A “leech” on the sidewalk. Photo credit: Tonya Flores
The only leeches I’ve ever seen in the eastern US were in water (in fact, we recently found some while flipping rocks in a local stream a few weeks ago – check out this video, starting at 3:00). I thought terrestrial leeches were largely confined to the tropics — I’ve heard some gruesome stories and have seen even gruesomer pictures thanks to friends who did fieldwork in the tropical rainforests of Southeast Asia and Australia.
Yet, something about Jason’s mystery “leeches” didn’t quite sit right with me. I couldn’t make sense of their suburban sidewalk location and their smooth shiny bodies.
So like any good nature nut, I decided to investigate further. With a little Internet sleuthing I learned that there are indeed terrestrial leeches in the eastern US. In fact, two terrestrial leech species can be found living under rotten logs in Great Smokey Mountain National Park; they eat earthworms.
Alas, the article that mentioned terrestrial leeches did not contain photos. So I contacted one of the world’s leading leech experts, Dr. Mark Siddall at the American Museum of Natural History to get his opinion. Who was this creature and why did Jason and his kids find it on the sidewalk?
Mark was kind enough to answer my query, very simply and directly.
It is not a leech, it is a flatworm, probably in the family Geoplanidae.
Doh! We missed that identification by a whole PHYLUM of life – Leeches are in the Phylum Annelida, the phylum of segmented worms, including your friendly neighborhood earthworms. Take a look at the pictures of Jason’s flatworm again – no repeating muscular segments there! Its smooth and shiny surface should have been our first clue to its identity.
Flatworms belong to the Phylum Platyhelminthes. Now that you are thinking about them, I bet you remember flatworms from high school biology – they’re touted as the most primitive example of bilateral symmetry (having distinct left and right sides). Flatworms lack a circulatory system and a body cavity – everything moves through their tissues by diffusion. Flatworms include parasites like tapeworms (see Dr. Eleanor’s recent post about her dog Lucy Bea’s butt worm) but also terrestrial predators, like the one Jason and family encountered on their sidewalk.
Similar to leeches, terrestrial flatworms show hunting behaviors – they use sensory organs near the front end of their body to detect food. Admittedly, it was Jason’s report of seeking behavior that first led me astray down the leech identification path.
The really cool thing about the terrestrial flatworm is that its mouth is on the underside of its body. When it encounters prey, the flatworm extends a muscular tube called a pharynx through its mouth. It then secretes digestive juices that dissolve prey outside of the flatworm’s body. With slime and that pharynx, terrestrial flatworms can take down earthworms, snails, slugs and insects.
Now must be the time of year when terrestrial flatworms are out and about. On the same day I heard back from the leech expert, I received an email from Carolyn, a Your Wild Life volunteer in Raleigh, describing flatworms she encountered on a morning walk with a friend:
They were very long – every bit the 10” I have read about. Dusty brown but no stripes that I remember. They were out at 7am in the shade last Saturday crossing the 8’ wide greenway on a stream of slime. It was in the low 50’s. My fellow walker who is terrified of snakes took off but I stayed to look. Strangest head I have seen, like a breathing fan. I knew it wasn’t a snake because of the slime and it didn’t take off from the vibration of our steps.
With my new and improved knowledge of terrestrial flatworms (and leeches), I’m pretty confident that Carolyn encountered a different species of terrestrial flatworm than Jason and his family. I suspect she spotted a “hammerhead worm” in the genus Bipalium, a group that looks quite alien-like with their broad heads.
As you go about the business of your daily lives, keep your eyes peeled for your own backyard mysteries, worm and otherwise. We’d love to hear about them and feature them on the blog!