Why torsion in gastropods

You might think the shell is a common feature of snails, but Gastropoda also includes the slugs those slimy little monsters without shelly homes. No the common feature is a sort of an anatomical do-si-do Gastropods play with their bodies. This process, called torsion, eventually places the organs previously on the right side on the left.

The central nervous system becomes a pretzel. Why evolution has favored this round robin of body parts in Gastropods, and how exactly it occurs is a fascinating story and one not yet fully understood.

In , Walter Garstang, proposed a hypothesis both for how and why torsion occurs. Garstang was born in in England and ascended the ranks of academia to work on everything from slugs to sand crabs to sea gulls.

As noted in his obituary, in , he was a poet and lover of nature. And although noted for many things, Walter Garstang, is probably best remembered for his poems about form, function, and development in invertebrates.

The veliger possesses two retractor muscles. One of these extends from the shell on right, over the gut, and attaches to the left side of head and foot. The other starts on the left and attaches to the right. Garstang proposed that these muscles were asymmetrical in their size and strength. Garstang suggested further that torsion was an adaptation for protection.

Without torsion, larval snails would retract into the shell tail first leaving the head and other important parts exposed to the teeth, claws, and tentacles of hungry predators. 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.

The mantle cavity was primitively posterior in position. The increase in the length of the ventral foot, which primitively was very short, tends to remove the mantle cavity and the pallial complex away from the head and its associated parts are shifted forwards. Before torsion, the anus, the ctenidia and the renal orifices point backwards, and the auricles lie behind the ventricle.

After torsion, the anus, the ctenidia and the renal orifices project forward, and the auricles lie in front of the ventricle. The original posterior face of the visceral sac becomes the actual anterior face, so that the visceral organs morphologically of the original right side become placed topographically on the left side, and vice versa.

The long, uncoiled pleura-visceral nerve connectives become twisted into a figure of 8. The right connective with its ganglion passes over the intestine to become the supra-intestinal, while the left connective passing underneath the intestine to become the infra intestinal.

The coil of the visceral sac and the shell, which primitively was dorsal or exogastric, becomes ventral or endogastric, after torsion. The anus is displayed towards the right side of all pallial cavity so that the original symmetry of the organization disappears.

Another characteristic feature involving asymmetry is the reduction or atrophy of the paird parts of the primitively left or topographically right side. The remaining gill may bear one row of filaments monopectinate gill.

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. Nevertheless, the diversity and success of the gastropods suggests torsion is very advantageous indeed.

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.

The changes occurring in torsion are to a certain extent reversible. This reversion is known as Detorsion and it is very characteristic of the whole group of the Euthyneura.

As a result, the pallial complex travels back towards the posterior end along the right side, the ctendia point backwards, the auricles move behind the ventricle, and the visceral loop becomes untwisted and symmetrical.

In this way, a secondary external symmetry is re-established. Torsion must be disadvantageous to adult snails, as many of them have undergone detorsion processes. Various degree of detorsion are met within the Euthyneura. In the least specialized Opisthobranchia and Pulmonata Acteon, Bulla, etc. Formerly, this condition was looked upon as an arrested stage in the torsion, but there is the same reduction of the paired parts of the pallial complex as in the specialized Streptoneura.