--- title: "Planarian (Schmidtea mediterranea) | AnimalTracks.wiki" description: "A freshwater flatworm whose tail can turn into a head when one gene is switched off." image: "https://animaltracks.wiki/images/animals/planarian-1.webp" published: "2026-10-11" updated: "2026-10-11" --- # Planarian (*Schmidtea mediterranea*) > **Status**: Checked against 9 sources, listed under Sources below. > **Tagline**: A freshwater flatworm whose tail can turn into a head when one gene is switched off. > **Soundtrack Vibe**: Krautrock & Electronic Pioneers // Laurie Spiegel, "Patchwork" > **Soundtrack Links**: [Spotify](https://open.spotify.com/track/4RLVoOmGiYhxUe9WVPr8t2) > **Vibe Match**: Planarians regrow rapidly from tiny tissue pieces. Patchwork rebuilds itself the same way, from a few short melodic fragments repeated and recombined into a whole piece. ## Biological Superpowers & Stats - **Taxonomy**: *Schmidtea mediterranea* - **Danger Level**: Low - **Conservation Status**: Not Evaluated ([IUCN Red List](https://www.iucnredlist.org/)) - **Superpower**: Planarians regenerate rapidly from tiny tissue pieces - **Habitat**: Fresh water in the western Mediterranean, from the Catalan coast to Tunisia - **Diet**: Planarians are carnivores known to feed on living or recently dead invertebrates - **Tags**: invertebrate, freshwater, regeneration ## Natural History & Behavior ## Cut it up, get new worms Schmidtea mediterranea is a freshwater planarian, a type of flatworm. In one lab study, the sexually mature worms used were about 1 cm long. Planarians can regenerate from essentially any type of injury, and they regrow rapidly from tiny tissue pieces. Stem cells called neoblasts do the work, and transplanting a single neoblast can rescue a worm given a lethal dose of radiation. By a 2007 estimate, these stem cells can produce the approximately 40 different cell types found in this animal. ## A gene that decides head or tail > **Tail into head** > When researchers silenced the gene beta-catenin, worms cut at the rear grew a head instead of a tail. In uncut adult animals, the tail turned into a head. The authors suggest beta-catenin works as a molecular switch that specifies and maintains head-to-tail identity, during regeneration and in the uncut animal. ## Immortal, with a catch Scientists have called this species immortal. Adults hold a large pool of pluripotent stem cells, and those cells keep making every adult cell type. There is a catch. A study found that the worms lose fertility as they age. The problem sits in the female reproductive system, and regeneration can restore it. With age, a signaling gradient that sets head-to-tail position shifts toward the tail, and tuning it with gene silencing slowed down or sped up the worms' reproductive aging. ## One worm, two ways to breed Hundreds of planarian species live worldwide, but this one has a restricted distribution in the western Mediterranean. Asexual populations live on the Catalan coast and the Balearic Islands. Sexual populations live in Corsica, Sardinia, Sicily and the Tunisian coast. No co-occurrence between the two strains has been reported. Planarians have a low dispersal ability because they dry out easily. Asexual worms reproduce exclusively by fission, which means splitting, followed by whole-body regeneration, and they never develop sexual organs. Sexual worms are hermaphrodites, with both male and female organs, and lay egg capsules. Researchers mainly study a clonal lab strain that started from a single animal collected from a fountain in Montjuïc Park, Barcelona, in 1998. Planarians in general are carnivores known to feed on living or recently dead invertebrates, including annelids, mollusks, crustaceans, and insect larvae. They extend a cylindrical eating tube, the pharynx, through an opening on the belly. Its muscles then pull tissue and fluid from the food. ## Frequently Asked Questions ### Can a planarian really turn a tail into a head? Yes, in lab tests on this species. Researchers shut off a gene called beta-catenin, and worms cut at the back regrew a head where a tail belonged. Even uncut adult animals had their tails changed into heads. *(Source: [Gurley, Rink & Alvarado 2008]())* ### Where does Schmidtea mediterranea live? In a small part of the world: the western Mediterranean, where it has a restricted distribution. Worms that split themselves in two live on the Catalan coast and the Balearic Islands. Worms that breed sexually live in Corsica, Sardinia, Sicily and on the Tunisian coast. *(Source: [Brand et al. 2026]())* ### What do planarians eat? They are carnivores. As a group they are known to eat animals without backbones, alive or recently dead: annelid worms, mollusks, crustaceans and insect larvae. Labs typically feed them organic homogenized beef or chicken liver. *(Source: [Pacis et al. 2025]())* ### How big is Schmidtea mediterranea? About 1 cm long, going by the sexually mature worms of the sexual strain used in one lab study. *(Source: [Collins et al. 2010]())* ## Sources 1. [Brand et al. (2026). Stepwise emergence of recombination suppression precedes fissiparous asexuality in the planarian Schmidtea mediterranea. Nature Communications.]() 2. [Gurley, Rink & Alvarado (2008). β-Catenin Defines Head Versus Tail Identity During Planarian Regeneration and Homeostasis. Science.]() 3. [Plass et al. (2018). Cell type atlas and lineage tree of a whole complex animal by single-cell transcriptomics. Science.]() 4. [Verdesca et al. (2026). Drift of positional identity drives reproductive aging in a long-lived regenerative animal. Current Biology.]() 5. [Pacis et al. (2025). Red Midge Larvae Are an Invertebrate Alternative Diet to Beef Liver for Planarian Husbandry. Biomolecules.]() 6. [Sánchez Alvarado (2007). Stem cells and the Planarian Schmidtea mediterranea. Comptes Rendus. Biologies.]() 7. [Collins et al. (2010). Genome-Wide Analyses Reveal a Role for Peptide Hormones in Planarian Germline Development. PLoS Biology.]() 8. [Wenemoser & Reddien (2010). Planarian regeneration involves distinct stem cell responses to wounds and tissue absence. Developmental Biology.]() 9. [Grohme et al. (2018). The genome of Schmidtea mediterranea and the evolution of core cellular mechanisms. Nature.]() *Published October 11, 2026. 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