I have a very boring-looking terrarium sitting in my basement workshop. It’s looks like it has nothing in it but some dirt, stones, leaves, and chunks of bark. However, hidden under some of the rocks are some very bizarre creatures. Last fall, during our “Simple Invertebrates” unit, I brought in some containers of dirt from my garden to use for a scavenger hunt. The kids were given magnifiers and were told to find as many forms of life as they could. Of course, they all found earthworms (some of them almost microscopic-- it is easy to forget that worms start out very, very small!), little gray “pill bugs” (or “rolly-pollies,” or whatever those little armadillo-like arthropods are called in your region of the world), and a selection of tiny millepedes, centipedes, spiders and insects.
Then one student found something amazing (and a bit alarming, as it turned out). She called me over to looked at a strange worm she had found. As soon as I saw it I was pretty sure it was not in the same family as the earthworm. Its body was not segmented. It was also too large to be a round worm. As I watched it under our stereo-microscopes, I saw it extended its head far enough that the shape of the head became visible. It was flat! The head was round and flat, like a plate. I suspected that we had found a land planarium.
As soon as I got home from my class, I began searching the web for pictures that matched what we had found. It was definitely a land planarian, a terrestrial “cousin” of the famous freshwater planarian that regenerates body parts. Cut a planarian’s head off and it will grow a new one. Split its head down the middle and it will grow two heads. Cut it lengthwise and each half will grow into a complete animal. Planarians are very handy in biology labs that study tissue regeneration. (You can get them from science supply companies like Carolina Biological. I had ordered some for this unit and the kids had a great time watching them slither around their jars. I highly recommend them.) But this planarian lived on land, not in the water.
I read some articles on land planarians and learned that what I had found was a recent invasive species. These planarians are not native to North America. They most likely had come over from Indo-China, hitching a ride on tropical greenhouse plants that had been shipped over. Before they got to America, they had been discovered invading England. They were first found in greenhouses at Kew Gardens in 1878, thus their scientific name, Biplaium kewense, and their nickname, the “Kew worm.” They have been in America since 1901, but have stayed mainly in greenhouses. But now they are on the loose. Maybe mine came from potted plants I had bought from greenhouses and planted in my garden.
I began looking around my property to see if I could find another of these rare flatworms. To my surprise I found lots of them hiding under rocks or bricks in soil that was very damp. Land planarians “breathe” by absorbing oxygen through their skin and they must have a layer of moisture around them to do this. Very wet clay seemed to suit them perfectly.
More research revealed that land planarians eat earthworms. They’ve been known to tackle earthworms that are larger than they are. And unfortunately, land planarians don’t till the soil. They don’t harm the soil, but they don’t benefit it, either. If the population of land planarians begins to increase and the population of earthworms begins to decrease, we could see the health of our soil declining. So basically these weird little things are invaders we don’t want in our gardens. Great-- I’ve probably got hundreds of them already. My excitement about my discovery was gradually turning to horror. These things should not be in my garden. But how to get ride of them? Researchers are still working on this problem.
I found a website that tracks where these critters are showing up. They have been found in quite a few US states, but not all of them. Apparently they have some really huge ones out in CA. I prefer the tiny PA ones.
I’ve lived in Pennsylvania most of my life and I’ve never seen one of these. I guess I just got lucky on one of my trips to purchase to a greenhouse to buy plants. Some people win the lottery. I got land planarians.
Here’s one of the articles I read:
http://entnemdept.ufl.edu/
(The close-up picture of the planarians was taken with my new Panasonic Lumix DMC-ZS8 camera.)
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