Fragmentation - Types of Asexual Reproduction
Fragmentation is a phenomenon where parent animal spontaneously (on its own accord) splits into two or more fragments. Each one of such fragments after that regenerates the missing parts to form a new individual. This method is reported in specific sea- anemones and some worms. Sea anemones occasionally break off part or whole of their pedal disc whereas the main body moves away (pedal laceration). The pedal disc may put out lobes that pinch off and regenerate into small sea anemones. Occasionally a sea anemone leaves behind its pedal disc along with a few mesenteries at the site of attachment, in such types of cases the part left behind regenerates into a new sea anemone and the another main part regenerates the missing pedal disc. Fragmentation may be considered like a type of fission. Moreover longitudinal and transverse fission takes place as a normal method of asexual reproduction in many sea anemones. Spontaneous separation is beautifully demonstrated in certain freshwater planarians (Platyhelminthes, Class Turbellaria). A general example is Dugesia. The fission takes place when the animal has grown to the maximum size. The fission plane usually forms behind the pharynx.
At that time the hind part of the animal is firmly attached to the substratum and the front part continues to move forward until the worm breaks at the constriction into an anterior and a posterior segment. Each segment next regenerates the missing parts to form a new small worm. In some planarians like Microstomum, chains of individuals may be formed as a result of repeated fission. When these individuals develop and acquire a certain degree of differentiation, they separate from the chain.