Strobilisation 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.
Neck-formation and segmentation are only separated for clarity purposes. In reality, the two processes are simultaneous, with segmentation to release new ephyras occurring at the upper end while neck formation spreads further down the body. Usually, a portion of the animal remains adhered to the
substrate and regenerates the body.
Examples
The
moon jellyfish (Aurelia aurita) 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.
Induction in laboratory conditions
Strobilation is successfully induced in laboratory conditions by intensive feeding and temperature lowering, and also by the effect of artificial compounds.
Strobilation in helminths
In
cestodes, the whole body except for the head and the neck undergoes strobilation continuously, reflecting the important role reproduction plays in the parasitic mode of life. The strobilating section is called strobila, or scolex, and each of its segment is a
proglottid. As they mature, proglottids are disposed of in the
feces of the host. [1]