Torsion (gastropod)

Torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle, and shell 180˚ with respect to the head and foot of the gastropod. This rotation brings the mantle cavity and the anus to an anterior position above the head.

In some groups of gastropods (Opisthobranchia) there is a degree of secondary detorsion or rotation towards the original position; this may be only partial detorsion or full detorsion.

The torsion or twisting of the visceral mass of larval gastropods is not the same thing as the spiral coiling of the shell, which is also present in many shelled gastropods.

There are two different developmental stages which cause torsion. The first stage is caused by the development of the asymmetrical velar/foot muscle which has one end attached to the left side of the shell and the other end has fibres attached to the left side of the foot and head. At a certain point in larval development this muscle contracts, causing an anticlockwise rotation of the visceral mass and mantle of roughly 90˚. This process is very rapid, taking from a few minutes to a few hours. After this transformation the second stage of torsion development is achieved by differential tissue growth of the left hand side of the organism compared to the right hand side. This second stage is much slower and rotates the visceral mass and mantle a further 90˚. Detorsion is brought about by reversal of the above phases.

During torsion the visceral mass remains almost unchanged anatomically. There are, however, other important changes to other internal parts of the gastropod. Before torsion the gastropod has an euthyneural nervous system, where the two visceral nerves run parallel down the body. Torsion results in a streptoneural nervous system, where the visceral nerves cross over in a figure of eight fashion. As a result, the parietal ganglions end up at different heights. Because of differences between the left and right hand sides of the body, there are different evolutionary pressures on left and right hand side organs and as a result in some species there are considerable differences. Some examples of this are: in the ctenidia (equivalent to lungs or gills) in some species, one side may be reduced or absent; or in some hermaphrodite species the right hand renal system has been transformed into part of the reproductive system.

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