Re: Is there a animal that can't move by itself?

Dear Nameless,

I hope your teacher is ready to dish out the extra credit points, because there are LOTS of animals that don't move! I can understand why he couldn't think of any, though, because the animals that we're most familiar with -- cats, fishes, snakes, worms, crabs, etc. -- are all mobile. That means they can move about on their own, although some of them are quite slow. In fact, I couldn't come up with any terrestrial (land-living) animals that are entirely incapable of movement.

However, if you were to visit a rocky shore at the beach during a low tide, you would see that many of the animals living there don't move. Biologists use the word "sessile" to describe animals that live like this. Barnacles, for example, live cemented to hard rocks. Once they choose a spot to settle on, they're stuck there until they die.

Here's a photo of some barnacles living on rocks:

Sea anemones and corals are also sessile. Some anemones are capable of moving around a bit, but are considered sessile because for the most part they tend to stay put. You can find sea anemones of all sorts living in tidepools on shores throughout the world:

Corals are animals that are related to sea anemones. You have probably heard of coral reefs, which are found in shallow tropical seas and are famous for their beauty and diversity. A coral reef is made up of the skeletons of thousands of coral animals; there are also corals that don't build reefs. Both kinds of corals are entirely sessile. Like barnacles, corals live their entire lives cemented to hard surfaces.

This is a photo of a temperate cup coral:

There is one more thing I should mention. I assumed that your question was asking for examples of animals that do not move their whole bodies, or relocate to new positions, and all of the examples that I have shown you fit into this category. However, all animals, even the sessile ones, are capable of moving at least some parts of their bodies, usually for feeding. For example, barnacles extend their feet from their shells to capture plankton from the water. Sea anemones and corals use stinging cells in their tentacles to kill small animals, then bring the food-laden tentacles to their mouths. So your teacher is sort of correct when he says that there aren't any animals that don't move at all.

Good luck with the extra credit!

Allison J. Gong
Mad Scientist