As a newbie to marine biology, I like to spend time on the internet finding out what dinoflagellates, marine angiosperms and anthozoa are. Most of the things I learn are too difficult to remember at once, so I keep notes and take online courses. If you’re interested, you can learn along. This time some basics about coral.
What is a coral reef?
A coral reef is a benthic ecosystem dominated by scleractinian corals. Benthic meaning the reef lives in contact with the bottom/substrate. Scleractinian corals are corals that build a hard skeleton during their lifetime. Apart from these stony corals, there is also another type of coral living on the reefs (or in close proximity): soft corals Alcyonacea.
Corals composed of polyps
A coral is a group or colony of identical polyps. Each polyp is only a few millimeters big. In a stony coral, when a polyp dies his skeleton remains and a new polyp will start to grow on top. A polyp looks like a flower, but it is an animal (of the phylum Cnidaria).
Let me explain the basic anatomy of a polyp. See my drawing above (:
The polyp catches particles with its tentacles and passes it through the mouth to the gastrovascular cavity. The gastrovascular cavity is the primary digestive and circulatory organ of corals (as well as in e.g. jellyfish and worms). The cavity is lined with the gastrodermis, which is a digestive tissue absorbing nutrients. The other tissue is the ectodermis, on the external surface of the polyp.
Polyps feed on plankton they catch with their tentacles, making them particle feeders and thus consumers. But the most important to know about polyps is that they live in symbiosis with dinoflagellates. Dinoflagellates are eukaryotic organisms, in this case, plankton – they are plants! The dinoflagellates live inside the gastrodermis, providing the polyp with photosynthetic energy from CO2 and H2O. In return, the inorganic nutrients broken down by the polyp are not going to waste but provided to the dinoflagellates – symbiosis! This makes a polyp also a producer.
Examples of coral
There are two types of coral.
- Octocorallia: organisms formed of colonial polyps with 8-fold symmetry.
- Hexacorallia: colonial polyps with 6-fold symmetry.
That’s it for now. A lot of information, also for me. Bye!
- Find Out: about corals
- EdX online course Tropical coastal ecosystems, University of Queensland