Molluscs: Torsion and detorsion in gastropods | Zoology Optional Notes for UPSC PDF Download

Torsion in Gastropods: A Twist in Development

What is Torsion?

Course of Torsion

• Initially, the pre-torsional larva is bilaterally symmetrical.
• The mantle cavity is situated posteriorly and ventrally, while the gut is straight and opens at the posterior end.
• A ventral flexure in the body creates a loop in the alimentary canal, bringing the mouth and anus into closer proximity.

• As development progresses, the saucer-shaped visceral mass and the shell transform into a cone shape and eventually become spirally coiled.

Mechanisms of Torsion

1. Muscle Contraction: In some gastropods, torsion occurs through a 180° rotation solely by muscle contraction, as seen in Acmaea.
2. Two-Stage Rotation: Other gastropods undergo a two-stage rotation, with the first 90° achieved through the contraction of larval retractor muscles and the second 90° by differential growth. Examples include Patella and Haliotes.
3. Differential Growth: Some gastropods achieve torsion through differential growth alone, as seen in Vivipara.
4. Differential Growth with Anus Relocation: In certain cases, torsion is driven by differential growth, resulting in the repositioning of the anus to match the adult configuration. Aplysia is an example.
5. Variation in Duration: The time required to complete torsion varies among different prosobranch species, ranging from minutes to several hours.

Factors Initiating Torsion

• Two primary factors initiate torsion: a) Asymmetrical contraction of larval muscles. b) Differential growth between the left and right sides of the larva.

Effects of Torsion

Pre-Torsional Stage Organ Position

• In the pre-torsional stage, the mantle cavity is situated posteriorly and opens backward.
• The mouth and anus are positioned at opposite ends.
• Ctenidia point backward, auricles are located behind the ventricle, and the nervous system is bilaterally symmetrical.
• The visceral sac and shell are dorsal or exogastric.

• After torsion, the mouth cavity opens in front, just behind the head.
• The gut adopts a U-shape, with the anus located near the mouth.
• Ctenidia shift to the front, with the right one moving to the left and vice versa.
• Auricles now appear anterior to the ventricle, with their positions reversed (right to left and vice versa).
• The nervous system becomes twisted into a figure of '8' due to the crossing of the pleurovisceral connectives.
• The visceral sac and shell become ventral or endogastric.

• In specialized forms, the ctenidium, auricle, and kidney on the original right side fail to develop. These organs are topographically placed on the left side.

Significance of Torsion

1. Advantage to Larvae: Torsion is believed to be a larval mutation that provides advantages to the larval stage. It allows the sensitive parts of the anterior end, including the head and foot, to be withdrawn into the anteriorly placed mantle cavity as a protective measure.

2. Stability in Adults: Torsion in adult snails, according to Morton (1958), promotes stability by bringing the snail's body closer to the substrate.

3. Respiratory Efficiency: The repositioning of the mantle cavity enhances the free flow of respiratory water current, aiding in efficient respiration and water circulation.

4. Enhanced Sensory Perception: The forward positioning of receptors (OSPHRADIUM) allows the snail to better scan its surroundings, test water and mud quality, and improve sensory perception.

5. Evolutionary Advantage: Torsion played a pivotal role in the evolution of gastropods, allowing for the development of a conical shell and facilitating the transition from an armor-like shell on the anterior side to a posterior position for more efficient creeping movement.

In conclusion, torsion is a fascinating process in gastropod development, impacting both larval and adult stages in various ways and offering unique advantages throughout their lifecycle.

Detorsion in Gastropoda: Restoring Bilateral Symmetry

Detorsion is a remarkable process in Gastropoda that leads to the partial or complete reversion of torsion, ultimately restoring bilateral symmetry in these fascinating mollusks. The loss of bilateral symmetry occurs due to the processes of coiling and torsion, and detorsion marks the reversal of this condition.

Detorsion During Evolution

Detorsion occurs during the larval stage as a response to the evolutionary loss of a shell or the development of a shell with openings on opposite sides. The Gastropod undergoes a transformation in which it regains its bilateral symmetry. This process is especially characteristic of the entire group known as the Euthyneura.

Key Transformations in Detorsion

Detorsion brings about several significant changes in the anatomy and structure of Gastropods:

1. Pallial Complex Reversal: The pallial complex, which includes the mantle cavity and other associated structures, moves back towards the posterior end along the right side.

2. Ctenidia Repositioning: The ctenidia, originally pointing forward, shift to point backward.

3. Auricles Relocation: The auricles move behind the ventricle.

4. Visceral Loop Untwisting: The previously twisted visceral loop becomes untwisted and symmetrical.

This process leads to the re-establishment of a secondary external symmetry in these Gastropods.

Evolutionary Significance

Torsion is considered disadvantageous for adult snails, and many have undergone detorsion processes to regain their symmetry. Various degrees of detorsion can be observed within the Euthyneura group.

• In less specialized Opisthobranchia and Pulmonata (e.g., Acteon, Bulla), detorsion is not complete, and the visceral loop remains partially twisted. The anus and ctenidia are directed laterally instead of anteriorly.

• In more specialized cases, such as the typical Opisthobranchia (e.g., Aplysia), total detorsion is accompanied by the reduction or disappearance of the shell.

• In extreme cases, such as Pterotrachea, the mantle, visceral sac, external shell, and even ctenidia may be lost, leading to a worm-like body form.

Detailed Insights into Detorsion

1. Reduction of Paired Organs: In some cases, the originally left ctenidium and the osphradium may be absent, contributing to detorsion.

2. Example of Eolis: Eolis demonstrates a veliger larva with a coiled visceral hump that undergoes torsion. However, in adulthood, there are no signs of torsion, and the pallial complex is posteriorly placed. Detorsion must have occurred during further development.

3. Pulmonata Example: In Pulmonata, the pallial complex shifts, but there is no chiastoneury due to the shortening of visceral commissures. The pleurovisceral mass and chiastoneury are secondarily lost.

In summary, detorsion is a remarkable evolutionary response in Gastropoda that leads to the restoration of bilateral symmetry. This process can vary in degree and may involve the loss or reduction of various organs and structures, ultimately resulting in a diverse range of Gastropod forms.