It would have been a waste to have a molecular coral biologist in my home for a week without embarking on a field trip. So, with that in mind, Chloe Boote (PhD Candidate, James Cooke University) and I packed our diving gear and set off for Coral Bay, which is quite possibly the most beautiful location in Western Australia. Located 1200km north of Perth, it is home to the Ningaloo Reef, which is a popular diving and snorkelling site and home to a diverse array of marine fauna and flora. The most popular reasons for visiting are to check off popular bucket-list items like swimming with manta rays, humpback whales and whale sharks. However, the coral reef itself is a large attraction, and the reason we made the 11.5 hour trek.
Probably the most fascinating thing I saw during our dives was two different colour morphs of the same coral species located side by side. We believed these to be Acropora cervicornis or “stag horn coral” because of the obvious branched structure.
The science behind this biological spectacle involves “polymorphism,” which is the occurrence of two or more different forms of a single species. These alterations can occur due to genetic deviations, environmental cues and/or random variations. Colour-morphism is a subdivision of this phenomenon found in coral.
Still with me? Good. Moving on.
Moving on to the tissue of healthy coral, we need to consider unicellular photosynthetic dinoflagellate called zooxanthellae. These are micro algae. The relationship between them and coral is an extremely important symbiotic relationship. The coral provides a protected environment, carbon dioxide and various organic nutrients and, in return, the zooxanthellae expel an abundance of oxygen and carbohydrates. Zooxanthellae can provide coral with as much as 90 percent of its energy. Active feeding, i.e. grazing, supplies the other 10 percent. Additionally, zooxanthellae are what gives the coral its vibrant colour.
Zooxanthellae usually belong to the genus Symbiodinium; however, some are not and are therefore often identified by clades “A – E.” A clade can be defined as a “taxonomic group of organisms classified together on the basis of homologous features traced to a common ancestor.” These clades are different in several ways and this can be advantageous to coral species during different global or local threats. Common differences include tolerance to thermal stress, photosynthetic productivity and efficiency, and pigmentation. Pigmentation of zooxanthellae becomes obvious once a dense cluster of this microalgae is held within the tissue of the coral. Therefore, coral can be found in different colours due to the clade of zooxanthellae captured from the water column and held within the tissue.
A Broken Reef
The above image looks a little like the aftermath of a battle, especially when compared with a ‘near-pristine’ Indonesian coral reef (below).
Coral formations can take tens of thousands of years to form. It is especially upsetting when you consider that coral is an animal, and not a plant. The most common culprits when it comes to the destruction of coral formations are storms and tourism.
Storms are a natural phenomenon and occur regularly in Coral Bay. Coral are often able to recover from these events; however, the frequency and intensity of these storms are increasing due to the impact of anthropogenic global warming (keep a look out for an upcoming article to learn more about global warming).
Tourism is an imminent and direct threat to the local health of coral reefs. Increased traffic of boats and people often cause physical damage to coral. Remember that because of the time taken for formation, minor damage repair may take many years of growth! Additionally, chemical pollutants, for example, from motorboats or even standard sun cream, can impact coral formations. To minimise chemical damage from sun screens, it is advised that we wait 20 minutes after application before entering the water.
Increased sediment in the water column may sound like a trivial matter, however when the health of coral is dependent upon the productivity of its photosynthetic companion, zooxanthellae, you can start to understand why this may become an issue. Suspended sediment is a source of light limitation for marine flora and fauna. As light enters the water column it is scattered by the particles suspended with the water. The intensity of this light reduces with depth and the concentration of suspended matter.
The high level of suspended sediment seen within the water column will likely reduce the light availability for the coral (and zooxanthellae). This may cause a reduction in productivity for both symbionts. A reduction in photosynthesis from the microalgae will reduce the energy supply to the coral. This will likely reduce the growth rate of the coral which in turn will provide less shelter for the zooxanthellae.
Other issues derived from increased sedimentation are:
– Smothering: Too much sediment will mean that the coral are not obtaining sufficient energy and will die.
– Terraformed substrate – The settlement of high concentrations of sediment will produce a soft, unconsolidated seabed. Larvae of most coral require a hard surface to settle and begin development. A soft seabed will limit the ability for coral to reproduce.
“Coral Bommies” (pictured above) are stand-alone structures located off the main reef and closer to shore. They are commonly dubbed “refuge areas” as juvenile fish and many other marine organisms seek refuge within these structures when the main reef experiences severe forms of stress and/or damage.
They are also capable of restoring damaged reef populations through “reseeding.” Using the reproductive methods of coral, they can deposit coral larvae with slightly different genetic material into the main reef area. This new, genetically diverse coral often stands a greater chance against the initial threat to the reef.
A Hostile Takeover
Like many animals, corals are in competition for food, land, etc. Most animals in the wild will fight others to get the desired reward and coral are no different.
The above image contains two species of hard coral, a larger bulbous coral, possibly from the Poritidae family, and a smaller plate-like coral, possibly of the Agariciidae family. The two types of coral are trying to grow over one another, with the encrusting plate-like coral growing over the sub massive colony. This shows how intraspecific competition occurs on a reef when viable living space is limited. The coral must use any opportunity available in order to thrive.
Fighting is a nocturnal activity, so at night you would see the coral mesentrial filaments – string like extensions of the internal folds of tissue – extent and try to digest and absorb one another by secreting toxins and enzymes that break the opposing coral down. What is visible of this conflict during daylight hours is a white band seen between the two species, where the fighting occurs. This is commonly termed ‘no-man’s land’, as neither coral owns this land yet. These battles can become epics, prolonging for 100’s of years.
This image is an example of a well-known process of coral called ‘bleaching’. Bleaching is a consequence of a stressed holobiont – host plus all microbial symbionts – and is also a common short term survival tactic exhibited by all species of coral. However long periods of bleaching, as the media has shown of the Great Barrier Reef, are dangerous and life threating.
When water is too warm coral will expel its endosymbionts from within their tissues. This occurs as the heat stressed zooxanthellae start to produce and release a chemical, ROS, from within the coral. This chemical is toxic to the coral host and as a result, leads to the expulsion of the zooxanthellae from the tissue of coral.
As previously explained, this microalgae carries the pigmentation often associated with coral and therefore the structures turn white, therefore it has bleached. We mentioned before how important the photosynthetic waste products of zooxanthellae are to the growth and respiration of coral, up to 90% of energy stores. Now without this symbiotic relationship all their energy needs have to come from active feeding or energy stores, an improbable feat for these small animals. Without the precious symbiosis the coral will begin to starve
Most coral can only survive a short period without their symbiont and must reabsorb a species in the immediate future. This is possible when the cause of bleaching is a period of natural phenomena, such as the El Nino Southern Oscillation. However when unprecedented and unnatural global warming occurs for long periods (and as a result larger, longer and more frequent ENSO events – published in Nature 2014), then these coral will begin to die. A single degree makes a huge difference.
Hopefully this coral hasn’t gone past the point of no return and can still recover.