Did you know when you're picking up a shell on the beach you're actually holding a lil critters skeleton? Seashells are kinda hard to define, but generally speaking they're the hard protective outer coverings of marine animals. Most seashells come from dudes called mollusks.
We have our skeletons inside, but mollusks have 'em outside - and are what what we know as a shell. So when you're walking along the beach admiring all the shells - you're actually walking along admiring a graveyard of marine skeletons. Cool, huh!?
Mollusks exude shells as a protective covering. The shells are excreted from the outer surface of the critter which called the mantle, and are made up of mostly calcium carbonate. So how come there are so many different types of shells? Well, there're over 110 000 species of mollusks!
After the lil dude dies, the durable shell remains. Ocean currents carry shells underwater, which is how they end up on the beach for us to find and wonder over.
Although there's a moer of a lot of species of mollusks, they come in 2 major types, bivalves and univalves. Remember latin class? "Bi" means "two", so bivalves are mollusks that have two shell halves that form a whole shell. Stuff like clams and oysters. "Uni" means "one", so univalves just have a one-piece shell, usually a spiral-type shell, often looking something like a larger, stronger and more elaborate snail's shell, guys like conch, whelks and nautilus.
So how come shells come in so many different patterns and colours? Some theories suggest this helps with camouflage against predators. Or that maybe bright colours imply to a would-be eater that the shell is poisonous,like a big sign saying "don't eat me!"
Another theory is that the colour patterns often align with the shell's spiral or axial sculpture. This sculpture, like corrugation in cardboard, strengthens shells against predators such as crabs, and pigment might further strengthen it. So instead of just trying to have a thicker shell, it might be more energy efficient for mollusks to make pigments to help with strengthening the shell.
So where do the colours come from? The food that the mollusk chows causes pigments to be produced within it's mantle. If the pigment secretion is continuous, then spiral or radial lines or bands will be laid down. If pigment secretion is periodic, then spots or flecks will appear on the shell. If the whole mantle is secreting pigment at the same rate, the shell will have uniform color, but if the process is interrupted, then axial or concentric lines appear. If the pigment is secreted in zones, then wavy bands or angular markings occur.
Why are there so many different shapes of shells? Mollusks' shapes are a product of habitat and life style for the most part. Shell shapes have evolved to make their lives easier. A snail that burrows through sand needs a shell that will move through wet sand easily, so a slender and gradually tapering shell, narrow end at the front, would work well. Whereas a mollusk that needs lots of camouflage may have evolved a shell that has a spiny or irregular surface which will catch and hold all sorts of camouflaging encrusting organisms.
The inhabitants of shell's can be some pretty crazy characters. Take for instance the muricid snail - he climbs aboard an oyster, drills a hole through its shell, then inserts its proboscis and uses the teeth at the tip to rasp up the oyster's flesh. The Cooper's nutmeg snail dines on shark! He works its way up through the sand underneath angel sharks resting on the bottom, then it threads its proboscis into a vein in the gills and sucks the shark's blood. For the shark, it's just like a gooey mosquito bite!
So before picking up that shell on the sand - make sure no-one is home, as you might just end up with a sting or a bite!