What Is A Marine Invertebrate?

Invertebrates are very important to marine aquarists, particularly those that own a reef system.

The word invertebrate covers all those organisms that have neither an internal skeleton nor a backbone. They live on land and in the seas.

There are around 2,000,000 different species on the planet and in the region of 97% of these are invertebrates*. Invertebrates that live on dry land include those that are well known to us such as worms that are often dug up by the gardener and spiders who weave there webs in many places.

Common examples of marine invertebrates include well known life forms often kept in aquariums such as shrimps, crabs, anemones, worms and sponges. In the seas and oceans the invertebrate is everywhere, not only on the wild reefs in warm areas but in cold and very cold areas as well. These invertebrates can be minute to very large.

Invertebrates that have a hard exterior shell, such as the very popular shrimps, have a ready made defence somewhat like a knight of old. It gives a level of protection against predators. The battle between predator and prey is ongoing, and defences need to be updated (over very long periods) as the predators adapt to overcome the defence strategy.

It would seem that hard-shelled invertebrates have developed an excellent defensive system without many drawbacks. The defence is not totally adequate because of the constant danger from predators as mentioned, and there is another major disadvantage. Living things need to grow, and the hard skeleton of, for example, a shrimp is fixed in size. So there is only one way that the shrimp can obtain the room to grow, and that is to get rid of the current shell. This is done fairly regularly and once the old shell has been abandoned there is a new one of larger size beneath. Unfortunately the new exoskeleton is soft and time is required for it to harden. During the hardening period the shrimp is extra vulnerable and needs to hide away in a secure spot until hardening is complete.

In order to produce a good exoskeleton the shrimp (just to use the example above) needs to draw on the seawater for some materials. Fortunately, they should be available in reef aquariums where the aquarist is already maintaining adequate calcium and other levels, and a problem shouldn’t arise.

Finding the abandoned skeleton of a shrimp shows how remarkable the process of creating a new exoskeleton is, for the abandoned one has all the details of a living shrimp. This includes the long thin antennae and legs. The abandoned exoskeleton can appear a bit ‘floppy’ though which gives the game away.

I remember keeping an aquarium years ago that among other things contained a large anemone and a clownfish. A cleaner shrimp was also resident. One morning I approached the aquarium and was horrified to see that the anemone had apparently got the shrimp. It was amongst the tentacles, perfect but lifeless. I was quite irritated even though I knew that the anemone had predatory abilities. I visually searched the aquarium and there wasn’t a sign of a shrimp, and was actually debating whether to obtain another. Later, as I came to the aquarium to supply food, there was the shrimp sitting on its normal rock. I did end up getting another so that the original had a ‘friend’, they are better off with more than one.

Just out of interest, science thought that sunlight is necessary to all life forms but has had to amend that view. As has been shown on television, very deep in the sea beyond light penetration there are a few small areas of mild volcanic activity. They produce ‘chimneys’ from where the volcanic output escapes, not lava but a stream of heat and minute debris that looks like smoke. Living near these ‘chimneys’ are crabs, filter feeding worms, shrimps and sponges. They have adapted to the alien world so far from the surface and have no requirement for sunlight. The sun’s energy is replaced by the emitted heat and chemicals.

If the aquarium is suitable (this really means a captive reef though a fish only system is suitable dependant on seawater quality, fish inmates and the aquascaping), invertebrates make excellent interesting inhabitants. Many such as cleaner shrimps are commonly available. They usually require very careful acclimatization to the home aquarium seawater and must not be exposed to air.

(*Reference: Marine Invertebrates. Martyn Haywood & Sue Wells)