iLabs: The Wonderfully Odd Flatworm
Have you ever heard of an animal that glides over surfaces by moving its cilia? Have you ever heard of an animal whose mouth is right in the middle of its body where it sticks out as a long muscular tube? Well if you have visited the Micro World Investigate Lab recently you may have seen one of these fantastic creatures.
They’re known by different names — flatworms, planaria, triclads, turbellarians. Often they are called simply flatworms, though scientists usually call the ones we have, “triclads.” ”Flatworms” is probably too vague because it includes freshwater, marine, and terrestrial species, microscopic as well as macroscopic.
The ones in our lab are macroscopic turbellarians, called either planaria, or triclads, named after the order they are classified in, Tricladida, within the phylum Platyhelminthes, which makes up all of the flatworms. Triclads are so named because they have a three-lobed gut, or gastrovascular cavity. One lobe extends from the mouth towards the head and the other two extend from the mouth towards the tail.
Here’s a picture of one that was taken out of our aquarium for display recently. It is most likely named Dugesia tigrina, which is the commonest species in the USA.
Someone asked us recently how long triclads live. The answer: “it depends.” It’s all in the reproduction.
Flatworm species vary in how they reproduce and this affects how long they live. Some reproduce sexually, others asexually by dividing, and some by alternating between the two depending on environmental conditions. For example, some reproduce sexually during the spring, when temperatures are cool. However, when temperatures rise in the summer, their reproductive organs degenerate and they switch to asexual division.
Species that reproduce asexually may live indefinitely. Some kept in lab cultures have survived for years. On the other hand, species that reproduce only sexually may live a few weeks to a few months. Those that can develop a cyst form during adverse conditions may live longer. Most large species divide about every 5–10 days while the smaller species can divide more often.
Another interesting thing is that all flatworms are hermaphrodites. That means they have both male and female sex organs in each individual. However when they reproduce sexually, they do not fertilize themselves but must obtain sperm from a different flatworm. In spring, egg capsules (called cocoons) are laid and fastened to a hard surface, like a rock, by a little stalk. Iowa State’s website has a good image of this cocoon.
When the baby flatworms hatch out they want food! But the problem is, so do the adults! This creates competition for food, and when supplies are not sufficient for everyone, the flatworms begin to shrink. They get shorter and shorter until either food becomes more abundant or they die. This phenomenon has been described by scientists conducting careful quantitative studies of flatworm population ecology.
In the photo given to us by a researcher, the scientist carefully removes Dugesia tigrina from the surface of an artificial substrate with a paintbrush so that they can be taken to the laboratory for measurements.
The really interesting thing about this “shrinking” action, is that the shrinking is due to the loss of cells, not by reducing the size of the cells they have!
Triclad flatworms viewed with a microscope seem to have two eyes, which appear cross-eyed. In reality, they are called eyespots because they do not form images. The eyespots help the flatworms detect light and dark. This allows them find dark areas to hide in, protecting them from predators. Some flatworms have numerous eyespots in a line along the front of the head.
Virtually all triclad flatworms live in freshwater where they typically lurk under rocks looking for suitable prey. Because flatworms move slowly they often rely on finding injured organisms or those that can’t otherwise escape. The flatworm will glide on top of its prey and seek an opening in the body where the flatworm can stick its long tubular pharynx. Then the flatworm secretes digestive enzymes into the prey so that it can suck out the partially digested contents of the prey’s body
Here is a video of a flatworm feeding:
For some additional information, check out the Animal Planet page on flatworms.
Given that they live in spots not usually seen by the average person, getting to see one up close is a rarity. We are pleased to have a healthy population living in our “pond tank” in the lab. So we hope you’ll come by to visit us and have an opportunity to view one of our fantastic flatworms on our large-screen TV!
Written by Todd Folsom & Deb Bailey