Welcome back to the world of biodiversity. Tonight we will pick up where we left off two weeks ago - taking a brief tour of the Great Barrier Reef. Last time we talked a bit about the ecology of the reef and about some of the fishes that live there.
Here's the reef at the surface reminding us of how vast it is.
The reef habitat is largely comprised of corals. Corals are animals, closely related to sea anemones. Their body is a polyp, basically a disk on top with a mouth in the center surrounded by tentacles.
This photo from part one (two weeks ago) shows that a coral is colony of polyps.
Another photo, also from two weeks ago, shows the scale of structure that can be built up over time by coral. Each individual coral secretes a calcium carbonate home for itself, and these build up to form the immense and varied environment of the reef.
This would be a good time to expand upon the mutualistic relationships within the coral. Inside of each polyp are organisms known as zooxanthellae which perform photosynthesis. The zoxanthellae belong to a group of single-celled organisms known as Dinoflagellates. Mostly they are roughly spherical organisms that when free-living move through the water in a tumbling motion because their two flagellae (whip-like organs like the tail of a sperm) are perpendicular to one another.
Many dinoflagellates can perform photosynthesis but their chloroplasts vary considerably and resemble those in several different groups of algae. This indicates that the dinoflagellates obtained their chloroplasts indirectly through mutualisms with algae (which in turn originally evolved chloroplasts due to mutualisms with cyanobacteria). So a coral organism is really a 4 layer mutualism!
Corals come in a wide variety of shapes and colors. There are hard corals and soft corals.
But reef invertebrates are a lot more than just corals. Here is a close relative of the corals, a sea anemone. It is more or less a larger polyp that lives individually or in a small colony. This is a pretty large anemone - all you can see are the tentacles, the rest of the body is hidden. Both corals and sea anemones are related to jellyfish and all are part of a group known as the Cnidaria which is highly distinct from other animals. One characteristic of this group are the nematocysts, structures inside cells on the tentacles which are microscopic harpoons. They are coiled up and are violently released when the appropriate cue arrives. They stab into objects and inject toxins. In most species they are unable to penetrate human skin and are harmless but some jellyfish are extremely dangerous.
Another group that is very popular with reef tank enthusiasts are feather duster worms. These are tube dwelling worms who have structures that are extended into the water for respiration and filtering food from the environment.
Below is one of the signature animals of the indo-pacific region - a giant clam. It was the sort of organism that I had read about since I was a child but never really expected to see. This photo shows inside the shell and the siphon which is part of the system moving water through the animal and filtering out food.
The rest of the organisms in the diary are ones that are, believe it or not, more closely related to ourselves. The next two are echinoderms, part of a large group of animals that is only found in the ocean. They are an example of radical evolution of the body. Although they are not closely related they have re-evolved the radial symmetry of the cnidarians we discussed before. Radial symmetry means that their body is organized like a pie or cake - it can be cut into equal halves in many planes. Most animals have bilateral symmetry with only a single plane dividing the body into equal halves.
Familiar echinderms include such things as starfish (which should really be called sea stars), sea urchins, and sand dollars. Here we see a feather star again filtering food from the sea.
Below is another echinoderm, a large sea cucumber our guide plucked off the bottom. It's hard to tell in this picture but it was about two feet long. Sea cucumbers are noted for their ability to disgorge their entire digestive system when threatened (not just the contents but all the organs). They can grow new ones. Note the surface covering that looks like a mass of sea stars. The five piece radial symmetry is a general characteristic of this group.
Finally we have a tunicate or sea squirt. This, believe it or not, is a close relative of the vertebrates. The larval stage resembles a tadpole and has structures developmentally equivalent to the formation of a spinal column. However as an adult it loses all of that and becomes the ecologcial equivalent of a sponge. These are fairly common animals in many oceans but I had never seen or heard of one so garishly colored before.