What's the Difference? Slugs and Snails

The Banana Slug. Go Santa Cruz!

Happy Friday the 13th!

So friend asked me a question about last weeks post: whats the difference between snails and slugs anyway?

Not only is this a great question, but it has inspired me to start a new series of posts called Whats the Difference, where I'll be addressing the differences (and similarities) between commonly mixed up animals and features. So if you've ever been confused about the difference between apes and monkeys, antlers and horns, whales and dolphins, or any one of the numerous other pairs that seem like basically the same thing. If you have a particular pair of things you want to know about, please let me know. Also, "What's the difference"is admittedly a weak name... so I'm taking suggestions!

Todays I'll cover the question that started this, what is the difference between snails and slugs?

Luckily this is a pretty easy distinction. Snails have shells, slugs don't.

Unimpressed? I am sorry to disappoint, but they are very similar in almost every other way. Both snails and slugs are molluscs, which meaning they belong to the phylum Mollusca along with all sorts of other cool critters like mussels, chitons and octopuses/ octopi (yes, both are correct).

But why would a slug ever give up it's shell? Well, there are the obvious disadvantages to the snail shell in terms of body's *ahem* orientation (see last post). But even more than that, slugs have found many ways around the disadvantages of lacking a shell without actually investing in making one. For one thing, slugs live only in humid climates, and even then only come out at night or when it's is especially wet out. Water is vital, and without a shell to protect them from some of that water loss, snails deal with it by choosing their environments carefully. Slugs also don't have to worry about collecting calcium. Calcium is one of the primary elements in building a snail shell, and for marine snails this is not a problem. Calcium is one of the many "salts" that make sea water salty, and if you are making a shell from calcium being surrounded by it makes your job considerably easier. But in a terrestrial environment calcium can be a lot harder to come by. Terrestrial slugs have tried to make the best of a bad situation and simply given up on calcium shells altogether.

However, there are marine slugs as well, meaning that finding calcium might not be the biggest driving factor behind shell loss. Many slugs have taken up burrowing in sediment in order to escape the heat of the day or to find food. As one might imagine, a large coiled shell is rather an inconvenience when burrowing, and it is much more convenient to go without.

Slugs are also not without their own forms of protection. Though they lack a shell, slugs depend greatly on the chemicals in their slime. Slugs are sticky, slimy, and taste terrible. Although different from the protection of a hard shell, the chemicals are never-the-less effective, as some slugs, like the banana slug (GO SLUGS!) have no major natural predators.

So this turned into a post about slugs more than about the difference between slugs and snails, but I think you get my point that there are really just a couple of major differences: Slugs don't have a shell, and a snail is ass-backwards.

Sources I trusted were surprisingly hard to find, but here are the ones I used:
http://www.andrewgray.com/essays/molluscs.htm
http://www.molluscs.at/gastropoda/index.html

The picture is my own

Snails Part 2

Well hello all, did you miss me?
I know that you must be chomping at the bit for more information on snails, and I am here to deliver. However, I should let you know that this is going to focus more on snails in general, rather than focus on the European garden snail from my last post.

While the notion of of a snail carrying it's home on it's back is often depicted as helpful or wise, popular culture seems to miss out on a very important drawback of the snail's shell. That is to say that a snail's butt it located right next to it's head. The drawbacks to such engineering are obvious. And though you might be thinking of a land snail right now, this system is seems especially ill designed for any marine or freshwater snails who live in habitats where everything can just sort of, float around...

Why then, you might ask, would such a creature ever evolve? Well as it turns out, pooping next to your head doesn't much matter if you aren't alive, and shells are especially good at keeping you alive. Evolution favored a shell that was small and compact, yet could be drawn over the entire body in case of emergency. The spiral shell of a snail achieved exactly that. It is small, yet because of it's stacked spiral loops, the snail is able to fit a surprising amount of body mass into it. This design, however, came with a side effect. The snails entire internal body was essentially rotated 180 degrees in a process known as torsion, putting it's anus in that unfortunate position. The exact reason for this torsion has been the topic of some debate by scientists. It was hypothesized that the rotation helped to place body parts so that they would be easier to draw into the shell when needed. This hypothesis, however, has recently been challenged by the "asymmetry hypothesis" which says that the current gastropod (that's snails and slugs) body plan developed from a single side of what was once a bilateral body.

Still, despite their drawbacks, the snails body obviously works. They have become so numerous that entire sates have taken action to try to stop them. Even if you don't seem them directly, the marks they leave on household plants, or national-bound crops, are obvious. Yet, they lack a jaw, or even teeth with which to chew, so how do they cause so much destruction?

Gastropods, and molluscs in general, have (as they so often do) developed a unique structure specifically for this purpose. Located just withing the mouth, the radula consists of rows of chitinous "teeth" (yes, I know I said they didn't have teeth. What I meant is that they don't have proper teeth made of calcium. Theirs are made of chitin). Pulled in and out of the mouth by the muscular odontophore , the radula is feeding organ for the snail. Its teeth can scratch, scrape and cut away at its food, allowing it to become the insatiable "pest" that it is today.

Lots of words that you don't recognize? Well, welcome to the world of Biology.
If you have any more questions about snails or suggestions for future posts please let me know in the comments.

Almost all the information in this post came from: Invertebrate Zoology: A Functional Evolutionary Approach by Edward E. Ruppert and Robert D. Barnes

Additional sources:

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7427.html

Modern Insights on Gastropod Development: Reevaluation of the Evolution of a Novel Body Plan

Louise R. Page

Integrative and Comparative Biology , Vol. 46, No. 2 (Apr., 2006), pp. 134-143

Published by: Oxford University Press

Article Stable URL: http://www.jstor.org/stable/3884787