aragonite crystals. Sub-micrometre aragonite fibres grow in the organic matrix, and these coalesce and aggregate into well-defined larger units. Each coral species has its own specific pattern of cells that secrete the initial organic matrix, and the pattern of the resulting limestone deposition matches the pattern of cells.
domes, branching thickets, table shapes, columns, and leaves. It matters whether the daughter polyp divides from its parent from within the ring of tentacles or from lower down on the body stalk of the parent: each will lead to different shapes of the ever-growing colony. In species in which tissue continues to connect all the polyps, a touch on one side can lead to a ripple of tentacle retraction across the whole coral, showing that a neural network extends between polyps.
Fertilized eggs become planulae and these can drift in the water column for several days or weeks. Planulae are elliptical and can actively swim for limited distances, although they are dependent on currents for their significant dispersal. They respond to chemicals and light in their environment—chemotaxis and phototaxis.
Chemotaxis might draw them towards locations with favourable chemicals, such as excretory products from their own species, which indicate that the site is probably suited to their kind.
These algae are visible in that their existence in great numbers imparts a greenish brown coloration to the entire reefscape. They are dinoflagellates—microscopic algal cells that are embedded in the tissues of the corals and in many of the shallow soft corals too. In a sense, when you look at a field of coral you are looking at a field of captive, single-celled algae. We do not see great stands of plants on a healthy reef, but photosynthesis is the basis of this ecosystem too.
When a polyp divides asexually, or when fragmentation of brittle forms occurs, zooxanthellae are already there, passed across in the parental tissues. But when reproduction is sexual then the zooxanthellae need to be acquired via the parent’s egg or later on from free-living algal cells in the water. Surprisingly perhaps, transmission of zooxanthellae into the egg appears to be difficult, and uptake from the water is more common. The process is poorly understood. Zooxanthellae can aid their own uptake by detecting chemicals secreted by their potential hosts; when they are in the free-living stage they have been shown to swim towards excretory waste products of corals. When they come into contact with the coral they must be absorbed, but not digested. Once established, the stability of the symbiotic arrangement is actively managed by the coral host, which can encourage or slow algal division and growth as it requires, and each polyp can digest or expel excess algae.
This coral–zooxanthellae relationship is therefore key, but it can be affected markedly by environmental stress. Under difficult conditions the coral will expel its algal cells, and because the coral’s own tissue is relatively transparent, the white limestone beneath it will then show through, giving a bleached appearance.
100 million bacteria can live in each square centimetre of a coral and in its surface mucus. Some are central to the nitrogen cycling processes for the coral, fixing nitrogen, or processing ammonium (a toxic compound of nitrogen) excreted by animal tissues to render it non-toxic. Other microbes process phosphate and sulphur compounds too, the total microbial community performing a nutrient and mineral cycling function, including waste treatment that is essential to the coral. However, as with the algal symbiosis, things can go horribly wrong, such as when excess nitrogen is added to the environment from sewage or agricultural run-off.
The fastest mechanism involves the dominant species exuding digestive mesenterial filaments onto a neighbour. This is a short-range but very rapid mechanism, which will be triggered within one or two hours if two corals are placed adjacent to each other—in an aquarium for example. It mostly takes place at night under natural conditions, and there is usually a consistently dominant and subordinate species in any one pair of species. The dominant coral digests the subordinate species, leaving only bare skeleton by morning
Some families of corals may be very aggressive, while others seem to be subordinate to just about every other species. This being the case, in order to survive, the subordinate species must have other attributes such as faster growth or greater reproductive rate.
sponge communities at 25 to 40 metres deep have been estimated to filter the entire water column above them every day. This capture of plankton is a key link between the water column and the benthic life (life on the seabed). This ‘bentho-pelagic coupling’ provides an important route through which particulate carbon and nitrogen are channelled between water and the reef.
Being so shallow, this area regularly experiences environmental extremes: high temperatures experienced during low tide, severe dilution of salinity by rain during the monsoon at low tide, alternating with dangerously raised salinity under the tropical sun when no rain occurs, not to mention severe desiccation from sun and wind on very low tides as well as damaging ultraviolet light, all ensure that only a small sub-set of reef life has become adapted to live there. Biologically, therefore, reef flats are relatively depauperate. They are an environmentally stressful ‘outpost’ of the heart of a reef.
A micro-atoll is a single colony that has grown into a ring with living sides but with a dead and eroding top. They cannot grow upwards any more, only outwards, expanding their girth,
Unlike, say, a flat sheet of limestone, irregular coral growth provides considerable volume and countless niches. Most striking in this respect is that most architectural of all coral genera, the Acropora.
reefs are remarkably porous, permitting considerable flow of water through them. Most caves experience the passage of thousands of litres of water per day through numerous small channels, pores, and crevices, and to a considerable extent it is this permeability of the reef rock which determines the amount of life that can live in a cave. This can be demonstrated easily, if unscientifically, by a diver inside the cave purging his or her regulator. The air disappears into the roof, and a dive buddy outside the cave might see, after a pause of maybe a few seconds or minutes, streams of air bubbles emerging
fragment further into sand and then silt. Much accumulates behind the reef and in its lagoons, but much also accumulates in the numerous small spaces on the reef itself, where it later becomes consolidated into more durable rock. Rubble that remains on the surface is very inhospitable to most forms of life that require a solid and stable base because its movement in storms has a similar effect to that of liquid sandpaper, producing a surface that is very hostile to most attached forms of
Parrotfish are responsible for the production of much of the sand on reefs. Their digestive systems are such that they require sand to aid digestion, and it is common to see a stream of fine white sand being defecated from a parrotfish as it swims.
cryptic reef fauna are estimated to have a biomass as great as that of the more visible fauna on the surface, and have a much greater diversity of species. Small, bioeroding organisms tunnel, bore, and chisel into the relatively soft limestone rock. Many bivalve molluscs use their shells to scrape out a tunnel in which they live. Other organisms secrete mild acid to dissolve the limestone. A particularly important bioeroding group is the sponges. These have no moving parts and so rely entirely on dissolving the rock.
The translucent quality of corals’ aragonite limestone means that light can penetrate sufficiently through both a thin layer of coral tissue and a couple of millimetres of limestone for sufficient photosynthesis to continue. A section through many massive corals will show a thin green band that is indicative of the presence of such algae.
Any unfortunate larva of a tunnelling or boring organism trying to settle on a live coral is consumed, so that the live surface of any coral is a most effective barrier to penetration of its skeleton. Mortality of the coral tissue exposes the skeleton to attack, and leads to greater sand and sediment production. It is not only a psychological gloom that makes a diver think that a dead reef has poor water visibility, but also the increased quantity of very fine silt suspended in it that has originated from such erosion.
sand underwater very often appears brown, grey, even purple, and mottled—not white as when it is piled up to form a beach. The colour is caused by expanses of filamentous algae and microscopic forms such as diatoms growing over the surface, photosynthesizing, and providing food for numerous other organisms. Many of these also fix nitrogen.
These microbial communities, especially bacteria, are key to the recycling of carbon and organic compounds on reefs, and this is known as the ‘microbial loop’: the invisible aspect to how a reef functions. Key nutrients such as phosphorus in the water over a reef can be turned over in minutes, and even complex organic materials are recycled in hours or days. For many years it was considered paradoxical that coral reefs could exist as oases of high productivity in the nutrient-poor vastness of the open ocean, but it is clear now that one reason why this is possible can be attributed to the microbial loop.
Some dinoflagellates are motile and entirely predatory. They have lost their chloroplasts, and feed on other protists, using their two flagellae to swim. Sometimes free-living forms undergo blooms, forming ‘red tides’, when they may produce neurotoxins that result in massive fish kills and produce ciguatera poisoning in humans who eat fish contaminated by them.
sharks are amongst the most visually impressive hunters on the reef. They have astonishingly acute olfactory senses and organelles that detect vibrations and electrical signals in water, including those coming from an injured fish.
the ‘fish pyramid’ on its own can even be upside down. However, one important point about a coral reef is that, as with plant biomass, it is not only biomass per se that is important but the productivity, or rate of turnover. Rapid growth and rapid consumption may lead to low biomass—and a reef does demonstrate a very fast pace of life. A second important point is that because fish are mobile many, including many of the numerically abundant species, roam over much larger areas than just the reef
predator release of prey is particularly significant on coral reefs in the present days of overfishing, and the explosion in algae growth that may result could swamp a coral reef.
For some reason, every now and again they appear in vast numbers, moving on to a reef from deeper water, consuming just about every coral they can get to (and showing some cannibalistic behaviour for good measure). Then, when there is absolutely nothing left to eat they disappear equally suddenly, leaving behind a spectacular white expanse of bare coral skeletons. A week or two later these skeletons become darkened as they are colonized by filamentous algae, and the dead corals become subject to countless burrowing, tunnelling, and boring animals which quickly exploit this newly available habitat.
a single 10 kg female might produce many millions of eggs per year, while ten 1 kg females of the same species would produce only a few thousand per year. If we remember that the larger fish are the most prized, we can immediately see that the damage done to the ecosystem by removing the largest fish is exponentially greater.
As the number of species in any ecosystem increases, so does the number of connections between them, and connections rise at a rate much faster than the number of species. Connections can be destroyed, as can the species themselves. The question of whether a reef ecosystem is more robust to outside pressures as a result of this huge number of species and connections, or made fragile because of them, has been endlessly discussed. The answer must be ‘both’
Different kinds of impacts have different consequences. Further, an impact that kills a reef can be different from the one that prevents its later recovery, so both aspects require redundancy for a reef to survive impacts.
Extremely degraded reefs are now common. The rubble left on many becomes sedimented and covered with a cyanobacterial and algal film. It has been said that the switch to such a state is like moving backwards in evolutionary time, to Precambrian conditions: a ‘slippery slope to slime’.
In Precambrian times, assemblages of simple marine organisms laid down reefs over aeons of time. These were stromatolites, built by photosynthetic cyanobacteria, depositing limestone but also trapping sediment to form layered domes. In that Archaean Eon, long before the advent of complex life, atmospheric oxygen was low.
Sponges never went away, and are still abundant on reefs. These were joined for many millions of years by stromatoporoids, which appear to be related to sponges, and which dominated from the Ordovician Period but began to decline in the Carboniferous—the time, some 350–300 million years ago, during which thrived the lush vegetation whose remains eventually became transformed into today’s great seams of coal.
coral reefs as a whole have been described as being the canaries in the coal mine—they are the part of the ecosystem that will be the first to succumb to overexploitation and abuse.
In the 1970s the shallowest 3 metres of just about all reefs there were covered with the world’s largest species of coral, Acropora palmata, known as elkhorn coral.
by the 1980s something called White Band disease had killed almost all of them, a disease linked to human sewage.
‘local factors’, which include impacts from sewage and industrial pollution, along with shoreline dredging, construction, and overfishing. Others are ‘global factors’ which are all related to the rising amount of CO2 in the atmosphere, which causes temperatures to rise and the seawater to become less alkaline (acidification).
southwards along the Great Barrier Reef, from well over 300 species in the northern and central regions to 244 in the south, and just 87 at Lord Howe Island at 32° South. Similarly, over 50 species of corals can be found in the Caribbean, but only 21 are known from Bermuda which, at 32° North, is the most northerly reef system. Importantly however, the vibrancy and productivity of a reef appears to be more or less unrelated to its diversity.
In the tentacles, the outer layer, or ectoderm, contains the stinging cells (cnidocytes) that define the entire Phylum, Cnidaria. These are used for catching food.
On the underside of the polyp, the ectoderm contains no stinging cells but instead structures needed for depositing limestone in a crystalline form called aragonite (Figure 7, bottom). These structures secrete an acidic organic matrix into the minute space between the polyp and the existing skeleton. This mixture forms a kind of ‘scaffold’. Proteins in the matrix bind on to dissolved calcium taken from the seawater. Carbonate ions in the seawater are then attracted to the calcium ions and combine to form aragonite crystals. Sub-micrometre aragonite fibres grow in the organic matrix, and these coalesce and aggregate into well-defined larger units. Each coral species has its own specific pattern of cells that secrete the initial organic matrix, and the pattern of the resulting limestone deposition matches the pattern of cells.
domes, branching thickets, table shapes, columns, and leaves. It matters whether the daughter polyp divides from its parent from within the ring of tentacles or from lower down on the body stalk of the parent: each will lead to different shapes of the ever-growing colony. In species in which tissue continues to connect all the polyps, a touch on one side can lead to a ripple of tentacle retraction across the whole coral, showing that a neural network extends between polyps.
Fertilized eggs become planulae and these can drift in the water column for several days or weeks. Planulae are elliptical and can actively swim for limited distances, although they are dependent on currents for their significant dispersal. They respond to chemicals and light in their environment—chemotaxis and phototaxis.
Chemotaxis might draw them towards locations with favourable chemicals, such as excretory products from their own species, which indicate that the site is probably suited to their kind.
These algae are visible in that their existence in great numbers imparts a greenish brown coloration to the entire reefscape. They are dinoflagellates—microscopic algal cells that are embedded in the tissues of the corals and in many of the shallow soft corals too. In a sense, when you look at a field of coral you are looking at a field of captive, single-celled algae. We do not see great stands of plants on a healthy reef, but photosynthesis is the basis of this ecosystem too.
When a polyp divides asexually, or when fragmentation of brittle forms occurs, zooxanthellae are already there, passed across in the parental tissues. But when reproduction is sexual then the zooxanthellae need to be acquired via the parent’s egg or later on from free-living algal cells in the water. Surprisingly perhaps, transmission of zooxanthellae into the egg appears to be difficult, and uptake from the water is more common. The process is poorly understood. Zooxanthellae can aid their own uptake by detecting chemicals secreted by their potential hosts; when they are in the free-living stage they have been shown to swim towards excretory waste products of corals. When they come into contact with the coral they must be absorbed, but not digested. Once established, the stability of the symbiotic arrangement is actively managed by the coral host, which can encourage or slow algal division and growth as it requires, and each polyp can digest or expel excess algae.
This coral–zooxanthellae relationship is therefore key, but it can be affected markedly by environmental stress. Under difficult conditions the coral will expel its algal cells, and because the coral’s own tissue is relatively transparent, the white limestone beneath it will then show through, giving a bleached appearance.
100 million bacteria can live in each square centimetre of a coral and in its surface mucus. Some are central to the nitrogen cycling processes for the coral, fixing nitrogen, or processing ammonium (a toxic compound of nitrogen) excreted by animal tissues to render it non-toxic. Other microbes process phosphate and sulphur compounds too, the total microbial community performing a nutrient and mineral cycling function, including waste treatment that is essential to the coral. However, as with the algal symbiosis, things can go horribly wrong, such as when excess nitrogen is added to the environment from sewage or agricultural run-off.
The fastest mechanism involves the dominant species exuding digestive mesenterial filaments onto a neighbour. This is a short-range but very rapid mechanism, which will be triggered within one or two hours if two corals are placed adjacent to each other—in an aquarium for example. It mostly takes place at night under natural conditions, and there is usually a consistently dominant and subordinate species in any one pair of species. The dominant coral digests the subordinate species, leaving only bare skeleton by morning
Some families of corals may be very aggressive, while others seem to be subordinate to just about every other species. This being the case, in order to survive, the subordinate species must have other attributes such as faster growth or greater reproductive rate.
sponge communities at 25 to 40 metres deep have been estimated to filter the entire water column above them every day. This capture of plankton is a key link between the water column and the benthic life (life on the seabed). This ‘bentho-pelagic coupling’ provides an important route through which particulate carbon and nitrogen are channelled between water and the reef.
Being so shallow, this area regularly experiences environmental extremes: high temperatures experienced during low tide, severe dilution of salinity by rain during the monsoon at low tide, alternating with dangerously raised salinity under the tropical sun when no rain occurs, not to mention severe desiccation from sun and wind on very low tides as well as damaging ultraviolet light, all ensure that only a small sub-set of reef life has become adapted to live there. Biologically, therefore, reef flats are relatively depauperate. They are an environmentally stressful ‘outpost’ of the heart of a reef.
A micro-atoll is a single colony that has grown into a ring with living sides but with a dead and eroding top. They cannot grow upwards any more, only outwards, expanding their girth,
Unlike, say, a flat sheet of limestone, irregular coral growth provides considerable volume and countless niches. Most striking in this respect is that most architectural of all coral genera, the Acropora.
reefs are remarkably porous, permitting considerable flow of water through them. Most caves experience the passage of thousands of litres of water per day through numerous small channels, pores, and crevices, and to a considerable extent it is this permeability of the reef rock which determines the amount of life that can live in a cave. This can be demonstrated easily, if unscientifically, by a diver inside the cave purging his or her regulator. The air disappears into the roof, and a dive buddy outside the cave might see, after a pause of maybe a few seconds or minutes, streams of air bubbles emerging
fragment further into sand and then silt. Much accumulates behind the reef and in its lagoons, but much also accumulates in the numerous small spaces on the reef itself, where it later becomes consolidated into more durable rock. Rubble that remains on the surface is very inhospitable to most forms of life that require a solid and stable base because its movement in storms has a similar effect to that of liquid sandpaper, producing a surface that is very hostile to most attached forms of
Parrotfish are responsible for the production of much of the sand on reefs. Their digestive systems are such that they require sand to aid digestion, and it is common to see a stream of fine white sand being defecated from a parrotfish as it swims.
cryptic reef fauna are estimated to have a biomass as great as that of the more visible fauna on the surface, and have a much greater diversity of species. Small, bioeroding organisms tunnel, bore, and chisel into the relatively soft limestone rock. Many bivalve molluscs use their shells to scrape out a tunnel in which they live. Other organisms secrete mild acid to dissolve the limestone. A particularly important bioeroding group is the sponges. These have no moving parts and so rely entirely on dissolving the rock.
The translucent quality of corals’ aragonite limestone means that light can penetrate sufficiently through both a thin layer of coral tissue and a couple of millimetres of limestone for sufficient photosynthesis to continue. A section through many massive corals will show a thin green band that is indicative of the presence of such algae.
Any unfortunate larva of a tunnelling or boring organism trying to settle on a live coral is consumed, so that the live surface of any coral is a most effective barrier to penetration of its skeleton. Mortality of the coral tissue exposes the skeleton to attack, and leads to greater sand and sediment production. It is not only a psychological gloom that makes a diver think that a dead reef has poor water visibility, but also the increased quantity of very fine silt suspended in it that has originated from such erosion.
sand underwater very often appears brown, grey, even purple, and mottled—not white as when it is piled up to form a beach. The colour is caused by expanses of filamentous algae and microscopic forms such as diatoms growing over the surface, photosynthesizing, and providing food for numerous other organisms. Many of these also fix nitrogen.
These microbial communities, especially bacteria, are key to the recycling of carbon and organic compounds on reefs, and this is known as the ‘microbial loop’: the invisible aspect to how a reef functions. Key nutrients such as phosphorus in the water over a reef can be turned over in minutes, and even complex organic materials are recycled in hours or days. For many years it was considered paradoxical that coral reefs could exist as oases of high productivity in the nutrient-poor vastness of the open ocean, but it is clear now that one reason why this is possible can be attributed to the microbial loop.
Some dinoflagellates are motile and entirely predatory. They have lost their chloroplasts, and feed on other protists, using their two flagellae to swim. Sometimes free-living forms undergo blooms, forming ‘red tides’, when they may produce neurotoxins that result in massive fish kills and produce ciguatera poisoning in humans who eat fish contaminated by them.
sharks are amongst the most visually impressive hunters on the reef. They have astonishingly acute olfactory senses and organelles that detect vibrations and electrical signals in water, including those coming from an injured fish.
the ‘fish pyramid’ on its own can even be upside down. However, one important point about a coral reef is that, as with plant biomass, it is not only biomass per se that is important but the productivity, or rate of turnover. Rapid growth and rapid consumption may lead to low biomass—and a reef does demonstrate a very fast pace of life. A second important point is that because fish are mobile many, including many of the numerically abundant species, roam over much larger areas than just the reef
predator release of prey is particularly significant on coral reefs in the present days of overfishing, and the explosion in algae growth that may result could swamp a coral reef.
For some reason, every now and again they appear in vast numbers, moving on to a reef from deeper water, consuming just about every coral they can get to (and showing some cannibalistic behaviour for good measure). Then, when there is absolutely nothing left to eat they disappear equally suddenly, leaving behind a spectacular white expanse of bare coral skeletons. A week or two later these skeletons become darkened as they are colonized by filamentous algae, and the dead corals become subject to countless burrowing, tunnelling, and boring animals which quickly exploit this newly available habitat.
a single 10 kg female might produce many millions of eggs per year, while ten 1 kg females of the same species would produce only a few thousand per year. If we remember that the larger fish are the most prized, we can immediately see that the damage done to the ecosystem by removing the largest fish is exponentially greater.
As the number of species in any ecosystem increases, so does the number of connections between them, and connections rise at a rate much faster than the number of species. Connections can be destroyed, as can the species themselves. The question of whether a reef ecosystem is more robust to outside pressures as a result of this huge number of species and connections, or made fragile because of them, has been endlessly discussed. The answer must be ‘both’
Different kinds of impacts have different consequences. Further, an impact that kills a reef can be different from the one that prevents its later recovery, so both aspects require redundancy for a reef to survive impacts.
Extremely degraded reefs are now common. The rubble left on many becomes sedimented and covered with a cyanobacterial and algal film. It has been said that the switch to such a state is like moving backwards in evolutionary time, to Precambrian conditions: a ‘slippery slope to slime’.
September 4, 2020
I absolutely loved this book! Charles Sheppard is clearly an actual expert. I particularly liked his short digressions about odd and unusual creatures (Osterobium-- "the other" symbiotic algae in corals), his discussion of "ecosystem services" ("a biologist might not much like reducing the idea of a reef to something that simply provides food, a mere larder with monetary value, but in this overcrowded and overexploited world that is increasingly the way of society"), and his honest and blunt descriptions of the threats facing reefs and myths people sell (that reefs can be sustainably harvested at high levels). I would like to go read the rest of his books now.
February 25, 2020
So-so intro to the ecology of coral reefs and the threats to them. Does not focus much on species, so needs to be coupled with a field guide. I didn't think the "very short introduction" format worked very well for this topic, which really needs more extensive visual aids and diagrams, not just text with a few monochrome photographs.
August 13, 2020
A very interesting introduction to Coral reefs.
December 30, 2020
A very short book indeed, at least mentioning all major topics on contemporary coral reefs and their troubles
May 7, 2023
Yawn. The whole point of these things is to make the subject matter accessible to laypeople. I'm not a layperson, and I still found it insanely boring, as did everyone else on the Marine Biology program I was on. And not because we knew everything in it (we didn't, education is a scam, etc.), but just because it was communicated so dryly. Bad book. Rip the coral reefs though :(
Displaying 1 - 8 of 8 reviews