There is a species of filefish that exclusively eats Acropora corals, absorbing chemicals from them which allow them to hide in plain sight. Although orange-spotted filefish are brightly colored, their predators often rely more on scent than vision. These filefish use the Acropora chemicals to hide their fishy scent. Read more.
Orange spotted filefish By jaredzimmerman [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/] via Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Orange_spotted_filefish.jpg.
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Corals have certain environmental conditions that they need in order to survive. These factors limit where corals can live. In this unit, we will learn about the abiotic factors that corals need to thrive.
These environmental conditions are referred to as biotic and abiotic factors. Remember, we can break down the word abiotic:
Therefore, abiotic means without life. Abiotic factors are non-living components of an organism’s environment. Do you remember what biotic means? It means life. Biotic factors are living or once living components of a community.
Coral reefs are complex ecosystems. There are many different biotic factors that affect coral reefs. They will be discussed throughout different units. For now, we will focus on the abiotic factors that affect corals.
Can you think of any abiotic factors that may affect coral reef ecosystems?
Here are some examples:
|Inorganic nutrients (phosphorus, nitrogen, etc.)|
|Dissolved gases – oxygen (O2), nitrogen (N2), carbon dioxide (CO2), etc.|
|Aragonite and calcium carbonate saturation
(we will discuss more about this in later units)
There are multiple abiotic factors that affect coral reefs (figure 8-1). The numbers in the diagram correspond to the numbered descriptions below. Here are the optimal parameters for corals to live in:
Salinity can be measured by using a hand-held refractometer (figure 8-2). When using this tool, a small drop of seawater is placed between a measuring prism and a cover plate. Light passes through the sample and the light is refracted through the prism to the scale. By looking through the eyepiece, the salinity can be read. There are other tools, such as hydrometers and conductivity meters, that are also used to measure salinity.
Figure 8-2. The light (red arrow) bends through a refractometer giving a salinity reading, which is seen through the eyepiece.
Corals need clear water (water with low turbidity) so that the light can penetrate through the water column, allowing the zooxanthellae to photosynthesize. When there is high turbidity, light can’t penetrate through the suspended particles. Additionally, corals can be smothered by sediments or particles that settle on top of the corals. Eventually, this could lead to the coral dying.
Secchi disks are lowered into the water with a rope. Light reflects off of the top of the Secchi disk. When the Secchi disk is no longer able to be seen, the person stops lowering the rope. Then they record the depth that is marked on the rope. Using the Secchi depth, scientists can calculate the turbidity of the water.
Figure 8-3. Secchi disk pattern.
Figure 8-4. pH scale
Corals prefer seawater to have a pH level between 8.0–8.3, which is the standard pH of saltwater. It is very difficult for corals to grow in acidic conditions. We will learn more about this topic and how it is related to ocean acidification in later units.
On a coral reef, space can be very limited and corals must compete with sponges, macroalgae (seaweed), and even other coral species. When space becomes limited, most corals can’t pick up and move. Instead, they battle it out with their neighbors. Some coral polyps do this by extending their mesenteries from their stomachs. Then the battle begins. The mesentery strands cover the intruder species and the coral releases digestive enzymes that begin to devour their neighbor. This battle for territory can be seen in the Coral Nighttime Battle. Click on video #5 to view.
Remember that these are general parameters and that there are corals that have adapted to different conditions. For example, Siderastrea radians, the lesser starlet coral, has adapted to living in very stressful conditions in Florida Bay (Lirman et al. 2002). This large body of water is an estuarine area where seawater mixes with freshwater. The average depth in Florida Bay is 5 feet (152.4 centimeters), which is one of the reasons that environmental conditions fluctuate greatly. During the dry season, when there is little to no rain and evaporation rates are high, salinity levels can reach hypersaline conditions (greater than 40 ppt). It has also been documented that certain basins in Florida Bay reach over 70 ppt. This would be a very difficult place to live if you were a coral. Remember that most corals can tolerate 23–42 ppt.
In another example, American Samoa corals have adapted to high temperatures that can easily exceed 89.6°F (32°C) for multiples hours and days at a time (Craig et al. 2001). This gives us hope that some corals will adapt to climate change and warming seas.
There are other abiotic factors that affect coral reefs. We have already learned how currents can affect distribution of coral reefs in Unit 7: Distribution. There are other abiotic factors that we will learn more about in later units (see Unit 9: Coral Growth). We will also learn about the biotic factors that affect coral reefs too (see Unit 16: Food Webs, coming soon).
Figure 8-1. Sun. 2014 via Clipart Panda. http://www.clipartpanda.com/clipart_images/sun-clipart-medium-5602503.
Figure 8-1. Thermometer. n.d. via Teacher’s Files. http://www.teacherfiles.com/clip_art_thermometers.htm.
Figure 8-2. Refractometer adapted from CEphoto, Uwe Aranas [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], 29 Oct 2013 via Wikimedia Commons. https://commons.wikimedia.org/wiki/File%3APortable-Refractometer-03.jpg.
Figure 8-3. By User: Mysid (Own work) [Public domain], 1 Nov 2005 via Wikimedia Commons. http://upload.wikimedia.org/wikipedia/commons/0/0b/Secchi_disk_pattern.svg.
Craig, P., Birkeland, C., & Belliveau, S. (2001). High temperatures tolerated by a diverse assemblage of shallow-water corals in American Samoa. Coral Reefs 20: 185-189.
Dubinsky, Z. & Falkowski, P. (2011). Light as a source of information and energy in zooxanthellate corals. In: Dubinsky, Z. and Stambler, N. (eds.) Coral reefs: an ecosystem in transition, Springer, Dordrecht.
Lirman, D., Orlando, B., Marcia, S., Manzello, D., Kaufman, L., Biber, P., Jones, & Tahzay, J. (2002). Coral communities of Biscayne Bay, Florida and adjacent offshore areas: diversity, abundance, distribution, and environmental correlates. Aquatic Conservation: Marine and Freshwater Ecosystems 13(2): 121-135.