Dugesia tigrina is called a flatworm because the body is flattened. It has a triangular shaped head, with two eyespots called ocelli on top of the head. These detect light. At the back of the head on each side are sensory lobes called auricles, and these are what make the head look triangular. Behind the head is a long, flat body that comes to a point at the end. The body is typically brown with white and yellow spots. The average length of this flatworm is 9 to 15 mm, but body size can be very different between individuals, due to their ability to regrow lost body parts. (Pickavance, 1971; Salo and Baguna, 1984; Sluys, et al., 2010; Smales and Blankespoor, 1978)
Dugesia tigrina is a free-living flatworm found across North America. Humans have brought these flatworms overseas into northwestern Europe and eastern Asia, with significant populations in Great Britain and Japan. (Cash, et al., 1993; Gee, et al., 1998; Pickavance, 1971; Sluys, et al., 2010)
These flatworms are typically present in lakes, ponds, and streams in temperate regions. They live under rocks, in plant material, and in other debris found on the bottom of lakes and streams. They are found at a maximum depth of 25 to 40 cm, and tend to prefer water temperatures between 13 and 25°C. (Folsom and Clifford, 1978; Gee, et al., 1998; Stokely, et al., 1965; Takano, et al., 2007)
Flatworms that are produced through sexual reproduction hatch from a cocoon, and are 2.0 to 4.5 mm in length when they first hatch. They are transparent, and have yellow yolk cells that contain nutrients which they use to grow and develop. As they grow, the yolk gets used up and is no longer visible. They have spots of color that become larger and darker. Individuals are considered to be adults after reaching 9 mm in length. (Folsom and Clifford, 1978; Vowinckel and Marsden, 1971)
These flatworms are hermaphrodites, with each individual worm having both male and female genitals. Only some populations reproduce with two individuals mating. When one individual encounters another individual, one glides on top of the other. They either both face the same direction or face opposite directions. The top flatworm moves its head back and forth over either the head or back of the bottom flatworm. After several minutes, both lift their tail ends, and arrange their bodies so that the underside of each tail is pressed against the other. Since both worms have male genitals, each inserts a penis into the other. This can last anywhere from 1 minute to 1.5 hours, and ends when the flatworms separate and glide away. Individuals can mate many times in their lives. (Vreys, et al., 2002)
Flatworms reproduce by two ways: with two individuals mating, or by fission. Fission occurs when one flatworm splits into two flatworms. An individual splits into two pieces, and the new piece becomes another flatworm, while the original piece regrows its missing parts. This happens more at higher temperatures (around 26°C), while mating takes place at lower temperatures (around 20°C). Mating produces a cocoon that attaches to a surface by a short stem. The cocoons are 1.3 mm wide, and each produces about 4 worms upon hatching. Multiple cocoons can be produced by one flatworm. Reproduction occurs the most during the summer. Some populations reproduce only by mating, others only by fission, and some populations do both. (Folsom and Clifford, 1978; Vowinckel and Marsden, 1971; Vreys, et al., 2002)
These flatworms produce a cocoon for their offspring, and provide yolk full of nutrients for the offspring to grow and develop. Otherwise, there is no parental care. (Vreys, et al., 2002)
There is no record of the lifespan of these flatworms. The difficulty in measuring their lifespan is due to their ability to regrow body parts. They do not age like most other organisms do. If a flatworm is continually provided food, they will keep living. Even if they are starving, the worms can reabsorb their body tissue and shrink to tiny sizes. Once they have access to food again, they return to their normal size. (Salo and Baguna, 1984; Sinko and Streifer, 1971)
Dugesia tigrina is often seen swimming through water. These flat worms can also glide along, with the help of mucus that they produce, as well as small hair-like cilia that cover the body and help the worms move. Individuals can be found either by themselves, or gathered in groups. They often forage for food in groups, which helps to increase the amount of food ingested. (Cash, et al., 1993; Pickavance, 1971; Smales and Blankespoor, 1978)
Despite being a primitive animal, these flatworms have a central nervous system that allows them to perceive and integrate information from the outside world. They have two eyespots called ocelli on the top of their head that detect light. They generally avoid light, however, which is why they can be found in the dark at the bottom of lakes and streams. At the corners of the head are two lobes, called auricles, that detect chemicals, and also sense touch. (Smales and Blankespoor, 1978; Takano, et al., 2007)
These flatworms are predators that will eat whatever they can catch. This usually includes small crustaceans, insect larvae (particularly those of midges and mosquitoes), small round worms, and the soft structures of some freshwater sponges. The flatworm uses the mucus it produces to catch prey. Feeding is significantly reduced or stops completely below a temperature of 6°C. (Cash, et al., 1993; Pickavance, 1971)
Common predators of these flatworms include freshwater fish, amphibians such as newts, and some insect larvae, including dragonfly and damselfly larvae. The mucus produced by these flatworms makes it more difficult for their predators to catch them. Foraging for food in groups is also another way to defend themselves, since more worms present mean theres a smaller chance any one worm will be eaten by a predator. (Cash, et al., 1993; Davies and Reynoldson, 1969)
Dugesia tigrina is prey to a variety of animals, including fish, amphibians, and insects. It is also a predator itself of insects, aquatic worms, and crustaceans. As a significant predator of insect larvae, particularly mosquitoes, these flatworms have been able to eat enough mosquito larvae to decrease their population size in some habitats. (Cash, et al., 1993; Davies and Reynoldson, 1969; Meyer and Learned, 1981)
These flatworms do not cause any problems for humans.
These flatworms are often used by scientists to do research. They have a simple body structure, but also have a central nervous system, and are able to regrow lost body parts. Scientists are studying this ability to regrow body parts, to better understand how body tissues can be regenerated. By understanding how this can be done in these flatworms, scientists may gain a better understanding of tissue growth, development, and aging in other animals, even humans.
Additionally, these flatworms have been introduced to some bodies of water to help control mosquito populations. The flatworms prey on the mosquito larvae, and in some cases have been able to eat enough larvae that the mosquito populations decrease. This is helpful since some mosquitoes can transmit harmful diseases to humans. (Meyer and Learned, 1981; Salo and Baguna, 1984; Takano, et al., 2007)
These flatworms are not an endangered species.
Dugesia tigrina is also known as Girardia tigrina.
It is thought that populations of these flatworms that have access to food do not age, and since they are able to regrow lost body parts, they are considered immortal.
Rosario Saccomanno (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Angela Miner (editor), Animal Diversity Web Staff.
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