Thursday, February 23, 2012

Are Corals Alive?

My boyfriend and I had been meaning to visit the Royal Ontario Museum for months now. We finally went, after they just recently made Tuesdays free for post-secondary students (and those who still have a student card). We mostly loitered in the natural history section, looking at the taxidermy and animal replicas. One of my favourite things there was a large saltwater aquarium that housed a beautiful selection of reef fishes, invertebrates, and corals. As I was taking in the view, someone beside me kept repeating the question, "Are corals alive?" to which his friends had no answer.

I've noticed repeatedly that the coral is something most people know very little about. To be honest, before my second year invertebrate zoology course, I wasn't too sure what a coral was either. Since finding out, I now think it's one of the most fascinating lifeforms on the planet, and I'd love to explain what exactly a coral is.


Corals are very closely related to jellyfish (and even closer related to sea anemones). They belong to the phylum "cnidaria," a group characterized by having stinging cells called nematocytes. Actually, cnidaria in Latin means "nettle-like," from "knide" (nettle) and "aria" (like).

Cnidarians come in two body plans: swimming medusa (B), and the sessile polyp (A). A jellyfish is an example of the former, while a coral the latter. Think of a coral as a jellyfish that cannot swim around, and plants itself the bottom of the sea. That's essentially what it is.


Unlike jellyfish, corals live in colonies. When you talk about "coral," you're really talking about a colony of several individual sessile polyps. When you say "coral reef," you're actually talking about a colony of polyps that secrete calcium carbonate to form a hard skeleton. This skeleton offers structural support and a place for polyps to retract their soft bodies when threatened.

Just as trees grow a new ring each year, reef-building corals grow a new layer of calcium carbonate skeleton each year. It can take hundreds of years to build a coral reef. The largest coral reef in the world, The Great Barrier Reef, is an estimated 6000-8000 years old. Just think about that for a minute.


Most corals are symbiotes. They harbour tiny photosynthetic protozoa called zooxanthellae. The zooxanthellae offer valuable energy, while in return the coral offers a home, physical protection and carbon dioxide and nitrogenous waste. Corals can also capture prey, such as plankton and small fish, by immobilizing them with their stinging nematocytes.


Corals can have one gender or two (gonochoristic or hermaphroditic), and can reproduce sexually and asexually. Most sexual reproduction happens by releasing eggs and sperm synchronously into the environment. Asexual reproduction happens by either splitting a small polyp from an adult (budding), or by splitting the colony (fission). Actually, coral reproduction gets a lot more complicated than this, but I won't go into any more detail now.

Actually, corals in general are a lot more complicated than what I've talked about here. I just wanted to explain their basic biology, and to make it clear that corals are indeed very much alive.

Photographs from: National Geographic.


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