TERRESTRIAL FLATWORMS EAT EARTHWORMS
Q. You often write about bizarre plants and animals. A few years ago, you wrote about some kind of odd invasive flatworm that eats earthworms. What was it, and is it dangerous to humans? If we find one, should we eliminate it?
A. You are probably referring to the land planarian, aka hammerhead worm, a soft, slimy, yellowish worm with black stripes down its body and an arrow-shaped head. This common nonnative flatworm was introduced from Asia more than a century ago, with U.S. greenhouse records reporting them as early as 1901. They now occur countrywide, probably by hitching transcontinental rides on plants intended for sale. As for their being harmful to humans, they are generally benign creatures, although some folks might take exception to their diet.
Flatworms are a major taxonomic group that includes internal parasites that can be bad actors, such as tapeworms and liver flukes. Land planarians, however, belong to a family of free-living flatworms that get their meals by preying on other animals. More than 800 different species are known worldwide. Most of them are scavengers or predators of earthworms, slugs and snails. A few prey on various insects and spiders. -The ones in the Southeast eat earthworms.
The common land planarians most people find are easy to identify by the triangular, spade-shaped head. These worms can stretch out a foot in length, and they leave a shiny, silvery trail reminiscent of a snail. They require moist conditions to survive; they are slimy to the touch but do not bite or sting. They are cool creatures to show off to folks, as my grandsons often demonstrate when they discover one in the backyard. Land planarians have a few odd characteristics. They sometimes lay eggs but commonly reproduce by breaking off the rear portion of the body. The individual anchors its tail in the ground and then crawls forward, leaving the end piece sticking into the ground to become another individual. If that’s not bizarre enough for you, they have neither a circulatory nor respiratory system, no legs and no external or internal skeleton. Some have eyes on the head and numerous eyes along the body. A few odd characteristics, as I said.
Land planarians may start off in a greenhouse or plant nursery, but some soon find their way outdoors and persist if environmental conditions are suitable. They can be extremely abundant in hot, humid areas of the country. I recall finding them in large numbers outdoors in New Orleans. They are ubiquitous in warm, humid habitats and have been established for decades. Trying to get rid of them is probably not worth the effort. I’m not aware of their doing any harm environmentally, aside from eating a lot of earthworms. Of course, earthworm farmers, as well as greenhouse owners for whom worms are important for soil aeration, might consider planarians a nuisance that should be eliminated. But many earthworms are not native to the United States either, so who’s to say that land planarians shouldn’t eat them?
Land planarians don’t always stop at eating earthworms. Some turn to cannibalism, dining on other planarians. They have virtually no major predators of their own. Some biologists surmise that the body slime is toxic. I am not aware of anyone who would ever consider eating it, but perhaps it discourages would-be predators. They are probably only limited in numbers when their normal earthworm prey in an area becomes depleted. They are also known for the ability to go for long periods without eating; they actually shrink in size.
Although they aren’t native to the United States, they are now firmly entrenched, persisting as one of the odd and intriguing forms of wildlife all around us. Many of them hide under rocks or ground litter to remain moist and are far more abundant than most people realize. During wet periods they sometimes prowl, especially at night, so keep an eye out for them. You might find one eating a nonnative earthworm.
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