Brief description of Torsion- an anatomical event seen in Gastropods of Phylum Mollusca.
(Zoology)
Introduction
Definition of torsion in gastropods
Importance of understanding torsion in gastropods
Overview of the impact of torsion on gastropod evolution and adaptation
Evolution of Torsion in Gastropods
Historical background of torsion in gastropods
Theories and hypotheses on the evolution of torsion
Fossil evidence of torsion in gastropods
Role of Torsion in Feeding
Impact of torsion on the development of radula
Torsion and the ability to access food sources
Specialized feeding behaviors enabled by torsion
Torsion and Locomotion
Influence of torsion on the development of the foot
Torsion's effect on gastropod movement and mobility
Adaptations in gastropod locomotion due to torsion
Torsion and Predator Avoidance
Torsion's link to shell morphology and defense mechanisms
Behavioral adaptations related to torsion in response to predators
Role of torsion in enhancing survival through predator avoidance
Conclusion
Recap of the significance of torsion in gastropod evolution
Implications of understanding torsion for studying other mollusks
Future research directions in studying the impact of torsion on gastropod adaptation
2. Introduction
Molluscs are typically bilaterally symmetrical animals, but this symmetry is lost in gastropods due to the
processes called coiling and torsion.
The Gastropod is the largest Molluscan class , containing at least 60,000 species.
The diet can be either herbivorous or carnivorous.
Gastropods can live for 20 to 50 years.
3. Cornu aspersum
Garden snail
Mapping Indian Snails and Slugs
India has nearly 1450 snails and slugs with the highest diversity in north-eastern
India followed by the Western Ghats. There is hardly any effort in India to
understand the distribution of these lesser-known animal group despite being
second largest phylum after Arthropods.
Limax maximus
Leopard slug Patella vulgate
Common limpets
SHELL
4. Body plan: Gastropods have an asymmetrical body plan and a large
foot.
,
: Gastropods have a single, usually spirally coiled shell, although this can sometimes be small or absent.
Habitat: Gastropods are found worldwide, in marine, freshwater, and terrestrial environments.
Head is distinct bearing tentacles eyes and mouth.
Osphradium is present
CHARACTERISTICS OF GASTROPODS
5.
6. The larval stage of Gastropoda
TROCHOPHORE VELIGER ADULT GASTROPODA (SNAIL)
7. TORSION IS…
It is an anatomical event during
the embryonic development of a
gastropod’s life.
The rotation of visceral mass
mantel and the shell 90-180
degrees counterclockwise with
respect to head and the foot of
the gastropod during its embryonic
development.
8. Factors causing
torsion
Asymmetric cell
proliferation: High cell
proliferation activity on
the right side of mantle
epithelium may be the
driving force of torsion.
Contraction of larval
retractor muscles: These
brings about the rotation and
the torsion.
Larval retractor muscle in Gastropoda
9. FEATURES OF TORSION
◦ Torsion is a distinctive characteristic of ALL gastropods during their laval
development.
◦ This process causes the anus and the mantle cavity to be positioned above the head,
resulting in a unique body plan.
◦ Torsion may occur within an hour or even minutes in some species.
◦ The phenomenon of torsion has great importance in evolutionary history and present
day biology of Gastropods.
10. Conditions before torsion
◦ 1. The mantle cavity is situated at the posterior side containing the pallial complex.
◦ 2. The ctenidia and two nephridiopores are located posteriorly.
◦ 3. The alimentary canal is straight with the mouth at the anterior side and anus at the
◦ posterior side.
◦ 4. The auricles are placed behind the ventricle.
◦ 5. The nervous system is bilaterally symmetrical.
11. ANATOMICAL EFFECTS IN ADULT GASTROPODS
Displacement Of MantleCavity
Mantle cavity was originally posterior in position
After torsion the mantle cavity opens just behind head and its associated parts shifted
forward.
12. 2. Change in relative position:
Before torsion, the anus and ctenidia are pointed backwards and auricles are
situated behind the ventricle
After torsion, The anus and ctenidia come forward and the auricles come to
lie infront of ventricle.
3. Loss of symmetry:
It is displacement of anus towards right side of the mantle cavity and loss or
reduction of paired parts of the primitively left or topographically right side
15. Debatable... TORSION AS THE “ GRAND
CONTROVERSY”
◦ The original advantage torsion gave gastropods is unclear. It is further complicated
by the fact that torsion brought with it a number of problems. A particular problem gastropods had to
overcome come was the location where wastes were excreted – above the head which
can potentially lead to fouling of the mouth and sense organs.
◦ Views against torsion There are many pelagic larvae of lamellidens which are
not twisted but still survive in pelagic larval life.
◦ Nevertheless, the diversity and success of the gastropods suggests torsion
is very advantageous indeed.
16. Advantages Of Torsion
• Protection: Torsion allows gastropods to retract their vulnerable head and foot into their shells,
providing a protective mechanism against predators and harsh environmental conditions.
• Locomotion: The twisted body plan enables gastropods to move in a more streamlined and efficient
manner, facilitating their ability to crawl and swim.
• Feeding: Torsion also plays a crucial role in the feeding process of gastropods, as it allows the mouth and radula
to be positioned in close proximity to the substrate, enhancing their ability to scrape and graze on
surfaces.
17. Effects of torsion on Gastropods
Change in Behaviour
• Torsion affects the behavior of gastropods, particularly in terms of locomotion and orientation.
• Some gastropods exhibit a characteristic crawling pattern due to the twisted body plan.
Changes in Feeding
• The twisted body plan can affect the positioning of the radula, a feeding organ, and alter the
way gastropods consume food.
18. GARSTANG VIEW
◦ Garstang advocated that Torsion is an adaptive feature that are useful to the
larvae for protection of soft parts against enemies of little direct use to the adult
◦ He suggested that before torsion, the untwisted larvae swimming the sea was
subjected as an easy prey to its predators because the mental cavity was at
posterior position and there is no place into which delicate head and velum can
be withdrawn at the time of danger So it is disadvantageous to the larval life.
◦ Therefore, at danger the larvae is able to withdraw its head and velum into the
mantle cavity Then the beating of Celia stops and the larvae falls to the sea
bottom This is a way they avoid predators
19. M o r t o n ’s v i e w
◦ Morton (1958) emphasis the importance of anterior location of
mantle cavity in both larval and adult molluscs.
◦ The anteriorly placed mantle cavity housing the head with the sense
organs, respiratory structures, etc.
◦ In adult, add positive advantage to test water and also to come in
intimate contact for gaseous exchange with the oncoming water.
20. DETORSION
Changes Occurring During Torsion Are Reversible To A Certain Extent .This reversion is known
as detorsion .
Whole group of Euthyneura shows detorsion.
Changes due to detorsion:
• Pallial complex may travel back to posterior end.
• Ctenidia point backwards again.
• Auricles move behind the ventricle again.
• Visceral loop may become untwisted & symmetrical
21. REFERENCES
Books referred:
Invertebrate Zoology by Ruppert Fox Barnes (7th edition)
Biology Of Animals (Volume 1 ) by Ganguly Sinha Adhikari
WEBSITES
https://www.biologydiscussion.com/invertebrate-zoology/torsion-and-detorsion-in-gastropoda-
with-diagram/29244
https://en.wikipedia.org/wiki/Gastropoda
https://www.britannica.com/animal/gastropod
Oxford academic
https://academic.oup.com/icb/article/46/2/134/646556
Research gate
https://www.researchgate.net/publication/51081719_Evidence_that_gastropod_torsion_is_driven_by_asymm
etric_cell_proliferation_activated_by_TGF-_signalling
