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Seashells description

The first forms of life on our planet appeared 540 million years ago.
Seashells are one of the oldest forms of animal life.

Below is the chronology of the main forms of life with the 12 geological eras
(Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Holocene(**)).

(**)We are currently living in the Holocene era which began in 9700 BC. The Anthropocene period (or Human era), which began in 1610 when the Old World met the New World, was recently added.

life evolution

There were 5 massive extinctions of species in -445, -370, -248, -201 and -66 million years ago.
The impact of human activity on the climate, ecosystems and the overexploitation of natural resources has triggered what scientists suspect as the sixth mass extinction of our planet.

1 ➫ Classification of seashells in the animal kingdom

In the animal kingdom, seashells are mainly found in 3 classes of the phylum molluscs (which includes 8 in total)

The animal kingdom is one of the 7 kingdoms of the living world

2 kingdoms without nucleus in their cells
5 kingdoms whose cells have a nucleus

1.1 ➫ Branch tree of the animal world (seashells are in the spiralians branch, dinosaurs and other extinct species are not represented)

(negative numbers in parentheses indicate the age, in millions of years, of the oldest fossils in the branch).
950x1603 950x1603 ecdysozoans
950x2394 950x2394 deuterostomians
950x1690 950x1690 spiralians

click a button inside the picture (Deuterostomians, Ecdysozoans or Spiralians) to switch between branches.

An old classification of the animal kingdom distinguishes:

1.2 ➫ List of the 33 branches of the animal kingdom

2 ➫ Anatomy of molluscs

When we speak of seashells we generally think of the shell, forgetting that it is first of all a living animal (a mollusk) which inhabited this shell.

Molluscs are invertebrate animals whose bodies include a visceral sac (internal organs), a mantle, a pallial cavity and a radula (except bivalves).

2.1 ➫ Mantle

The mantle forms a sort of cape draping the shell of the animal. It has the crucial task of containing visceral mass. For conchifers, the mantle has another function: to build and maintain the shell.

deployed Calpurnus verrocus mantle

Calpurnus verrocus mantle

2.2 ➫ Pallial cavity

The pallial cavity is a space formed by the folds of the mantle. For most molluscs it contains the gills and acts as a respiratory chamber. In bivalves it is used to suck food. In cephalopods and scallops it is also a locomotion organ.

2.3 ➫ Siphon

Mollusc siphons are tube-like structures in which water circulates. Water flow is used for one or more purposes such as locomotion, feeding, respiration, and reproduction.

Most marine gastropods have a siphon. Some bivalves do not have a siphon, for others the siphons are always paired. In cephalopods, there is a single siphon or funnel called the hyponome.

2.4 ➫ Radula

Molluscs are the only living organisms with a radula, it is a preponderant organ for their classification. It's a kind of tongue bristling with micro-teeth. It allows the animal to feed by scraping micro-algae (or plants) and bringing them towards the mouth like a conveyor belt.

radula system of a snail

radula system

A few rare gastropods do not have a radula: the Tethydidae , the genus Clathromangelia (family Clathurellidae) and, in terrestrial environment, the species Careoradula perelegans (family Streptaxidae). Most cephalopods have a radula with the exception of the spirulae. Bivalves don't.

The radula consists of several transverse rows of teeth.

The number of rows, the shapes and the arrangement of teeth can characterize a whole superfamily (Epitonioidea : ptenoglossa radula).

The Sacoglossa order characterizes molluscs whose radula has only one row of teeth.

A row can have three types of teeth:

  • a central tooth
  • one or more lateral teeth (possibly with a dominant lateral tooth) on each side of the central tooth.
  • one or more (sometimes even a very large number) marginal teeth beyond the lateral teeth.

There are seven types of radula depending on the configuration of these three types of teeth:

docoglossa ou stereoglossa: the most primitive form, the central tooth can sometimes be absent, 3 lateral teeth (the most external dominant) and 3 marginal teeth.

docoglossan radula

rhipidoglossa: a central tooth, five lateral teeth (the most external sometimes dominant) and a large number of marginal teeth acting as a kind of broom to recover the remaining food debris.

rhipidoglossan radula

hystrichoglossa: a central tooth , 27+14 lateral teeth and a large number of marginal teeth.

hystrichoglossan radula

radula (left side) of Perotrochus caledonicus (1 central tooth, 22+14 lateral teeth and 53+7 marginal teeeth)

ptenoglossa: no central or lateral tooth but a large number of marginal teeth.

radula ptenoglosse

radula of Janthina janthina

taenioglossa: seven teeth in total including a central tooth, a lateral tooth and two marginal teeth (on both sides) or 2x (1 + 2) +1.

taenioglossan radula

stenoglossa or rachiglossa: three teeth in total, one central tooth and two lateral teeth (sometimes only one central tooth). This type of radula makes it possible to dig powerfully into the shell of a prey and, using acid secretion, to pierce it.

stenoglossan radula

toxoglossa: a very small central tooth (if not at all) and two long pointed side teeth with barbs. They contain a channel for the venom and work separately. As their connection to the base of the radula is very loose, they can be ejected like a harpoon on a prey. The radula of certain Conidae (Gastridium geographus, Cylinder textile) contains a venom that is lethal to humans.

toxoglossan radula

2.5 ➫ Feeding

The different types of radula show the evolution of gastropods in their mode of feeding: herbivores, detritivores, scavengers or carnivores. Scraping algae requires many teeth, according to the first three types. Carnivorous types need fewer teeth, especially lateral and marginal teeth. The ptenoglossus type is located between the two extremes and is typical of gastropods parasitic on polyps.

Carnivorous gastropods use different techniques to capture their prey:

  • acid: the combined use of the radula with acid secretions makes it possible to perforate the shell of a prey to devour it from the outside
  • asphyxiation: by enclosing a prey in a pocket formed by their foot and then waiting for its agony to devour it
  • poison: either injecting it directly via a small harpoon (toxoglossan radula of Conidae) or indirectly by diffusing toxins into the water
  • some just vampirize their prey: they make a small cut with their radula, and pump the blood. The salivary glands then produce blood thinners that prevent the pump from clogging.

bivalves feed on phytoplankton and microorganisms by filtering the water.

All cephalopods are carnivorous. They have a two-part beak and most have a radula, although it is small in most octopuses and completely absent in Spirula. They feed by capturing prey with their tentacles, then grab them into their mouths. They have a mixture of toxic digestive juices, some of which are made by symbiotic algae, which they eject from their salivary glands onto their captured prey. These juices separate the flesh of their prey from the shell.

3 ➫ Shell construction

To create their shells, the seashells acquire the elements (calcium, carbon and oxygen) from their food but also from the water around them.

These elements are taken up from the blood stream by specialized cells in the mantle, they are then combined with specific proteins which produce secretion on the outer surface of the mantle.

These secretions are then deposited by the movements of the mantle on the surface of the shell, harden quickly and thus make the shell enlarge.

The shell consists mainly of calcium carbonate (CaCO3).

The formation of the shell is more than a simple continuous addition of material to the edge of the opening. Mollusc shells are complex structures made up of three layers, each created separately.

The first two layers are made by the front edge of the mantle, at the outer lips of the shell opening.

The mother-of-pearl layer, on the other hand, is constructed by the entire surface of the mantle. Throughout the life of the animal, the mantle adds mother-of-pearl to the inner surface of the shell, making it thicker and thicker.

NB: The mantle of cowries secretes a fourth layer above the periostracum: the epiostracum which gives them this smooth and shiny appearance.

The precise description of a shell uses a very specific lexicon that it is important to know in order to fully understand the descriptive sheets for each family.

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