Land Planarians

A break in the much appreciated rain and I found myself out in the garden. [Where else?] When I moved a large plant container, I saw something I had never seen before.

‘Bipalium adventitium’ or yellow land flatworm found in Northern California (Sanjay Acharya) CC BY-SA 4.0

Curled up in a perfect spiral, under what had been the very center of my container, I saw a flat yellow worm with a dark stripe down the middle of its back. Of course, I had to collect it for identification. Unfortunately, this particular specimen escaped and is, I believe, lodged in my moisture meter.

Land planarians, also known as land flatworms, or arrowhead flatworms, are a family of flatworms known as Geoplanidae. There are nearly 1,000 different species of flatworm worldwide, broken down into 4 subfamilies, but we know very little about them. What we do know is simply too strange not to share.

Valdivian Rainforest flatworm (José Grau de Puerto Montt) CC BY-SA 3.0

Invasive planarians

Land planarians are native to Indo-China. Sometime around 1901, soil containing these flatworms was transported to the U.S. At first, land planarians were only found in greenhouses. Now they are found in several states including: Alabama, California, Mississippi, New Jersey, New York, North Carolina, Ohio, Oklahoma, South Carolina, Tennessee, and Texas. If you find one in a state not listed, I’m sure that your local County Extension Office would love to see it. At this point, land planarians are only found in places where nursery plants go. They withstand freezing temperatures by hiding in protected areas. They cannot, however, tolerate low humidity or drought.

Land planarian description

Land planarians are flat, slimy worms. Apparently, the slime helps them move and is the only way they can maintain internal moisture levels. That slime is said to taste terrible, though I don’t know how or who figured that out. That bad taste means they have few, if any, natural predators. 

​Planarians can range from less than one inch to nearly a foot in length. Most planarian species tend to be brown or brownish-gray, but they can also be yellow, green, black, or even bluish-green. Most planarian species have dark longitudinal lines that start at the head. Heads tend to be triangular or crescent shaped. Planarians do not have a mouth, per se. Instead, they have a single opening on the underside of their body.

If the outside of a planarian looks strange, the inside is even more bizarre. Planarians are a mass of squishy tissue and nerves, with a layer of locomotive hairs on the underside. They have no brain, circulatory system, respiratory system, or digestive system. So, how do they eat?

Black Brazilian flatworm (Piterkeo) CC BY-SA 3.0

Land planarian feeding

Cousin to parasitic tapeworms, planarians are nocturnal predators that feed on slugs and snails, pillbugs, millipedes, spiders, and earthworms. They use chemical signals that are produced in folds of their skin to detect prey. Some land planarians use physical force to hold their prey, while others have a sticky mucous that entraps their victim. 

Now, when I said they feed, it isn’t feeding as we know it. When a land planarian feeds, it slimes over top of a potential food, attaches its “mouth” opening, and vomits digestive juices , liquifying its prey. Then, it sucks up the soupy nutrients. Land planarians do not have an anus, so waste products are released through the same opening used to bring it in. If that wasn’t weird enough, land planarian reproduction is even more odd.

Iberian flatworm (Eduard Solà) CC BY-SA 3.0

Flatworm reproduction

Like many other flatworms, land planarians are able to reproduce either sexually or asexually. Sexual reproduction culminates in eggs being placed in cocoons that hatch in 3 weeks. A single planarian will, every couple of weeks or so, attach its tail to a rock or some other immoveable object and slime away, tearing its tail from its torso. A new tail grows from the wound, as we would expect of a flatworm. The tail segment left behind, however, does the same thing, growing a new torso and head within 10 days. [When food is scarce, it is not uncommon for land planarians to eat their own reproductive tissues.] Scientists love studying flatworms because of those reproductive habits. In one study, it was found that decapitated flatworms retained the memories of their parent worms. [I can’t make this stuff up.]

As far as invasive pests go, planarians are not a significant problem, unless you have a greenhouse or practice vermiculture. [Vermiculture refers to raising worms.] In most outdoor gardens, fluctuations in humidity help keep land planarian populations in check. If you do have a greenhouse, or raise worms, flatworms can wipe out your entire worm population in short order.

The next time you see slime trails, don't assume they were made by snails or slugs. It may be that those garden pests are on the run from something far more terrifying (to them).

Earthworm rescued from flatworm. Note the swollen area of the worm, 1/3 of the worm's length from its head - that was where the flatworm had started feeding. (Kate Russell)