Wednesday, August 22, 2012

Undomesticated Peeves Part 3

Or, “Hermit Crabs vs. Snails”

This is not an “Alien vs. Predator” kind of setup; instead, I simply want to highlight an extremely important, fundamental difference between hermit crabs and snails—a crucial distinction that so many people, even those with graduate degrees in biology, are ignorant of.

[moon-snail shell with moon snail inside]

[moon-snail shell with hermit crab inside]

Hermit crabs, as I have mentioned before, have wimpy little abdomens, soft and vulnerable and just waiting to be snacked on—but those abdomens are perfectly developed to grip the inner column of a snail shell so that they can wear an armored layer at all times. As hermit crabs grow, they need to find larger shells to fit into, and so they exist in a perpetual state of envy and crisis, convinced that somewhere nearby some other crab has a better shell than they do. They also spend an inordinate amount of time investigating empty shells around them, sometimes switching back and forth between or among shells multiple times before finally making a decision.

All this to say: hermit crabs make their own carapaces, the tough shell coating they share with other crustaceans like lobsters and shrimp, but to protect their abdomens, they must rely on snail shells that they find. Hermit crabs, then, do not make snail shells.

Snails, on the other hand, do.

You would think some hint of this would be evident in the name “snail shell,” but many people are convinced that snails find their shells lying along the sea bottom—perhaps left there by the Shell Fairy—and gratefully pick them up.

This is not the case.

Just as lobsters make their shells, we make our bones, and turtles make their bones and shells, so too do snails make their exoskeletons, adding on to these shells as they (the snails) grow, so that the tiniest pointiest part of a snail shell, the tip of the spire, is always the oldest and earliest shell, the part formed when the snail was just an itty bitty little gastropod. To learn more about the details of how snails actually form shells, you can read this brief and fascinating article from Scientific American written by a shell researcher. Basically, snails secrete a protein matrix on top of which form a couple of layers of calcium carbonate.

Among other things, this means that:

1. If you remove a snail from its shell, that’s it: it can’t run out to the store and get another—so well done, snail-killer.

2. Because snails (and other mollusks like clams) are taking up calcium and other elements from the surrounding seawater to build their shells, shell analysis can tell researchers about the composition of the marine environment in the area that snails are living.

3. Because of ocean acidification—an effect of the huge excess of CO2 our fossil-fuel emissions are releasing and that the ocean is absorbing—and its changes to ocean chemistry, calcium carbonate will be less available for snails (and other organisms) to take up, and could lead to snails with thinner, weaker, smaller shells.

But most importantly, what this means is that snails make their shells. There’s no snail-shell Kmart of the Sea producing them, and all those stunning colors and shapes are created by these unassuming creatures.

How cool is that?

{A note: I do write all text and take all pictures. Please do not reproduce either without my permission.}


biobabbler said...

I'm amazed people didn't know that. How very odd.

Thanks for the world edification! =) And lovely shots, and usu. interesting goodies.

Anca said...

I grew up singing to snails and convinced that my melodic (NOT!) voice brought them out of their shells, so to speak.

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