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The shells

The “houses” of molluscs

Shells are the most famous and used marine objects in the world, but what exactly are they?


How to make a shell

The shell is not something already present in Nature for millions of years like rocks but is produced by marine and terrestrial animals called molluscs. Do you know snails, mussels, clams, newts? This entire enormous group of animals produces the shell throughout its life.

Marine mollusks are able to remove calcium ions dissolved in water by transporting them through the blood to a special organ called the mantle. This is able to process calcium by transforming it into calcium carbonate crystals, but the real touch of genius is the production of a special protein substance called conchiolin that cements the calcium carbonate crystals together, creating the shell itself.


What a shell looks like

The shell is composed of three layers:

Photo taken in Nosy Be with the spider shell in the foreground in all its splendor.
Spider Shell
  • Periostracum: organic membrane made of conchiolin that constitutes the outermost layer and is generally brown. Not all shells have this layer, for example it is absent in cowries. (We will see who they are shortly)

  • Ostracum: is the intermediate layer, quite hard, made of crystalline structures

  • Hypostracum or mother-of-pearl: it is the innermost layer, less thick and with a laminar structure, it is the layer that attracts our eyes the most, as it takes on wonderful colours and shades.


This is the standard situation for the vast majority of shells, but we are talking about molluscs, the second largest group of living animals in the world, so there will certainly be no shortage of surprises!


The diversity of shells

Molluscs that have shells are part of the large group of Gastropods and can be either marine or terrestrial.


Let's imagine the classic snail, here it is, a gastropod!


This large group can create extremely original and different shells, let's see some examples...


The cowries

Think of an animal that spends most of its time polishing the outside walls of its house every day! Strange, isn't it? And yet it's true, there are mollusks called cowries that have a very large mantle and can evert it out of the shell to enclose and polish it. Precisely for this reason, cowries are highly sought-after shells for their smooth, shiny surface and rich in unique colors.

Surely you have already seen them, they are among the most famous shells: they grow to a maximum of ten centimeters in size and look like a kind of bag that ends at the bottom with a serrated opening, from where the mollusk's mantle comes out.

The largest cowries can be encountered while snorkeling or diving, they often hide among the coral, coming out more bravely during the night. Instead, it will be easier to find only the shells on the beach, but I recommend leaving them there!

7 cowries on the beach of different shapes and colors.
Cowries on the beach in Nosy Be, Madagascar.


Triton's trumpet shell

They are among the largest marine gastropods in existence, they are part of the Ranellidae family with which they share…. Their shell is tapered and of different colors, very common in the stalls but now rare in the sea. The largest representative is the giant newt (charonia tritonis) with its 40 cm in size, it is the classic shell used by the ancients as a “trumpet”, in fact it is also called Triton’s trumpet (hence the name of newts).

In addition to their magnificent beauty, they are also fearsome predators of a large enemy of the coral reef… Find out who…


Glaucous cone Shell

In the sea, the main rule to follow is "don't touch anything", especially when we don't know! Some wonderful cone-shaped shells, tapered and graceful, with bright colors; are actually hungry predators.

Glaucous cone shell Nosy Be, Madagascar.
Glaucous cone shell Nosy Be, Madagascar.

These animals have a venomous harpoon, developed from a modified tooth. The venom is made from a toxin that paralyzes potential prey such as small fish and invertebrates.

Only a few species of conidia could be dangerous to humans…


In any case, it is better not to touch them!


If you think conidia are really scary, wait until you hear about the next two groups of ruthless predators…



Parasites yes, but with class

The Epitonidae family is a particular group of small mollusks and they have a spiral shell that is very similar to scales. Some species are indeed parasites of corals but...

Today we tell you in particular about the ingenuity of Epidendrium billeeanum, also called yellow sea slug by us mortals.

This, at first glance, cute little slug, is a parasite of the coral Tubastrea coccinea, a wonderful orange/reddish coral commonly called orange cup coral. The curse of this coral is however to have this external skeleton made of a tube, in which the polyp resides and from which the tentacles emerge to feed.

So far everything normal, except that the yellow sea slug is able to eat the polyp directly inside its tube, with a long proboscis that sucks it completely. As if that were not enough, this super mollusk of a few centimeters, lays its eggs in the empty cavity, faithfully copying the tentacles of the polyp itself, which until a short time before was alive and well in its home! In this way, the mother snail guarantees the survival of her offspring, protecting it from predators!


Now that we've made 30, let's make it 31! This story will also make you see the "innocent" kingdom of shells in a different way.


Il Conte Dracula delle conchiglie

This special little group of mollusks called Colubraria, has lost the peculiar chewing apparatus of mollusks (called radula) and in its place has developed a special “proboscis” capable of everting and introflecting. But that’s not the sensational thing about these little creatures: these small mollusks, in particular Colubraria obscura are just a few centimeters long while the length of the proboscis can reach three times the length of the animal itself!


What do they do with this “elephant” proboscis? Like good night-owl Count Draculas, these mollusks insert their proboscis into the open mouths of sleeping fish (for example the parrot fish) and suck their blood!


How many things happen at night in the sea? Amazing, isn’t it?!

Photo taken in Maldives, during a night dive, parrotfish inside its bubble.
Parrotfish inside its bubble

What can a shell tell us?

The shell is not only the home of the mollusk but also its external skeleton, the exoskeleton, so it can give us interesting information about the life of its “owner”.

During the juvenile phase of the mollusk, the shell grows faster and then slows down in maturity, so the larger shells will host “older” animals!

In many shells you can see the growth rings, a bit like trees!


Photo of a forellate cone shell
Perforated cone

The shell is not only a skeleton but also a place to take refuge during the they can in fact present scars or holes that could tell us what its owner has suffered such as predation or accidents. Very often, the shells that we find on the beach have small holes on the surface, they are precisely holes made by predators to be able to attack the animal inside.






Even just walking along the beach we can “listen and touch” so many stories about the custodians of marine beauty… Let’s look beyond the shell itself, close our eyes and imagine what an extraordinary journey it made before arriving on that beach!


What do we and shells have in common?

Can we do something for them or they for us?


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