VARIATIONS IN SHELL STRUCTURE THAT HAVE OCCURRED IN THE PHYLUMMOLLUSCA

Molluscs have evolved in the world 600 million years ago during the Ordovician period. “The first molluscs were simple warm like having segments” [1] and it use crawling mode of locomotion. During the Ordovician period seven class molluscs which are present to day were present, they only underwent modification because of their vulnerability to predators. Molluscs are composed by soft bodies, because of predators they underwent modification to develop the hard shell to protect themselves.

Because of the soft bodies that molluscs have, “they produce an external skeleton known as exoskeleton” [1]. It is a hard part that cover the back of molluscs, it also serve as the protective organ, because is not simple for predators to break it. This exoskeleton or shell is formed by calcium carbonate (CaCO3).”In most of the molluscs there is flesh mantle that secretes, modifies and line the shell with mucous” [1] for better locomotion and for protection when there is a predator it allows the body to flip inside the shell smoothly.

Adaptations of the molluscs determine the difference of shell structure to occur in molluscs, these determined by the environment that they are living in. some molluscs chose to live in terrestrial, while other chose to live in the aquatic environment. This adaptation bring difference to the shell structure, some grow large while other looses their shell completely.

Molluscs which have niche in the aquatic environment have different shell structure to the molluscs that live in the terrestrial. Most of the molluscs that have habitant on the ocean they have dome shape shell, while the terrestrial molluscs have the spiral shell structure. This modification and shape of shell structure depend on the environment and the capability of movement, the shape of the shell can also determine the mode as the weight that they have can weigh more and push the molluscs to move in the other way.

The shape of the molluscs that are found in the ocean, they have a dome shape, which is formed by plates. There are eight plates which overlap each other in a very tight way to protect the molluscs from violet serge of ocean waves. Primitive molluscs such as polyplacophora have under gone modification and adopt this type of shell structure, dome shape. It has habitant in shallow water in the ocean.

While those that found on the bottom of the water or ocean have adopted different modification of shell structure. Class Monoplcophora which are found in the deep water have modification on their shell structure, which enables them to walk on the ground level of the sea. They have bilateral shell with a disk shape valve less than 25mm, this shell structure is fragile and thin. The body of molluscs of class monoplcophora does not have segment, but the shell structure develop coiled chambers which is covered with Horney sheath.

Other molluscs develop cyndrical tube which opens at the both end, which resemble the tusk of an elephant. It is elongated, symmetrical and tubular, this type of shell found on the molluscs that found burrowing in the mud, they use their shell to create the burrowing niche.

While other class of molluscs such as Aplocophora do not have shells and there is no fossils that indicate that they were having one. The flesh mantle that they have does not produce shell, but their modification tends to influence “flesh mantle to produces calcareous spicules”. [1] These spicules protect them from predators as it become less palatable.

In all molluscs gastropod underwent a very important modification in the evolutionary part, this influence the change in shell structure. Before its shell was symmetrical coiled like garden horse, but because of modification it change to central axis were coils are laid down, in order to balance out weight of the shell. This modification that gastropod underwent was influence by the habitant they occupy they live in both environment terrestrial and aquatic.

In nutshell the importance of the shell in all molluscs is to give protection to the molluscs as they are vulnerable to predators and their body are soft in such away that it cannot stand the disturbance that can involve when it moves .it also helps them to adapt different environment. Molluscs that live in terrestrial have hard shell that protects them from radiation and predators. While those that found in the aquatic have soft shell that protect them from ocean wave.

Today because of modification in shell structure that molluscs have it allows them to occupy almost all environments. The different that the shell structure has is the one that identify the habitant of the molluscs.

References

1. Bourquin.A.2000. The Phylum Mollusc: [internet] [cited 2006-May-4] Available from: http://www.manandmollusc.net/advanced_introduction/advanced_intro_printable.html

2. Wikipedia contributors. Aplacopra [Internet]. Wikipedia, The Free Encyclopedia; 2006 April 20, 05:55 UTC [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Aplacophora


3. Wikipedia contributors. Polyplacophora [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 12, 14:07 UTC [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Polyplacophora

4. Wikipedia contributors. Monoplcophora [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 21, 01:25 UTC [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Monoplacophora

6. Wikipedia contributors. Scaphopoda [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 23, 12:22 UTC [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Scaphopoda

7. Wikipedia contributors. Gastropoda [Internet]. Wikipedia, The Free Encyclopaedia; 2006 May 20, 06:36 UTC [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Gastropoda

Mr Elelwani Muanalo
NISL- Ecological Informatics Student
CSIR Pretoria
Tel: +27 12 841 2133
Fax: +27 12 842 7024.
Email: emuanalo@csir.co.za
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