Torsion or twisting is a significant process that happens during gastropod larval development. This process rotates the visceropallium counterclockwise through 180° from its original position, bringing the mantle cavity and pallial complex in front of the body in adults. Here, let’s learn more about the process of torsion in Mollusca.
Table of Contents
- Torsion in Mollusca
- Mechanism of Torsion
- Effects of Torsion
- Significance of Torsion
- Frequently Asked Questions
Torsion in Mollusca
Early in the development of the gastropods, one of the most important changes to the molluscan body morphology takes place. It is torsion. Torsion is a process by which the mantle, mantle cavity and visceral mass are rotated 180° in a counterclockwise direction. The mantle cavity and the anus are positioned anteriorly above the skull as a result of this rotation. The digestive tract is also twisted into a U shape together with the gills, anus, and apertures form the excretory and reproductive systems. They are positioned right below the brain and nerve cords.
Mechanism of Torsion
Torsion is initiated at the rear of the head-foot complex. The duration of a complete torsion is usually very short. But it could also take a few hours or days. After rigorous research, Thomson (1958) identified five possible ways in which torsion occurs:
- The only known animal that can rotate completely or 180° using just muscle contraction is Acmaea (Archaeogastropoda).
- A 90° rotation was achieved in two stages, the first by the contraction of the larval retractor muscles and the second via slower growth. It is the most prevalent mechanism currently understood. It is seen in the Patella and the Haliotis.
- Rotation 180° solely via differential growth mechanisms, such as in Vivipara.
- Rotation caused by distinct growth stages, whereby the anus attains an adult-state-appropriate position, such as in Aplysia.
- Torsion is no longer recognised as a movement of the viscerapallium, as the organs have been in the post-torsional posture since their initial development. It is observed in Adalaria.
Stages of Torsion
Torsion is achieved in two stages:
- Stage 1 – It takes into account the 90° rotation caused by muscular contraction and the shifting position of the mantle cavity from the posterior to the anterior end.
- Stage 2 – It takes into account the remaining 90° and involves differential growth.
See more:Difference between Cephalopods and Gastropods
Effects of Torsion
The following are some of the events that occur during torsion:
- Looping of the digestive or alimentary canal – It was originally straight from the mouth to the anus and is now thrown into a loop after torsion.
- Displacement of the mantle cavity – It is primitively posterior and is now shifted forwards.
- Loss of symmetry – Due to torsion, the bilateral symmetry of molluscs is lost.
- Chiastoneury – The pleurovisceral connectives become twisted into a figure 8.
- Changes in position – Before torsion, ctenidia, renal orifices and anus point backwards and the auricle is positioned behind the ventricle. After the twist or torsion, ctenidia, renal orifices and anus project forward and the auricle lies in front of the ventricle.
The visceral organs that originally belonged to the visceral sac move from its initial posterior face to the anterior face. Thus, the visceral organs originally belonging to the left side become topographically placed on the right side and vice versa.
Significance of Torsion
Torsion’s adaptive importance is hypothetical but has three important significances.
- First, if there were no torsion, the foot would withdraw into the shell first and the more vulnerable head would withdraw last. By entering the shell through torsion, the head is less vulnerable to prospective predators. An operculum, a proteinaceous coating on the dorsal, posterior border of the foot in some snails, improves defence. The operculum covers the entrance of the shell as the gastropod pulls the foot into the mantle chamber, protecting the snail from desiccation in dry environments.
- The second benefit of torsion is that the mantle cavity has an anterior aperture that permits pure water from the front of the snail to enter, rather than water that has been tainted with silt from the snail’s movement.
- The third benefit of torsion is that it increases the snail’s sensitivity to stimuli originating from the direction in which it moves by twisting the sense organs of the mantle around the head region.
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