Everybody has a pretty good feeling for what a coral reef is. It is a reef where lots of corals grow, right! And there are lots of fish there as well. But what are corals, and what else lives on coral reefs? The fact is that coral reefs are among the richest ecosystems on planet earth. Seventy percent of the planets surface is occupied by oceans, but only 0.1 percent of the ocean floor is occupied by coral reefs. Yet, these amazing ecosystems account for about a quarter of all known marine life forms, providing homes for a majority of the Phyla (major groups) of animals. A coral reef tank aims to emulate a real tropical reef, albeit at such a miniscule scale that only some of the organisms can successfully be kept there.
Even so, the diversity of animals in a coral reef tank can be astounding. When you look at the UNBC coral reef tank , you see the many corals (Phylum Cnidaria), a fair number of fish (Phylum Chordata), some shrimp and hermit crabs (Phylum Arthropoda), brittle stars, sea urchins and if you look carefully a sea cucumber (Phylum Echinodermata) and some snails (Phylum Mollusca). That is five of the 30+ Phyla of multicellular animals that you can see without making any effort whatsoever. If you look a little closer, you will also see sponges (Phylum Porifera), and several types of worms (Phylum Annelida), e.g., a Hawaiian feather duster fan worm and cryptic bristleworms that may come out at night, and on some of the corals you will see small acoel flatworms (Phylum Acoelomorpha), distant relatives of the true flatworms which include flukes and tapeworms. Take some samples and look under a microscope, and now you will find roundworms (Phylum Nematoda) and rotifers (Phylum Rotifera). That takes us to ten Phyla, or 1/3 of all existing types of animals living in that tiny space. If we add the various protozoans and more cryptic organisms that lurk in various crevices, the diversity becomes even more impressive of course. And within each Phylum there are many different sub-groups (Classes) of animals, e.g., sea urchins, sea stars and sea cucumbers represent different Classes of this Phylum. Finally, there are organisms that are not animals, e.g., diatoms, as well as green and red algae, and various bacteria that are all critical for the well-being of a reef aquarium. Keeping this system in balance is the trick in maintaining a healthy system. Because of the small volume of water, this can be difficult at times, with even relatively minor perturbations causing chain reaction leading to a more or less complete crash. Usually such events are caused by a drastic water quality change.
In order to establish such a rich environment in a reef tank, a key component is the use live rock. This is coral rock that has been harvested from, or placed in a natural environment to allow organisms to occupy them. The organisms that are deliberately introduced into a reef tank generally come from the frist five Phyla mentioned above. All the rest come in with live rock. Live rock is critical for establishing the proper balance in a reef tank, but on occasion undesirable hitch-kikers may be introduced. That is a risk one has to accept, however.
One of the key features of a coral reef is the close interaction between different types of animals. The term for this is symbiosis, which essentially means “living together”. Parasitism is one type of symbiosis, where the host is harmed by the parasite, which gets some kind of benefit from the association. The acoel flatworms may be parasites on mushroom corals, but it is unclear if they actually harm them or simply live on them, in which case they would be called commensal symbionts. Most of us associate the type of relationship that exists between clownfish, or anemone fish as the are also called, and the sea anemone as an example of symbiosis, and this is called mutualism, because both the host and the symbiont derives a benefit from the association. But there are a number of fascinating relationships that may not be that obvious unless you know what to look for, and I will describe some of these in upcoming blogs.