Friday, November 7, 2008
There are curious creatures all around us and one of these is the land planarian or terrestrial flatworm. Aquatic planarians are native to the U.S. Land planarians are native to more tropical latitudes but were introduced to the U.S. in the soil of potted plants from Asia around the turn of the last century. After becoming established in greenhouses, they moved out into the environment and have been able to survive even through freezing winters.
When and where found. On October 27, I turned over a log in the back yard and, among the millipedes, slugs, earthworms, a couple of black beetles and the occasional spider, I found a single land planarian. I spend quite a bit of time fossicking around in this kind of habitat and this is the first time I have seen one of these.
Classification. Land planarians are in the phylum Platyhelminthes and class Turbellaria. This one is in the genus Bipalium from the Latin meaning “a pick-axe or double mattock.” I can’t be sure of the species but it may be B. kewense. Bipalium planarians are also known as arrowhead flatworms. Distinguishing characteristics. The Bipalium planarian is a yellowy-brown color with a dark line down the center. It is flat and exudes a lot of slime which helps prevent it from drying out and also makes it unpalatable to predators. The most striking characteristic is the pick-axe shape of the head. At first my worm seemed to have a slender pointed head end but when I disturbed it, it lifted its head which expanded on both sides into the shape of hammerhead shark’s head. The worm was about two inches long but could stretch and retract. They are known to reach lengths of about 12 inches.
Food. The Land planarian feeds on earthworms and other soil-dwelling animals like slugs and insect larvae. It is also cannibalistic and even able to use some of its own reproductive tissue for food when times are tough, although it can do without food altogether for several weeks. It detects prey with chemoreceptors. Then it extends its pharynx right out of its mouth and latches on tightly to the victim which is soon enveloped in digestive slime. The food is broken into small pieces externally before being consumed. The worm has a single opening for both intake of food and output of waste.
Reproduction. Land planarians have two methods of reproduction. One is by mating and producing eggs which hatch into little planarians. The other is by attaching the tip of its tail to some object and leaving it behind when it moves on. The parent is unharmed and the tail develops a new head in about ten days. The parent can repeat this once or twice a month. I know there are humans who would be happy to shed part of their rear ends now and then, but if it created a clone population I think that would be more change than we can cope with right now.
Do we have anything in common? In the evolutionary scheme of things, flatworms are thought to be the oldest living creatures to have developed bilateral symmetry. The very earliest multi-cellular animals had radial symmetry – like sea anemones. These radial animals had only two layers of embryonic cells. The flatworms, like humans, have three layers – a quality called triploblastic. What we don’t have in common (but might be able to profit from) is the extreme regeneration ability. With growing interest and advances in stem cell research, scientists are looking again at flatworms since they are one of the simplest triploblastic organisms that regenerate tissue in a manner more like higher animals.
Status as pest. Land planarians can become pests in greenhouses and in earthworm-rearing beds because they have such a voracious appetite for earthworms. I found some very dire warnings at some websites hosted by worm growers. The university websites were significantly more moderate, suggesting that these creatures are not a menace in the environment at large. After all, most of our earthworms are also introduced species.
Sources. Clemson University Extension Service, Land Planaria by Jenny Staeben; University of Florida Institute of Food and Agricultural Sciences, Land Planarians, by P. M. Choate; Sustaining Life – How Human Health Depends on Biodiversity, Eric Chivian and Aaron Bernstein; ScienceDaily.com, March 22, 1999.
© 2008 The Carlisle Mosquito