Planarians are non-parasitic members of the class Turbellaria (phylum Platyhelminthes). There are thousands of species–they live in lakes and rivers, the ocean, and even on land (if the soil is moist enough). We use freshwater species, largely for the ease of care. We keep them at room temperature, in the dark (as they are photophobic), in plain mineral water (think bottled water just like you probably drink), in tupperware containers. They have a single opening, through which they extend their pharynx (or feeding tube) to eat. In the lab, we feed our planarians liver pate (paste) that we make especially for them.
We use mainly two species in the lab: Schmidtea mediterranea, which has a sequenced genome and for which there exist both an obligate asexual strain and a sexual strain (planarians are hermaphroditic); and Dugesia japonica, which are larger and hardier and perfectly suited to chemical genetic screens.
Additionally, for our evolutionary research, we also keep small colonies of several other planarian species in the lab. As you can see in the image on the right, they come in a wide range of colors, sizes, and head shapes. One species, Polycelis felina, even has over 20 eyes.
This natural variation in body morphology is one of the reasons that planarians are great for studying how animal shape is re-established during regeneration (all of our planarian species are regenerative).
Planarians are a powerful regeneration model because they can reproduce asexually, by transverse fissioning. During fissioning, worms basically attach themselves by their tail and then stretch until they pull themselves apart and separate into two pieces!!! The head fragment will regenerate a new tail, and the tail fragment will regenerate a new head. And 7-14 days later, you have two worms instead of just one…both genetically identical! Planarians are able to do this remarkable feat because roughly 25% of their body is made up of adult stem cells, able to regenerate all tissues the worm may need to replace. In the lab, we utilize this ability to further our understanding of regeneration.