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Torsion and Detorsion in Gastropod Molluscs: Developmental Biology Explained.Solved UPSC_Zoology CSE Mains previous year questions

Hi I am Dr. Mugdha giving detailed explanation of torsion and detorsion processes in gastropod molluscs, their evolutionary significance, and implications for anatomy and locomotion, tailored for UPSC Zoology exam preparation. UPSC Zoology previous year questions on Torsion and De-Torsion 2019, 2025 This crisp precise answer carter to the need of UPSC Aspirants, UG and PG Zoology students.

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Torsion and Detorsion in Gastropod Molluscs By : Dr. Mugdha Assistant Professor, Zoology UPSC CSE Zoology Mains Previous Year Questions Q1. What do you understand about Torsion and Detorsion in Mollusca? Elucidate. (IAS 2019/15 Marks) Q2. Torsion and De-Torsion ( UPSC 2025 , 10 Marks )
Introductio n Unlike other classes of Mollusca where adults are bilaterally symmetrical, the adult Gastropods are asymmetrical due to the processes called Torsion which happens in its veliger larvae. Torsion or twisting is an evolutionary conserved morphogenetic movement during larval development of Gastropod whereby, visceral- pallium rotates anti-clockwise through 180° from its initial position, so that mantle cavity with its initial pallial complex is brought in front of the body in adult. While the head and foot remains intact. It occurs at sites like neck, behind the head, foot through which oesophagus, rectum, aorta, visceral nerve loops & shell muscles pass.~90% of rotation is due to contraction of larval retractor muscles & 10% of it is contributed by differential growth. As a result of torsion, mantle cavity becomes anterior, renal orifices, ctenidia, anus becomes anterior. The nervous system appears in figure of 8. Alimentary canal twists in visceral mass & opens by anus.
Continue writing  The bulky visceral mass goes close to the substratum. It becomes important as Gastropod shell has only one opening to serve as entry & exit to all visceral mass. Accumulation of buccal mass to the anterior side provides stability during locomotion. Torsion allows foot to be retracted after head for better protection of head. The presence of osphridium in anterior side allows/ensures chemical tasting of water before entering the mantle cavity.  Detorsion is reversal of torsion seen in the larvae of mollusc which have lost their shell secondarily or shell has two openings both in anterior & posterior region negating necessity of torsion. E.g. in Acteon, Bulla, Aplysia.
G phradium G phradium Explaining phenomenon of Torsion
Thank you About the author : Hi I am Dr. Mugdha a professor and avid student of Zoology/Bio Sciences. I am an alumni of Hindu college university of Delhi and two times qualified CSIR_UGC JRF NET , GATE Life sciences and Bio TECH . I also made through several Assistant Professor jobs by BPSC , MPPSC, UPHESC (Rank 6). Since I am an ex-civil service aspirant and have appeared for several civil services mains and interviews organized by UPSC, BPSC UPPSC I have huge interest in teaching and content writing in Zoology/Life Sciences/ Biology/Environment. For Free online demo classes for UPSC Zoology mains/ Environment , Agriculture and Biotech prelims, PGT, TGT, Scientific officer, CUET_UG/CUET_PG Zoology/Life Sciences , NEET Biology. You can also reach me for teaching in your online coaching or content writing even as research methodology trainer. contact me Mobile : 6204164408 Email : panchpran5life@gmail.com