More often than not, we humans tend to think of ourselves as perfectly evolved and impeccably suited to life in all parts of this planet. However, observations of small “lower” organisms can easily demonstrate that we are in no way alone in planetary domination. Planarians have colonized vast marine and terrestrial habitats due to their simple but extremely efficient anatomy, coupled with their incredible reproductive fitness.
Life style (environment/feeding)
Planarians are small flatworms that live in ponds and rivers on almost every continent and some have adapted to a terrestrial habitat in the moist undergrowth of rainforests (see blog 1). Smaller aquatic planarians like Girardia (or Dugesia) tigrina require submerged environments to prevent dessication and to facilitate movement utilizing thousands of beating cilia on the animals ventral surface. These tropical land dwelling planarians ride on mucous trails produced by modified secretory glands on the body surface through body wall muscle contraction and snake like undulation. On a recent excursion to the arboretum in Guelph, I couldn’t help but wonder how these organisms survive the harsh winters and hot dry summers when all of the swaps dry up.
Planarians have amazing regenerative properties and it has been demonstrated that the entire organism can regenerate itself from a fragment 1/279 smaller than its full body size. Here is an interesting study by Felix et.al on head regeneration: http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1000915
This gives them an obvious advantage when dealing with predators, as well as providing them with an easy way to reproduce. Cloning one’s self is not an ideal method of reproduction as it does not allow for any genetic diversity, but it requires little energy and is a sure fire way of ensuring the production of offspring when there are no mates available. Planarians are hermaphrodites meaning that they possess both male and female gonads. When two planarians do meet they swap both sperm and eggs.
Planarians have a unique method of growth using specialised cells called neoblasts. They are essentially stem cells that float around and they can turn into skin, neurons, reproductive cells and muscles. They are being studied to better understand stem cells in larger organisms.
Planarians possess very simple but effective internal organs. Their pharynx (mouth) is actually an extendible tube halfway down the body, although most people often assume it’s in the head region. Digestion begins in the pharynx, much like it does in you and I. The rest of the food is processed in the gastrovascular cavity, which is analogous to the stomach in “higher” organisms. Planarians receive oxygen and expel carbon dioxide through diffusion over their skin. They have a surprisingly complex neural networks, they have a small brain and a ladder shaped nervous network that allows them to make coordinated movements.
Here is another fascinating blog about terrestrial planarians: http://shadygardens.blogspot.com/2010/09/planarian-eater-of-earthworms-and-enemy.html
Written by: Mike Morra
Felix, D. and Aboobaker, A. (2010) The TALE Class Homeobox Gene Smed-prep Defines the Anterior Compartment for Head Regeneration. PLoS Genetics. Published online 22 April 2010. DOI:10.1371/journal.pgen.1000915.
Anatomy photo: Alvarado S.A. Current Biology Magazine #R738. 14(18)
Tree of life: Mike Morra (adapted from Z00*2700)
Planarian eye mutations: Felix, D. and Aboobaker, A. (2010) The TALE Class Homeobox Gene Smed-prep Defines the Anterior Compartment for Head Regeneration. PLoS Genetics. Published online 22 April 2010. DOI:10.1371/journal.pgen.1000915.