To avoid becoming a squishy heap on the ground humans have a skeleton. Quite a good invention really, not having a skeleton would cause quite a few difficult problems.
Humans aren’t alone with the possession of a skeleton of course; there are all sorts of life forms that have one too, how about the elephant or man’s best friend the dog as just two examples.
Skeletons are not essential to life however; there are creatures that don’t use one at all. As a non-scientist I can say that they use a different method to maintain their body shape, I’m not sure how a scientist would put it. For example a shrimp has a hard outer shell rather than an inner skeleton.
Vertebrates, those that have a skeleton, are vastly outnumbered in the world. It is estimated that vertebrates are around 3% of the life forms in the world. It follows that a massive 97% (I just had to demonstrate my mathematical abilities) are invertebrates.
There are land dwelling insects such as spiders and worms, examples of well known invertebrates. In the cold sea there are huge numbers of plankton, anemones, sponges etc. The warm seas contain many better known (to the marine aquarist) invertebrate species, including filter feeding worms, shrimps, corals and sometimes very large anemones. It is with some anemones of course that the representatives of the vertebrate/invertebrate world have combined forces to mutual advantage; these are the clown fish with their selected hosts. There are other examples of this life style.
Invertebrates are very adaptable as demonstrated by the discovery of volcanic activity deep in the sea well beyond the sun’s light. Scientists used to accept that life basically depends on the availability of light from the sun, an example of which is that plant life uses the sun’s light to photosynthesize and grow, and then the plant life is eaten by a herbivore from which energy is obtained. Herbivores are then the prey for carnivores. They all basically depend on the plants and the sun. This belief has been changed by the life found near this deep volcanic activity, including filter feeding worms, mollusks, bacteria and crabs which depend not on the sun’s light but on heat and chemicals.*
Anyway marine aquarists, or rather those who maintain a reef system, keep various types of invertebrate including shrimps, corals, sponges, anemones, filter feeding worms etc. These very interesting life forms only ask for a few things for success, including high quality seawater, for the majority of corals and some others sufficient light of the correct spectrum, space to grow and freedom from predation. It is usually important to place for example corals in a position where they receive sufficient seawater flow.
It is worthwhile noting that all marine aquariums, big or small, fish only or reef, depend on invertebrates for their health and life. The bio-filtration system, whether this is live rock or canister filters, contains bacteria that deal with the toxins ammonia and nitrite. Without these bacteria the life in the aquarium would die. With live rock, further bacteria should be able to deal with the sometimes troublesome nitrate.
Those aquarists who make use of natural live rock could discover that they have invertebrate life forms in their aquariums that they didn’t import themselves or at least not intentionally. Some such as filter feeding worms could be welcome. Others such as the Aiptasia anemone are not welcome. The Mantis shrimp is another, though in this case it is worth keeping but needs re-housing to an area where problems won’t arise.
Invertebrates provide the aquarist with a chance to create a living reef at home, be this large or small, though compared to Mother Nature’s creations all home reefs are miniscule. Nevertheless, a successful home reef is wonderful to see.
(* Reference: Marine Invertebrates. Martyn Haywood & Sue Wells)