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Name: Damien
Status: other
Grade: 4-5
Location: NC
Country: USA
Date: N/A 


Question:
Why does a snail have a shell and a slug does not?



Replies:
You are asking one of the magic questions in biology: How is it that a particular animal has the features it has today? You seem to find it peculiar that there is this huge difference between the snail and the slug in having or not having a shell, but in other respects are very similar creatures. Well, rest assured, you are not alone, and some great scientists sorted through lots of data to build a model we call evolution to answer the question. The most famous of these scientists is the one whose name is closely associated with evolutionary theory: Charles Darwin. Darwin developed his ideas from observing closely related species of birds that were isolated from one another on islands in the Carribean. From those observations, Darwin derived the ideas of natural selection where those individuals of a population of a species that are more fit to live and have offspring will out compete those individuals that are less fit. From those thoughts, Darwin coined the phrase: Survival of the fit.

So how do these ideas help you understand how it is that essentially the same slimy animal came to be separate species where one remains a slimy being but the other is housed in a shell it creates? No doubt, the snail and the slug have a common ancestor. That common ancestor probably looked very similar to a slug, without a shell. Going along basic ideas in evolutionary biology, you can imagine that different groups of the ancestral population came under different environmental pressures. For example, one group may have been in a warm, moist environment where the snail/slug ancestral creatures did not have difficulty maintaining their moist condition, and another group may have migrated to a drier climate where the environment selected for creatures that could better retain their moisture. Alternatively, or in addition, one group may have had the good fortune to live in an environment where it could easily hide from predators that were keen to eat them, whereas the other group may not have been in an environment where lots of plants were available by which to hide. Bit by bit, the selective pressure of the environment creates pressure on the creatures where those that can better withstand the pressure are in a better position to mate and generate offspring. One strategy that clearly developed for better maintaining moisture and/or frustrating predators was the "invention" of the outer armor that we call a shell. So, the rare individual that had the ability to collect calcium and use it to create a shell would have been a very popular mate, more readily held its moisture and maintained its vitality and vigor, and left more and more offspring.

This view of creating new species, a snail from a slug-type forebear, is certainly far more simplistic than what actually happened, but it gives you a framework for thinking through how it is that different organisms have the peculiar characteristics that they have. And the characteristics that are common to all organisms, such as how energy is converted from food, would be characteristics that were developed early in evolution and maintained. Mess with such a characteristics and the likely result: Death. That is the deal in evolution - the stakes are very high, where the more fit an organism is, the more offspring it survives to have, the less fit it is, the less offspring it will generate, and the greater likelihood it dies. Over time, these pressures change what the overall population is, and it is literally how populations that are isolated from one another end up creating different species!

Don Silvert.

Donald J. Silvert, Ph.D.


The different body construction of snails and slugs is adaptation over time to finding food and surviving in different habitats and by different life styles. No one can really say "why" they evolved that way, just that over time they are successful and survive. The shell of a snail provides protection, while the slime that covers a slugs body protects it. This site has a lot of interesting information:

http://www.weichtiere.at/english/gastropoda/index.html

J. Elliott


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