motile: 1. Biology Moving or having the power to move spontaneously: motile spores.

How does this differ from mobile?

Mobile simply means that it is able to move. Motile means that it can move of its own accord. Thus, a rock rolling downhill can be said to be mobile, but the rock needs a push of some sort to get it going, so it is not motile.

Both words refer to movement in living organisms,but I think mobile refers to voluntary movements and motile to involuntary ones. Mobile is also applied to inanimate objects meaning "capable of movement", but I don't think that motile can properly be applied to inanimate objects.

Tinman

While looking up motile, I came across an AWAD site with the following definition:

mo.tile a. [See Motive.] 1. (Biol.) Having powers of self-motion, though unconscious; as, the motile spores of certain seaweeds.

2. Producing motion; as, motile powers.

But on that same site was a discussion of "words relating to body parts". Here are the words:

glabella
lunula
uvula
frenum
axilla
canthus
gular

Some of these words have been discussed before. If you don't know them, look at AWAD.

Tinman

I think of 'motile' as referring to things like flagella and cilia that can move in place, either to create feeding currents or to cause the organism to move from place to place (be mobile). A motile cell would generate movements on the spot, a mobile cell would move around the organism.

Interesting distinctions. I agree that "motility" refers to involuntary movement. Probably the most common use of "motile" is with testing the motility of sperm.

How does automobile fit into all of this? It means automotically (or self) mobile, right? Is "automobile" ever used to refer to "mobile," not related to cars?

We seem to have many separate thoughts of how to distinguish 'motile' from 'mobile': self-power (arnie), volition (tinman, kalleh), movement in place (aput). I found 'motile' defined on a medical site as "having spontaneous but not conscious or volitional movement."