When thinking about corals, probably the first thing to spring to mind is their beauty, color and in some cases their tranquility. As they have a soothing and calming effect for such a beautiful and tranquil structure growing and living in an environment that is constantly at war with itself with rough oceans and seas constantly changing around them. They are a steadfast in an ever shifting and churning world.

However what probably does not come straight to the forefront of your mind is that corals are constantly waging war upon each other for space to grow by engaging in coral cannibalism. Corals also fight not only for space but for self preservation against an algae Chlorodesmis fastigiata that can carry harmful and life threatening bacteria. Corals in this case of self preservation will release a chemical response, signalling for help against a threat. The chemicals released would attract species of fish called Gobies, which would help clean the surface of the invasive an threatening algae. Enabling coral metabolic rate to recover.

Corals weapon arsenal

Corals despite their stationary  state have developed quite the defensive mechanisms and arsenal against attacks, and also to provide attacks of their own to advance themselves across the reef.

  • Sweeper tentacles – The most common defense mechanisms in the hard coral species. These
    Nematocyst batteries on the tentacles of a large Cynarina polyp. Great Barrier Reef, Australia. Photograph: Charlie Veron
    Figure 1- Vibrant nematocysts on the tentacles of Cynarina polyp found on the Great barrier reef Australia. photo credit: Charlie Veron. (source)

    mouth-less elongated tentacles form the outermost portion of the coral colony and act as a sentry patrol along the periphery. When a sweeper tentacle comes into contact with an opposing and competing coral, it may attack and and cause burning towards the competing coral to the point of killing it or causing serious damage. The burning sensation achieved by the sweeper tentacles is the result of specialized stinging cells called nematocysts. Corals may rlease their sweeper tentacles if a coral is placed in close proximity of another aggressive coral.

 

  • Corals such as Scolymia, Pavona and Cynarina all have the ability to produce and release mesenterial filaments. The mesenterial filaments are the internal tissue folds that create
    Figure 2 – Acanthastrea echinata secreting its mesenterial filaments to attack a neighbouring brain coral. (source)

    structure within a coral polyp’s body so essentially spill their guts quite literally. They contain a large quantity of nematocysts, which enable the coral to capture and kill prey within their filaments. The mesenterial filaments are used by corals to attack and digest coral competitors creating more space for them to expand and increase in size. Allowing for new polyp’s to take the place of the recently deceased coral that it was neighbouring.

 

 

 

  • Terpenoid compounds is a soft coral mechanism used to compete against hard corals. The compounds are released into the water to injure or inhibit growth. Once the chemical compounds have taken effect on the hard corals the soft corals can grow over the inhibited individuals, this process is called “allelopathy“. By growing over the hard substrate corals in their weakened state, it prevents light from reaching the coral which they are dependent on for respiration, so overtime kills the hard stony coral.

 

Other threats to corals

Outside of coral space competition which sees corals maiming and killing one another for space and resources. Other far more detrimental threats are out there which devastate coral reefs in far more accelerated process.

Coral bleaching is the fast and upcoming threat devastating the worlds corals. It is caused by increased water temperatures of 1 degree Celsius or more. This temperature causes the coral to expel the zooxanthellae living within their tissues, turning them completely white. corals subjected to bleaching can recover but they are under far more stress increasing chances of mortality.

Hurricanes and Cyclones although only occur at specific times of the year between the months of August and November, can have some drastic and permanent consequences on corals. As offshore structures they provide some protection against wave action preventing the full energy force of Hurricane strength waves from hitting the shore which could cause major damage to towns or villages located near the coastline. An example of coral reefs acting as a wave barrier, was the coral reefs located in Discovery bay Jamaica, that got decimated by Hurricane Gilbert in 1988. Projects run by the neighboring marine lab which sits within 300m of the remaining coral reef crest, aim to rebuild the coral reef system there. They employ techniques such as using a calcium carbonate exoskeleton structure being fastened to the floor enabling a fixed structure for polyp’s to build and colonize on.

Figure 3 – coral reefs at shallow and mid depths found in Discovery Bay. 10 years apart from each other. 1988 photographs shows the devastation carried out by Hurricane Gilbert. Both top and bottom photos were taken from the exact same locations. photo credit : Bob Steneck. (source)

 

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