The Importance of the Spiral: It might just determine who you end up with

Nature can be really impressive.

Take, for example, the humble snail. Many people are aware that a snail’s shell is actually it’s external skeleton, but take a moment to think about that. Could you imagine having your bones outside your skin?

Even animals we associate with having protective shells, like turtles, don’t really have bones outside of their skin. A turtle’s shell is actually an internal skeleton. The shell is essentially a modified spine and ribs, and is covered with scutes, composed of keratin, the same thing as your hair and fingernails. And between those scutes and bone is skin tissue. Even though it looks like a turtle’s shell is outside of its body, it’s actually an internal structure.

Turtle shells are  internal skeletons . The shell is actually a modified spinal column and ribs. Image: Fritz Flohr Reynolds (CC)
florida box turtle
Florida box turtle (Terrapene carolina bauri). Notice the hard plates of scutes on the turtle’s shell. The scutes of most turtles are made of keratin, the same protein as your hair and nails. Notice the ridges on these scutes, similar to those you’d find on your finger nails.  Image: Jonathan Zander (CC)


Snails, on the other hand, have a truly external skeleton. There is no tissue on the outside of it. And that makes it even more interesting.

molting crab
Other armored sea creatures, like crabs and lobsters can molt out of their old exoskeletons when they get too small. Image: NOAA Photo Library (PD)

Other animals that have exoskeletons, like crabs and lobsters, need to shed those structures when they grow out of them. But snails don’t do that.

So how do snails – and their shells – grow?

Snail shells are composed primarily of calcium carbonate over a thin protein layer. Or, as biologist Francis Horne says: “think of laying down steel (protein) and pouring concrete (mineral) over it”.

We often make the analogy of a shell as an outer skeleton. But actually, our skeletons are comprised of different materials. Snail shells are comprised of about 3% protein and 97% of the mineral calcium carbonate. Endoskeletons, on the other hand are about 33% protein and 66% of the mineral calcium phosphate (and small amounts of other minerals).

But now let’s think about how shells are actually made. There’s been a lot of research on this topic, but scientists still aren’t sure exactly how it happens. We do know that animals like snails and clams need to extend the ends of their shells to grow larger. The newest part of the shell forms where the animal’s head sticks out and they actually grow in that characteristic spiral shape.

Snail shells are formed when a special tissue called the mantle secretes a protein mixture at the edge of the shell. Image: James St. John (CC 4)

First, a special tissue in the snail called the mantle secretes a mixture of protein and chitin.

The mother of pearl pattern on the inside of this rainbow abalone (Haliotis iris) is due to high concentrations of aragonite. Image: H. Zell (CC 4)

And this is where it gets tricky. Currently, the accepted hypothesis is that this protein mixture attracts and binds to calcium that is in the water column, creating a layer of calcified crystals – this is what makes it so tough.

Lastly, on the inside of the shell, a mother-of-pearl layer is formed due to high concentration of an substance called aragonite which diffracts light into the luminescent pattern we all know.

And just think – these shells come in a vast array of shapes and sizes! And there was an animal that had to make each one of them. And while shells produced from different species can look very different, shells produced by the same species tend to look quite similar.

For example, most species of snail have shells that always spiral in the same direction. When viewed from above, they either coil to the right or to the left. And for very good reason. It turns out that the location of a snail’s reproductive organs are affected by how its shell whorls. Because of this, it’s physically difficult for individuals with opposite shell shapes to mate.

So ladies, next time you put your hair in a bun, which direction will you spiral? If you were a snail, that’d be a life-changing decision.

Feature Image: Moon Snail by Nick Hobgood (CC 4)

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