Alright, so now I have your attention with the headline, I promise there will be corals attacking in this post! There will also be less aggressive aspects of life as a coral.
The polyps, once attached to a surface, grow by depositing calcium carbonate (CaCO3) at the base and sides, whilst the living part of the coral is at the top of the reef.
So, how do corals feed, why are they these awesome colours, and what do they do apart from sit there looking very pretty!
The majority of the energy used by a polyp is produced via photosynthesis. This is something that is most associated with trees, flowers and other green plants. There are some animals which use photosynthesis, but they cheat a bit, and use chloroplasts (The cells which are responsible for photosynthesis) from algae which they eat. The ones we know about which do this are in a group known as Sacoglossa, or endearingly, sap sucking sea slugs. There is a brief New Scientist article on them here and a link to the paper referenced in the New Scientist article is HERE in case anyone wants to read further on it.
So, since coral polyps are not sap sucking sea slugs, or plants, how do they perform photosynthesis? In a previous post I linked some pictures showing a coral egg with something called Zooxanthellae in it. These are tiny micro-organisms which live in symbiosis with the polyp. It is not certain how mutual the benefit is, but, from the perspective of the coral, there is a great benefit to these organisms living within it, and the Zooxanthellae gain a benefit from being protected within the coral.
Why do corals get a big benefit? Because the regions in which we find coral reefs are waters which are low in nutrients, and the photosynthesis provided by the Zooxanthellae enable the corals to survive in this environment, and this in turn leads to the coral reef areas being among the most productive regions of the ocean, despite having very low nutrient availability.
This symbiosis between the coral and the Zooxanthellae is also the reason why we find coral reefs in very shallow, clear waters, as the organisms need sufficient light to perform photosynthesis. This however, causes its own problems: Exposure to high levels of sunlight also means exposure to UV which is harmful, and could damage both the coral, and the Zooxanthellae. It appears that some corals have a defence against this, they are able to absorb UV at certain wavelengths. This means, that despite having a bright white skeleton, which would usually reflect back the light, some corals absorb UV light in their skeletons, whilst reflecting back the light in the wavelength used for photosynthesis (known as PAR, or photo-synthetically active radiation). So, instead of getting a double dose of incoming UV radiation, the coral skeleton protects the tissues by absorbing these wavelengths. The result is, that under UV light, the corals fluoresce, as the image below shows.
This flourescence is usually yellow, but there are some striking examples of other colours, as shown below.
So, apart from all this photosynthetic goodness, what else do corals do to get nutrients? Well, as they are Cnidaria, they have stinging cells which can fire, so they also hunt, well, hunt as much as a stationary organism can! The video below shows corals preying on passing small animal at night, and also leads nicely onto the final part of todays post, and the promised coral attacks!
What the video does not mention, and I have tried to find a clip covering it, but have not been able to, is that as well as firing stinging cells at each other, the video (at around 1:51), shows corals extruding digestive filaments. Yes, this is as gross as it sounds, they are effectively turning parts of their stomach inside out to digest the other coral! I have seen footage of this a number of times now, and it still strikes me as one of the most fascinating, yet gross things I have seen.
If you want to see more about coral reefs, and the awesome creatures which live among them, and the amazing connections which these organisms form with each other, I highly recommend watching David Attenborough, Blue Planet, Episode 6, Coral Seas (BBC).
There are some fascinating creatures which find shelter within the reefs, and some of these provide direct benefits to the reef, such as the guard crab, a small (less than 5cm) crab which helps defend the reef from a large (over 50cm) starfish (the crown of thorns starfish) which feeds on the reef.
Then there are the fish which eat the coral, the bumpheaded parrot fish being one, as shown in this excerpt from Blue Planet
Even though everyone knows about the amazing colourful fish which live in reefs, for me the most interesting parts are the bits I have briefly covered above, the bits that not many people get to know about, which happen at night. I hope I have managed to show these in a clear way, and if you get the chance,do check out the Blue Planet episode I mentioned, it is really worth it.
References (direct links to the journal articles :
“Coral Skeletons Defend Against Ultra-Violet Radiation” Reef, Kaniewska, Ove Hoegh-Guldberg: http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0007995#pone-0007995-g002
“Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica” Rumpho et al: http://www.pnas.org/content/105/46/17867.full.pdf+html