How Do Snails Grow Their Shells?

A snail's shell is its permanent home -- but how do these wee, soft, slimy creatures grow hard, beautiful shells? Learn how it works in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff from how stuff works, Hey, brain stuff,
Lauren Vogel bomb Here. Snails are very attached to their shells. Literally,
these slow moving mollusks grow protective shells to which they
are physically connected. Eviction means death. No snail can survive
being torn away from its private calcified fortress. But how

(00:24):
exactly do snails shells develop and what sets them apart
from other types of animal armor? Before we get to
that A bit of snail one oh one. The total
number of snail species could be as high as forty
three thousand. Most of us are familiar with land based
varieties or those that live in ponds, but marine snails
also exist, and when it comes to procreation, certain species

(00:46):
reproduce sexually, while others are self fertilizing hermaphrodites. Whatever gets
the job done right. They all do have one thing
in common, though. Snails all hatch from eggs. Generally, the
parents lay these eggs in loo soil or affix them
to rocks, though a few species are over viviporous, meaning
their eggs hatch inside the mother's body. The newborns then

(01:07):
squirm out and face the world, but this brings us
back to that question about the shell. It's during the
gestation period that a snail's shell begins to form, and
here's where the mantle comes in. This is a crucial
organ possessed by mollusks such as snails. Its function is
making and developing shells. Calcium carbonate is the primary ingredient

(01:28):
in snail shells. Those small amounts of protein also go
into the mix, so in order to build these shells,
the mantle creates an electric current that helps the organism
push calcium ions into the right places. Before hatching, a
baby snail grows a protoconch, the first component of its shell.
Once our little snail leaves the egg, healthy eating becomes

(01:48):
a priority. The mantle will require additional calcium to strengthen
and expand the shell. Instinct compels newborn snails to devour
what's left of the calcium rich egg that they've just escaped,
and thus begins a lifelong habit. Snails of all ages
and sizes need to keep eating foods that are high
in calcium. This is one reason why snails are so

(02:09):
often considered pests. A few of them like to chow
down on spinach, broccoli, turnip, and other calcium loaded crops.
They can also get their calcium fixed by swallowing soil
or gnawing on limestone. That protoconch comes with a small
opening or mouth. The mantle organ adds new layers of calcium,
carbonate and proteins to the mouth from below. As that

(02:30):
new material hardens at the mouth, the shell grows, Spiraling
coils form around the protoconch, which gradually spins around and around,
becoming the apex, or uppermost tip of the snails widening shell.
Depending on the species, the protoconch will either remain there
permanently or break off at some point. Snail shells always

(02:51):
coiled to the right or to the left. That's another
thing that varies from species to species. Most snails possess
right word pointing shells, but there are a few you
whose shells orient towards the left, and in some species
these shells can coil in either direction. If you were
to cut open a snail shell, you've noticed several main
layers in the cross section. At the surface, there's the pereostracum,

(03:13):
a thin outer layer usually made with organic material. Below that,
you've got a layer of hard calcium sitting atop a
blanket of noker that's a resilient composite material better known
as mother of pearl. One thing you won't find, however,
is a network of nerves or blood vessels. Turtle shells, meanwhile,
contain both. In most cases, the shells we find in

(03:35):
these much adored reptiles are really networks of modified bones
like ribs, vertebra, and pelvic elements that have been fused
together and covered by hard plates. When a turtle's shell
gets cracked, the body dispatches cells to try and repair
the damage. Snails have a different method for fixing cracks.
Calcium and protein secretions from their mantles can be used
to help strengthen the damaged areas. Of course, having a shell,

(03:59):
even one that can heal it self, does not guarantee safety. Ironically,
a lot of nesting birds like to eat snails because
of their protective shells and the calcium that they contain.
To keep one step ahead of predators, many snails have
seriously up to their self defense game. For example, the
deep sea species Chrystomalin squire if um is covered in
iron sulfide, giving the entire animal a strange metallic appearance

(04:23):
and acting basically like battle armor. In fact, researchers have
looked into the possibility of using this animal's amazing exterior
to develop better armor for human soldiers. Today's episode was
written by Mark Vansheny and produced by Tyler Clang with
the kind engineering assistance of Ramsay youngt. For more on
this and lots of other perhaps slimy but battle ready topics,

(04:46):
visit our home planet, how stuff works dot com.