UPSC Zoology Notes: Torsion and De-Torsion in Gastropods

UPSC Zoology Notes: Torsion and De-Torsion in Gastropods

These detailed notes cover the phenomenon of torsion and de-torsion in gastropod molluscs, a key topic for UPSC Zoology (UG/PG) examinations.

What is Torsion?

• Definition: Torsion is a 180° twisting of the visceral mass and shell during larval development.

• Process: It occurs in two stages – first 90° due to muscular contraction (velar muscle), second 90° due to differential growth.

• Result: The anus and mantle cavity come to lie anteriorly, above the head.

Effects of Torsion

• Coiling of the visceral nerve loop.

• Anterior placement of mantle cavity and anus.

• Overlapping of digestive and reproductive systems.

• Reduction of right-side gill and kidney (in many forms).

What is De-Torsion (Detorsion)?

• Definition: De-torsion is the reversal of torsion, untwisting the visceral mass back to a more bilateral or asymmetrical arrangement.

• Occurs in opisthobranchs and pulmonates.

• Result: Mantle cavity moves posteriorly, restoration of bilateral symmetry in some organs.

Comparison: Torsion vs. De-Torsion

• Torsion – 180° twist, affects larval development, leads to asymmetry.

• De-torsion – partial or full untwisting, restores symmetry, linked with secondary aquatic life.

Significance for Exams

Clear, point-wise explanations with diagrams and comparison tables. Ideal for UG/PG Zoology and UPSC optional preparation.