Torsion in gastropoda

definition
thomsons explanation
effects of torsion- displacement of mantle
theories of torsion
detorsion

1. TORSION IN GASTROPODA Bollapalli Ashok Kumar M. Sc ., B.Ed.,M.A (Lit)CSIR-NET,( Ph.D ) LECTURER IN ZOOLOGY KRK Govt Degree & PG College ADDANKI-523201 Phone:[phone] Email: [email protected]

2. introduction All the molluscs show bilateral symmetry except the Gastropods which are asymmetrical body with mantle c avity lying anteriorly and the shell and the visceral mass coiled spirally and directed posteriorly This is because of torsion or twisting of the body or visceral mass at the time of development

3. Definition and site of torsion గాస్ట్రోపొడ జీవులలో TORSION ఒక ప్రత్యేకమైన లక్షణం ఈ దృగ్విశ్యంలో అంతరంగ సముదాయం మరియు పాల్లియల్ కుహరం 180 డిగ్రీలులో rotation జరుగుతుంది Due to this movement the mantle cavity and pallial complex is brought to the front side in the adult The head and foot remain fixed and do not take part in torsion The part behind the head that includes the neck through which oesophagus, rectum, aorta and visceral nerve loop undergo torsion

4. Coiling and torsion are two different processes that occur in gastropods Coiling takes place in the shell and accordingly the body of the animal which lives inside the shell Torsion is limited to certain parts of the body and it is no way related to general coiling of the shell and the body

5. Coiling of the shell is of two types namely Planospiral Conispiral Plano spiral in which the whorls of the shell lies outside the preceding coil Conispiral in which the whorls of the shell coils round a pillar like structure called columella . Conispiral coiling took place in evolution because it gives more balance of visceral cavity and solve problems caused due to torsion

6. In course of evolution gastropods became asymmetrical in body except in head region The reason is that the shell increases in size to accommodate the growing body and it topples to one side So the mantle cavity and pallial complex shifted forward from posterior position The foot of the gastropods grew and expanded while viscera expanded into a big hump on the back If this hump grows vertically into a broad cone, it hinders the locomotion of the animal So the viscera becomes spirally coiled to compactly placed

7. As the growth advances the viscera becomes voluminous, it looses its position and leans more to one side This causes unequal growth of the body and right side develops more than that of left side This requires adjustment of structures of the body. So in the larval stage a torsion occurs in the parts like visceral hump and the pallial complex in 180 degrees around a vertical axis

8. It occurs in between head and anus in anticlockwise in the dorso ventral direction As a result the mantle cavity is shifted forward and makes the anus lie by the side of mouth In this torsion, the original left ctenidium topples over the right side. The intestine is thrown into the loop and pleuro visceral connectives form into a figure of “8” by undergoing twist The right connectives become the supra intestinal while the left becomes infra intestinal This twisting of nerves is known as streptoneury

9. The above explanation is an old one and now it is not totally accepted Because in Acmaea the whole process is completed in 2 or 3 minutes Now it is supposed that it is due to asymmetrical position and pull of the larval retractor muscles which run from velar lobes to shell They are present only on right side whose contractions causes torsion

10. In this process only narrow neck of the larvae twisted . So that everything present between head and anus undergoes an anti clock rotation through an angle of 180 degrees in a dorso ventral direction So by torsion, originally dorsal or exogastric shell becomes ventral or endogastric .

11. Thomson’s(1958) explanation Complete or 180degrees rotation , achieved by muscle contraction is limited to Acmaea only 180degrees rotation is achieved in two ways .i.e. first 90 degrees movement is brought about by the contraction of larval retractor muscle and the remaining 90 degrees movement is by difference in growth. It is seen most common way in molluscs like Haliotes and Patella B 180 degrees rotation may occur only by differential growth as in Vivipara Rotation by differential growth processes with anus coming to a position appropriate to the adult state as in Aplysia Torsion no longer recognisable as a movement of Visceropallium , the organs in the post torsional position from their first appearance as in Adalaria

12. EFFECTS OF TORSION Effects of torsion were first realised by SPENGEL in 1881. He coined two terms regarding this phenomenon Streptoneury - nerves undergo a twist in the form of figure ‘8’ and Euthyneura - nerves become straight after torsion The effects are as follows Displacement of mantle cavity Change in position The alimentary canal forms loop due to torsion Chaistoneury Exogastric position of visceral sac and shell becomes endogastric due to torsion Change in position of various organs

13. Displacement of Mantle cavity Due to increase in the length of foot, the mantle cavity and pallial complex from posterior region are shifted forward and mantle cavity opens behind the head.

14. Change in position Before torsion the Anus, Ctenidia and renal openings face backwards and they face forwards after torsion The auricle which is placed behind the ventricle before torsion comes to front of it after Organs of viscera of original right side becomes placed on left side and vice versa

15. Chaistoneury Long , uncoiled pleuro - visceral nerve connectives are twisted to form a figure of “8”. So that the right pleurovisceral becomes supra intestinal nerve and T he left pleuro visceral becomes the infra intestinal nerve

16. Organs like ctenidia , kidney, osphradium etc present on left side move to right side and persist While the organs on right side atrophy and disappear thus losing the original symmetry

17. Theories on Torsion Garstang’s theory Morton’s theory

18. Garstang’s theory It is useful to larva rather than adult as per Garstang He explained that the mantle and head are at the posterior to the foot before torsion and hence when attacked by enemies the delicate parts like mantle and foot are exposed to them But after torsion the mantle cavity become anterior so that the sensitive parts .i.e. head and velum could be withdrawn first and then foot When operculum sealed the aperture of the shell the animal fell down to bottom and escaped from the enimies

19. Objections to Garstang’s theory Many pelagic larvae of bivalves are successfully living pelagic life in which case why not a gastropod larvae is a big question Secondly, the movement of velar cilia is under the control of N.S. and their movement can be stopped at any moment without retracting into the shell In some animals like Haliotes only Partial torsion i.e. 90 degrees occur during larval period at which time head can not be withdrawn first into mantle cavity. The remaining 90 degrees of torsion occurs only after the larvae settles at the bottom

20. Morton’s theory This is quite opposite to Garstang’s theory According to this theory, the advantage is to the adult than the larvae He says that due to torsion the bulky pallial complex comes nearer to the substratum thus increasing the stability of animal When the animal moves up the stream free flow of water into the mantle cavity could not take place because the water should enter from the posterior side of the animal which is difficult because the water flow is against the movement of the animal. But after torsion as the entire pallial complex is shifted anteriorly the inflow becomes easier.

21. Due to torsion organs like osphradium come closer to the head and along with the tentacles of the head it can continuously monitor the water that is entering the body so that fresh and pure water can only be directed in.

22. detorsion When the changes occurred during torsion are reversed, it is called detorsion This causes straightening of the nerves and the group in which it occurs is called Euthyneura Because of this detorsion , the pallial complex goes back to the posterior end, the auricle occupies the place behind the ventricle And the visceral loop untwines and becomes symmetrical and hence the nerves become straightened. So the external symmetry is brought back

23. The extent of detorsion varies in different groups In groups like Opisthobranchiata and Pulmonata the detorsion is incomplete So the visceral loop remains partially in twisted condition The anus and ctenidium are directed laterally instead of forwards. However in Opisthobranchiata group( Aplysia ) there is complete detorsion In groups like Nudibranchiata , the mantle cavity, visceral hump, external shell and ctenidia are lost completely Eg : Doris, Aeolis etc