Mitey slug killers

Warning: If you're squeamish, you'd probably be happier to go away and come back tomorrow.

I don't know why this shocked me. I know that almost anywhere you look, there are mites.

"Mites are among the most diverse and successful of all the invertebrate groups. They have exploited an incredible array of habitats, and because of their small size (most are microscopic) go largely unnoticed. Many live freely in the soil or water, but there are also a large number of species that live as parasites on plants, animals, and some that feed on mold. It is estimated that 48,200 species of mites have been described.[2] Scientists believe that we have only found 5% of the total diversity of mites." From Wikipedia.

If an estimated 5% adds up to 48,200, the number of species could be up to 964,000. 

Mites are found almost everywhere. They feed on birds, on dust, on fabric, on mammals, on plants and beetles and butterflies and cheese. They swarm in our carpets and hang on our eyelashes. (I've seen these, under the microscope, on mine; I wanted to sterilize my eyes. But I got over it.)

But this didn't seem right, somehow:

Sick grey slug.

I found this small slug (1 1/2 inches) a couple of days ago, lying inert under our drying beach shoes. He wasn't moving, and I was about to toss him on the compost, when I saw the white dots running around on his back and belly. If you squint, you can see a group here, about mid-point underneath him.

I brought him inside for a closer look. He was still alive, but barely. The mites were very much alive, racing back and forth, pausing ever so often to take a bite of slug. (Ewww!)

Three mites on the head.

Mite along the side.

Zooming in on that mite.

The slug was exuding quantities of mucus, possibly in an attempt to rid himself of the pests. It didn't seem to slow them down.

I thought maybe I could trap one to get a better photo. I washed the slug several times in clean water, until he was free of them. Then I examined the water, trying to catch one with an eyedropper.

I couldn't see even one. Not on the slug, not in the water, not in the eyedropper, not on a slide that I smeared mucus onto, not when I added stain to the water. Nothing moved, even under my hand microscope. Where did they go? Curious.

Rid of the mites, the slug perked up a little, moved as if to crawl, gave up. In the morning, he was dead. There were no mites on or around him.

So I'm left with questions. Where did the mites disappear to? Did they kill the slug? Or were they just an accessory?

One thing I learned; slugs and snails are the usual hosts for these mites. I found them on BugGuide, on a slug just across the border in Washington. They're slug mites, possibly Riccardoella limacum.

"Slug mites are very small (less than 0.5 mm in length), white, and can be seen to move very rapidly over the surface of their host, particularly under the shell rim and near the pulmonary aperture. While once thought to be benign mucophages, more recent studies have shown that they actually subsist on the host's blood,[2][3] and may bore into the host's body to feed." Wikipedia

A couple of websites mention that they can be found inside the gill, entering by the pneumostome, the breathing hole visible in the third photo above. So my poor slug could have been infested even after I thought he was clean. As few as 6 mites are enough to seriously weaken a slug, leaving it open to infection by other parasites.

This is from a German site, a Google translation, I think. "After our own observations however already 6 mites are sufficient, in order to make from a before life-glad and lively snail a appetiteless and apathetic misery, which goes into few weeks to reason."

I don't quite understand the translation, but "apathetic misery" describes my dying slug perfectly. I washed myself and my working equipment down with alcohol after I was done with the mites. They gave me the heebie-jeebies.