Write short notes on/Answer the following in about 150 words each : (d) Torsion and detorsion in gastropods

Torsion in Gastropods

Torsion is a defining characteristic of gastropods, occurring during the veliger larval stage. It involves a rapid, usually anticlockwise, 180-degree twisting of the visceropallium (visceral mass and mantle) on the head-foot axis. This rotation is primarily driven by the contraction of larval retractor muscles and, in some cases, differential growth.

Process and Anatomical Changes:

• Rotation: The visceral mass, mantle, and shell rotate 180 degrees. This moves the mantle cavity from a posterior to an anterior position, above the head.
• Organ Rearrangement:
  • The anus, gills (ctenidia), and excretory and reproductive openings are relocated to the anterior end, near the head.
  • The digestive tract becomes U-shaped.
  • The nervous system undergoes chiastoneury (a figure-of-eight crossing of pleurovisceral connectives).
  • The auricles of the heart shift to lie in front of the ventricle.
• Symmetry Loss: The bilaterally symmetrical larva transforms into an asymmetrical adult.

Adaptive Significance of Torsion:

• Protection: The anterior mantle cavity allows the gastropod to retract its head and foot first into the shell, providing rapid defense against predators.
• Sensory Enhancement: Relocation of the osphradium (chemoreceptor organ) to the anterior position allows for sampling of water the gastropod is entering, aiding in locating food or detecting predators.
• Respiration: Anterior positioning of the mantle cavity may improve water flow over the gills, especially in aquatic species, preventing fouling by sediment stirred up by locomotion.
• Shell Balance: Torsion helps to re-center the gravity of the asymmetrically coiled shell, providing better balance during movement.

Detorsion in Gastropods

Detorsion is an evolutionary process observed in some gastropod groups, particularly within the Opisthobranchia (sea slugs) and Pulmonata (land snails), where there is a partial or complete reversal of the torsional twist.

Process and Anatomical Changes:

• Reversal of Twist: The visceral mass and mantle cavity partially or completely untwist, moving the mantle cavity and associated organs back towards a more posterior or lateral position.
• Restoration of Symmetry: Detorsion often leads to a secondary restoration of bilateral symmetry, reducing the asymmetry caused by torsion.
• Nervous System: In completely detorted forms, the nervous system may lose its figure-of-eight pattern and become more symmetrical.
• Organ Repositioning: Ctenidia may point backward, and the visceral loop becomes straightened.

Adaptive Significance of Detorsion:

• Improved Respiration: Moving the gills away from the head can reduce the risk of fouling by expelled waste, enhancing respiratory efficiency.
• Streamlined Body: Detorsion can result in a more streamlined body shape, advantageous for specific lifestyles such as burrowing or swimming (e.g., sea slugs).
• Shell Reduction/Loss: Detorsion often accompanies the evolutionary reduction or complete loss of the shell, as the protective advantages of torsion become less relevant.

Feature	Torsion	Detorsion
Definition	180° twisting of visceropallium during larval stage.	Partial or complete reversal of torsion.
Symmetry	Changes from bilateral to asymmetrical.	Secondary restoration of bilateral symmetry.
Mantle Cavity	Moves from posterior to anterior (above head).	Moves back towards posterior or lateral position.
Nervous System	Develops chiastoneury (figure-of-eight).	Untwists; becomes more symmetrical.
Adaptive Role	Protection, enhanced sensory perception, improved respiration, shell balance.	Improved respiration, streamlined body, linked to shell reduction/loss.
Occurrence	Universal in gastropods during larval stage.	Occurs in specific gastropod groups (e.g., Opisthobranchia, Pulmonata).