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 brings the mantle cavity and anus to an anterior position above the head. In some species there is a degree of secondary detorsion or rotation towards the original position, this may be only partial or full detorsion.

Development
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 a much slower stage 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 if 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.

Evolutionary Roles
The original advantage torsion gave gastropods is unclear. It is further complicated by the fact that torsion brought with it a number of problems. A particular problem gastropods had to overcome come was the location where wastes were excreted – above the head which can potentially lead to fouling of the mouth and sense organs. Never the less, the diversity and success of the gastropods suggests torsion is very advantageous indeed.

One role which is seems a likely candidate for the original purpose is that of defence against predators in adult gastropods. By moving the mantle cavity over the head, the gastropod can retract its vulnerable head into its shell. Some gastropods close the entrance to their shell with a tough operculum which is attached to their foot. In evolutionary terms the appearance of an operculum occurred shortly after that of torsion, a possible link with the role of torsion, though there is not sufficient evidence either for or against this theory.

There are many other advantages torsion provided gastropods. For aquatic gastropods the anterior positioning may be useful for preventing sediment getting into the mantle cavity, which is more likely with a posterior positioning due to sediment being stirred up by the motion of the gastropod. In terrestrial species, ventilation is better with anterior positioning. This is due to the back and forth motion of the shell during movement which would tend to block the mantle opening against the foot if it was in a posterior position. Another possible advantage for aquatic species is the osphradium (olfactory sense organs) are moved to an anterior position and are able to sample the water the gastropod is moving into to rather than from, this may help the gastropod locate food or avoid predators.

Evolution of an asymmetrical conispiral shell allowed gastropods to grow larger but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod’s body and so helps prevent it falling over.

What ever the original role of torsion, it is clear that further adaptations linked to torsion provide modern gastropods with many advantages.