In general, species such as stony corals require free space to settle and grow. While this may sound easy in principle, free space—whether it is on the ocean floor itself or on top of another organism—is an extremely limited resource in the marine environment. As a result, species often compete with each other or exhibit aggressive behavior to secure or maintain a given plot of substrate. [a]
Indirect encounters between corals are considered competitive strategies to compete for space and most often involve overtopping. [a] This method is primarily utilized by fast-growing branching species. Without directly interacting with neighboring colonies, a branching species can outcompete its stout, slow-growing neighbors by virtue of its upright, faster growth rate. The underlying corals suffer light deficiency, affecting the ability of their zooxanthellae to conduct photosynthesis, and also causing the underlying coral to come into contact with fewer food particles. [b] Over time, overtopping by fast-growing species can kill the slower-growing species underneath. [c] more...
To survive and thrive in a space-limited environment, some corals have developed several specialized mechanisms for direct interaction for self-preservation and competition with other corals and other organisms for space in an ecosystem. The most common aggressive mechanisms include sweeper tentacles (detect and damage adjacent coral colonies), mesenterial filaments (enabling external digestion of neighboring colonies), and terpenoid compounds (coral chemical warfare). [d] These methods are not necessarily mutually exclusive as some species of corals employ each of these mechanisms in concert to prevent losing their space on the reef. [e] It has been estimated that, on the reef, between 22 and 38 percent of all coral colonies are engaged in battle or are within range to engage. [f]
Sweeper tentacles are the most common defense mechanisms in the hard corals, and also occur in some soft corals. Specialized stinging cells called nematocysts are present in these tentacles and can attack a competing coral and literally burn it to the point of either killing it or severely damaging it. The length of these sweeper tentacles is not correlated to the length of the normal coral polyp and may, in fact, be many times longer. [e] Sweeper tentacles are also utilized by some species to detect adjacent coral colonies that are encroaching on it.
In addition to sweeper tentacles, several hard coral species can produce mesenterial filaments from their stomachs (corals of the genera Favia, Favites, Scolymia, Pavona, and Cynarina all have this capacity). [g] These filaments can kill or devour other coral polyps through a process similar to digestion. Some corals even have the capacity to produce both sweeper tentacles and mesenterial filaments, enabling them to fight a battle on several fronts. [e]
Soft corals generally compete with the hard corals by conducting chemical warfare; they release terpenoid or sarcophine compounds into the water to injure or impede the growth of neighboring corals. Like their name implies, these compounds are similar to turpentine in chemical structure and in most instances, are just as toxic. These chemicals also prevent other unwanted organisms from settling on or near the corals that produce them. [e] By releasing these compounds, the soft coral injures neighboring stony corals and can thus grow above them, eventually blocking out the light that they are both dependent upon and thereby killing the underlying hard coral. The process of influencing the growth and development of other organisms through use of chemical compounds is called 'allelopathy.' [h] Like their name implies, these compounds are similar to turpentine in chemical structure and in most instances, are just as toxic. These chemicals also prevent other unwanted organisms from settling on or near the corals that produce them.