Strobilation
Strobilation or transverse fission is a form of asexual reproduction consisting of the spontaneous transverse segmentation of the body. It is observed in certain cnidarians and helminths. This mode of reproduction is characterized by high offspring output, which, in the case of the parasitic tapeworms, is of great significance.
Strobilation in cnidarians
- The process starts with preliminary morphological changes. In particular, the cnidarian's tentacles tend to be reabsorbed.
- Neck-formation: transverse constrictions appear near the upper extremity of the animal. A strobilating polyp is called a strobila while the non-strobilating polyp is called a scyphistoma or scyphopolyp.
- Segmentation: the number of constriction sites increases and migrates down the body length, transforming the body into a sequence of disks. The fissures intensify until the initial body is divided into equally spaced, separate segments. The oral end of the polyp becomes the oral end of the ephyra.
- Metamorphosis: neurosecretory products of the two previous processes now disappear.
Examples
- The moon jellyfish reproduces both sexually and by strobilation. This latter process occurs during the colonial polyp stage and produces either polyps or juvenile Medusae called ephyra. Strobilation tend to occur at specific periods, typically early spring. As ephyra size remains constant regardless of the polyp size, larger polyps produce more numerous ephyras.
- Some scyphozoans, such as Nausithoe aurea, cnidarians also strobilate in their solitary polyp form, producing either ephyra or planuloids. Strobilation does not happen periodically, but is thought to be induced by external stimuli, such as iodine, light regime, temperature, or food availability.