1. WALLACE RESOURCE LIBRARY
Module 01 - Ecosystems: Coral Reefs
D01 – The Effect of light on coral morphology in the Caribbean

2. What is a coral reef?
“Mound/ridge of living coral, coral skeletons and calcium deposits from other sea organisms that resides in the ocean”
Many different plants and animals contribute to the reef’s construction
The most important are corals, which is why they are known as ecosystem architects
The main type of organism that contributes to the structure of a reef are the hard corals. However other organisms contribute to reef formation, in particular encrusting coralline algae which is often referred to as the glue of the reef as it covers and binds many of the larger skeletons together to form the reef.
The important thing to realise is that a coral reef is in constant battle with the process of erosion, which is constantly acting on the reef to reduce it in size (like a cliff being washed away slowly by waves). A healthy reef will experience growth which is higher than erosion (i.e. calcium carbonate is laid down by corals and other organisms faster than it is being eroded by the oceans). If conditions change (e.g. ocean acidification), erosion can increase, or corals can start to grow slower meaning the reef grows slower. If this happens, the reef will start to shrink instead of grow, and will not be able to keep up with sea level rise (for example).

3. What is a coral reef?
Certain species of coral produce a calcium carbonate skeleton
Successive generations of these corals lay down their skeleton on those of previous generations
A coral reef is a result of thousands of years of coral skeletons
Note: Most corals grow only a few cm per year!
Corals that produce a calcium carbonate skeleton are called “Hard Corals”, and belong to the “Scleractinian” group. Note that some species of coral produce a skeleton, but this skeleton doesn’t contribute to a reef, as the coral is free-living. Those corals whose skeletons contribute to reefs (reef building corals) are known as “Hermatypic”, and those that aren’t are known as “Ahermatypic”.
Over approximately 170 million years this relationship has produced the worlds coral reefs and coral islands by countless generations of polyps growing on the skeletons of earlier generations

4. Heterotrophy AND Autotrophy
Calcareous “cup” - the corallite
This is a diagram of a coral polyp (living in a colony next to another identical polyp). The coral itself lives within a cup formed out of calcium carbonate which forms the coral skeleton. When this polyp dies, a new polyp will lay down its cup on top of this one, and in that way the colony grows larger.
The coral has tentacles around its mouth, which contain stinging cells called nematocysts, similar to jellyfish. These tentacles and nematocysts are used to capture prey which the coral feeds on. This type of feeding (on organic matter) is known as heterotrophy. Food sources include zooplankton, invertebrates and even small fish (if the polyp is large enough). Tentacles often catch the prey directly, but the polyp also secretes a mucus which small particles stick to. The tentacles will then pull in the mucus with the attached particles into the mouth and digest.
However, most corals also gain energy from sunlight. Photosynthesis is not performed directly by the coral, but by microscopic algae called zooxanthellae which live in the coral tissue. Acquiring energy through photosynthesis is called autotrophy. The coral provides a safe and optimal environment for the zooxanthellae to live, and in return take any excess carbon which the zooxanthellae have assimilated through photosynthesis. This can be up to 90% of carbon moving from the zooxanthellae to the coral polyp.

5. Coral distribution
There are a few key things to note from this map (see if the students can spot any of them – this is good practice for data analysis and interpretation):
That corals are restricted to within the tropics (i.e. close to the equator). This is due to the temperature requirements of reef building corals – they generally can’t survive in temperatures below 19°C.
The three centres of coral diversity are found in the Western areas of the three main oceans: (1) The Caribbean (Western Atlantic), (2) the Indo-Pacific (Western Pacific Ocean), (3) the Red Sea and East Africa (Western Indian Ocean). This is due to the prevailing currents in the tropical oceans being in a western direction.
There are large areas of the tropics with very little or no coral diversite (e.g. West Africa and South America). This is because these regions experience upwelling of cold nutrient rish waters, so you find totally different ecosystems there.
The Caribbean is much less diverse than other areas of the world (see the next slide).
There are three main centres of coral diversity, (1) the Caribbean, (2) the Indo-Pacific and (3) the Western Indian Ocean

6. Factors influencing coral reef distribution
Light Intensity
Required for photosynthesis of algae living symbiotically within corals.
This limits corals to shallower waters (<60m)
Temperature
Requires temperatures above 19C, elevated temperatures (>30C) can cause bleaching
Abiotic
Factors
Salinity
Corals cannot tolerate low salinities (e.g. from rivers), but they can tolerate high salinity
Abiotic factors are and factors which are physical rather than biological. The abiotic factors listed here are what determines where you find coral reefs and where you don’t. Once you have gone through all these, ask the students to think about what climate change is doing to these factors – which ones are being made worse?
Light Intensity
Many corals have symbiotic algae called zooxanthellae (covered in following lecture) that use sunlight to photosynthesize to create carbohydrates. The corals then take a portion of these carbohydrates. Therefore corals need access to sunlight to enable photosynthetic algae to function.
Because of this dependence on sunlight corals are limited to a depth range of about 60m, although this will vary with location as variables like sediment in the water will influence the depth to which light can penetrate. Rough estimates put this level at 1-2% of surface irradience levels.
As you go deeper, you will find that corals that depend less on photosynthetic algae will start to dominate. This leads to coral species having a depth distribution. Have a look on your next dive and see whether you can notice a change in the corals you find at different depths.
Sedimentation
Sediment resting on its surface is one of the main stresses for a coral as it blocks light to the algae, blocks feeding structures and can smother the coral. For this reason corals can remove this sediment through various mechanisms such as ciliary action and secreting mucus, however such mechanisms require energy expenditure so areas with high sedimentation will require higher energy expenditure and this can be prohibitive. For this reason corals are commonly found in high energy areas where waves, swell and currents prevent sediment settling on corals, and conversely its uncommon to find corals in low energy areas such as lagoons and estuaries where sedimentation is high.
Water with high levels of suspended sediment (lower visibility) will also have much lower light penetration which will also restrict coral growth.
Salinity
Corals have quite a high range of salinity from about 27 to 40 ppt (parts per thousand) in which they can survive. This is a wide range and often not an important variable as it is usually very constant with changes in salinity uncommon in the oceans. Locations with high salinities are those where evaporation is high and mixing is low resulting in high salinity such as the Persian Gulf, while low salinity is caused by high levels of freshwater input so usually near river mouths. This is also a problem in rockpools, where evaporation increases the salinity hugely.
Temperature
Ideal temperatures for corals is mid 20’s 0C although corals can tolerate ranges between 18 to 340C. However, long exposure to high temperatures can lead to bleaching (when the coral ejects all of its symbiotic algae from its tissues) and potentially death of the coral. This has been seen with El nino events that have led to higher sea temperatures and some coral bleaching.
Emersion
Corals cannot be exposed to the air for long or regularly. Some corals will be exposed by extreme low tides and this can be tolerated for a couple of hours due to the protection afforded by a protective mucus layer but this is only short term and emersion will be fatal if for much longer.
Emersion
No coral species can withstand regular or prolonged time out of the water
Sedimentation
High sedimentation reduces light levels,
and smother corals

7. Tasks for this data set
You are going to investigate the effect of light on the coral morphology (form/structure) of the coral Montastrea cavernosa
You will do this by analysing photographs of the coral taken at different depths.
This data will be used to construct graphs so that you can attempt to answer TWO important research questions.
Background to the research:
Corals are an incredibly important group of organisms as they are the ecosystem architects for coral
reefs, which are one of the most biodiverse and productive biomes on the planet. The complex 3D
structure of a coral reef, which is created almost entirely by the corals themselves, provides a range
of habitats to support many thousands of fish species. In turn, these fish support tropical fisheries,
which are the major source of protein for around one billion people around the world. However,
corals are under severe threat from factors such as climate change, and understanding how they will
react to future changes in conditions such as temperature and light are vital if we are to predict how
coral reefs will change in the coming decades.
Corals can get their energy either through feeding on particles in the water column (heterotrophy)
or by using light for photosynthesis (autotrophy). This is because the coral itself is a small animal,
similar to a jellyfish or sea anemone, with stinging tentacles that it can use to catch prey and pull it
into a small mouth at the centre of the coral. But most corals live in a symbiotic relationship with
microscopic single-cell plants called microalgae. The coral provides the microalgae (known as
zooxanthellae) with protection and a suitable environment to survive inside their tissue. In return,
the coral takes a large portion of the energy which the microalgae produce through photosynthesis.
Most corals use both heterotrophy and autotrophy to satisfy their energy requirements. One good
example of how they maximise the benefits of this opportunity is when corals pull their tentacles in
during the day to allow more light to reach the microalgae, but then extend their tentacles and feed
heterotrophically during the night when there is no light available. This ability is particularly
important, as the light available in the ocean is incredibly variable, and by changing depth by only a
few metres, the quantity and quality of light reaching the coral will change dramatically. The ability
to utilise multiple feeding strategies is therefore an important tool in allowing corals to cope with a
highly varied environment.
Most corals live in groups called colonies, which can be many metres in diameter, although each
individual coral polyp is usually less than 1cm in diameter. These colonies come in a range of shapes
and sizes, but so do the individual polyps themselves. The polyp sits in a calcium carbonate cup
which it creates itself, known as a corallite, and the size of corallites (and therefore polyps) can vary
greatly within a single species, in particularly in response to environmental conditions. With light
levels expected to change in the coming years as climate change worsens, studying how the size of
coral polyps varies between areas of different light levels gives us a valuable insight into how coral
morphology will change in the future. One of the easiest ways to estimate polyp size is to measure
the density of corallites on the surface of a coral colony. By comparing this between depths, we have
a ready made gradient of light levels to compare.

8. Research questions
1. How does the common Caribbean coral Montastrea cavernosa modify its morphology in response to different light conditions?
2. Can we use corallite density data to predict a maximum depth at which Montastrea cavernosa would be expected to be found on this coral reef?
Answers/discussions to research questions:
1. Our data clearly suggests that the morphology of Montastrea cavernosa polyps changes with
depth. As light is the major changing factor between different depths, and based on our
knowledge of how reef-building corals gain their energy, we can suggest (although not
prove) that these morphological variations are in response to changing light conditions.
Those corals found deeper on the reef have lower corallite density (and therefore larger
individual polyps), whilst those corals found shallower have higher corallite density (and
therefore smaller individual polyps). This indicates that corals living under low light
conditions (i.e. deeper) invest more energy in larger structures needed for heterotrophic
feeding (e.g. tentacles) which require a larger polyp to contain. On the other hand, corals
living under high light conditions have less need to supplement their autotrophic energy
with heterotrophy, and therefore don’t require such large polyps.
2. The relationship between depth and corallite density suggests that eventually corallite
density will reach zero. This could be used to predict a maximum depth distribution of
Montastrea cavernosa corals on reefs around Cayos Cochinos. By using the regression
equation, an exact maximum depth can be calculated. To do this, the regression equation
will need to be rearranged to make x the subject, which would give the following equation: x
= (y – ) ÷ By solving this equation where y = 0, the maximum depth our
relationship predicts M. cavernosa to be found at is 23.28m. Remember that this is only a
prediction, and if we visited the study reef and surveyed below this depth, we may find that
this species is able to survive deeper, and that no further decrease in corallite density is
observed below a particular depth.

9. Summary Conclusions
The data will suggest that the morphology of the coral polyps change at depth and as light is the major changing factor between different depths that the morphological changes are in response to changing light conditions.
The relationship between depth and corallite density suggests that eventually the corallite density will be zero. This could be used to predict the maximum depth distribution on the reefs sampled in this part (Cayos Cochinos) of the Caribbean.
1. Our data clearly suggests that the morphology of Montastrea cavernosa polyps changes with
depth. As light is the major changing factor between different depths, and based on our
knowledge of how reef-building corals gain their energy, we can suggest (although not
prove) that these morphological variations are in response to changing light conditions.
Those corals found deeper on the reef have lower corallite density (and therefore larger
individual polyps), whilst those corals found shallower have higher corallite density (and
therefore smaller individual polyps). This indicates that corals living under low light
conditions (i.e. deeper) invest more energy in larger structures needed for heterotrophic
feeding (e.g. tentacles) which require a larger polyp to contain. On the other hand, corals
living under high light conditions have less need to supplement their autotrophic energy
with heterotrophy, and therefore don’t require such large polyps.
2. The relationship between depth and corallite density suggests that eventually corallite
density will reach zero. This could be used to predict a maximum depth distribution of
Montastrea cavernosa corals on reefs around Cayos Cochinos. By using the regression
equation, an exact maximum depth can be calculated. To do this, the regression equation
will need to be rearranged to make x the subject, which would give the following equation: x
= (y – ) ÷ By solving this equation where y = 0, the maximum depth our
relationship predicts M. cavernosa to be found at is 23.28m. Remember that this is only a
prediction, and if we visited the study reef and surveyed below this depth, we may find that
this species is able to survive deeper, and that no further decrease in corallite density is
observed below a particular depth.