In this lecture, we will focus on coral reef ecosystems, beginning with some very basic questions such as “What is a coral reef?”
We will then explore the different types of reef systems and will take on board a series
of definitions that are associated with the different types of coral reefs.
This will lead to a discussion about the distribution and abundance of coral, particularly the physical
and chemical variables that determine where carbonate coral reefs flourish.
You will learn as part of this that there is a great variety of coral reefs – from those
that are growing at the limits of light penetration to those that have adapted to live in the warm,
shallow and highly sunlit waters of the tropics. This information will allow you to address
the goal of being able to describe the factors which drive the distribution and abundance
of healthy coral reef ecosystems. Let’s begin with the first question – “What
is a coral reef?” In the broadest definition, coral reefs are benthic ecosystems which are
typified by a dominance of Scleractinian corals. As we will develop later, there are two major
types of coral reef communities. Firstly there are reefs where calcium carbonate has built
up over time to create reefs and islands like the one here on Heron Island on the southern
Great Barrier Reef. Reef-building corals belong to the order Scleractinia
within the class Anthozoa. The corals we are referring to here belong
to the simplest multicellular phylum of the animal kingdom – the Cnidaria.
As you know from previous lectures the Class Anthozoa includes organisms such as
sea anemones, sea fans and corals. Here is a cross-section of a polyp.
The gastrovascular cavity is lined with a digestive tissue known as the gastrodermis – which is
one of only two tissue which are typical for these simple organisms.
The other tissue is the ectodermis, which covers the external surface of the polyp.
There is a single opening which functions as both a mouth and an anus into that cavity.
When the polyps are feeding, they capture particles using their tentacles and pass them
to the gastrovascular cavity through the oral opening where they are digested and absorbed by the gastrodermis.
These animals have a variety of growth forms – with the polyp being the most widely found
structure. Polyps have a very simple body plan – with a ring of tentacles in multiples
of six or eight around the mouth. Now within the tissues of corals, there is
a little bit of magic and this takes the form of tiny single-celled algae known as dinoflagellates.
These tiny separate organisms live inside the cells of the gastrodermis where they
are able to trap sunlight and provide their animal hosts with abundant photosynthetic
energy. We are going to come back to this amazing mutualistic symbiosis later in this
lecture. Now take a break now and familiarise yourself
with the differences between the two coral-like organisms: the Hexacorallia and
the Octocorallia. When you feel you have mastered this difference, begin the next section of