Torsion and Detorsion in Gastropods

Torsion and Detorsion in Gastropods:- Today in this article I’ll discuss Torsion and Detorsion in Gastropods, which is a very important topic from an examination point of view because this question is frequently asked in the exam, whether it is a Competition examination or college examination of BSc Zoology Honors. So Please This article i.e “Torsion and Detorsion in Gastropods” till the end.

Torsion in Gastropoda

  • Mollusca is typically bilaterally symmetrical animals but this symmetry is lost in Gastropoda. 
  • Gastropods are asymmetrical bodies with a mantle cavity lying anteriorly, and the shell and the visceral mass coiled spirally and directed posteriorly. This is due to the process called as torsion or twisting of the body or visceral mass at the time of development.

What is Torsion?

Torsion (twisting) is the rotation of visceral organs in an anticlockwise direction through an angle of 180° on the rest of the body during larval development. The phenomenon takes place in the free-swimming (veliger) larva of gastropods.

  • It converts the symmetrical larva into an asymmetrical adult.

Process of Torsion

(A) Conditions before Torsion

  • The alimentary canal is straight with the mouth and the anus are at the opposite ends.
  • The mantle cavity is situated at the posterior side containing the pallial complex. Also, the ctenidia and two nephridiopores are located posteriorly.
  • The auricles are placed behind the ventricle. 
  • The nervous system is bilaterally symmetrical.
  • The embryo is bilaterally symmetrical in the veliger stage when a foot and a planospiral shell are formed first in this stage.

Torsion and Detorsion in Gastropods

(B) After torsion 

  1. Ventral Flexure: First of all a ventral flexure appears in the larva. It converts the straight alimentary canal into a loop. The shell and visceral mass become cone-shaped first and finally become spirally coiled.
  2. Differential Growth: On the right side of the larva, the growth is retarded; but on the left side it is accelerated.
  3. 3. Anticlockwise rotation: The mantle and pallial complex rotate to an angle of 180° in the anticlockwise direction. The process shifts the organs from the left side to the right side.

Events in Torsion

Torsion produces the following changes:

  1. Looping of the alimentary canal: The alimentary canal becomes loop-like. The anus is brought forward near the mouth.
  2. Twisting of the nervous system: The pleuro-visceral connectives becomes twisted into a figure 8. This phenomenon is called chiastoneury.
  3. Displacement of mantle cavity: The mantle cavity is shifted forward.
  4. Change in the position of ctenidia: The ctenidia are brought and directed forward.
  5. Displacement of auricles: The auricles are shifted to the front of the ventricle.
  6. Coiling of shell and visceral mass: The shell and visceral mass are coiled.
  7. Degeneration of structure on the left side: The organs located on the left side of the larva degenerate.
  8. Loss of symmetry: The bilateral symmetry of the larva is lost as a result of torsion.

Torsion and Detorsion in Gastropods

Significance of Torsion

  • As a gastropod shell has only one opening, it has to serve as an entrance as well as an exit for all visceral organs. Both mouth and anus must open on the anterior side. 
  • The mantle cavity also must open on the anterior side for easy respiration.
  • Respiratory current opposes locomotion after torsion which increases the availability of water inside the branchial chamber. 
  • The visceral mass has to undergo rearrangement so that openings of kidneys, gonads, and anus should migrate to the front side which is the only opening of the shell. 
  •  The small chemoreceptor osphradium also migrates to the front side so that it can chemically analyze the water current entering the mantle cavity.
  •  The bulky buccal mass migrates to the anterior side which provides stability during locomotion. 
  • Torsion allows the foot to be retracted after the head for better protection of the head.

Detorsion in Gastropoda

  • The reversion of torsion is called Detorsion
  •  During detorsion, the visceral mass shifts to an angle of 180° in the clockwise direction; the mantle cavity is brought forward, the alimentary canal is untwisted; the anus is brought forward. 
  • Detorsion occurs in Euthyneura Eg. Aplysia.
Torsion and Detorsion in Gastropods

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