Mollusc shell
The ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid, octopus, and some smaller groups such as the caudofoveata and solenogastres.
Within some species of molluscs, there is often a wide degree of variation in the exact shape, pattern, ornamentation, and color of the shell.
When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle often ceases to produce the shell substance.
[4]: 475 The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but also, in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur.
[4]: 475 Because the extrapallial space is supersaturated, the matrix could be thought of as impeding, rather than encouraging, carbonate deposition; although it does act as a nucleating point for the crystals and controls their shape, orientation and polymorph, it also terminates their growth once they reach the necessary size.
On the whole, the transcription factors and signalling genes are deeply conserved, but the proteins in the secretome are highly derived and rapidly evolving.
[20] Perlucin operates in association with Perlustrin,[20] a smaller relative of lustrin A, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking.
[4] A wide range of enzymes are expressed during the formation of the shell, including carbonic anhydrase, alkaline phosphatase, and DOPA-oxidase (tyrosinase)/peroxidase.
In molluscs whose ecology changes from the larval to adult form, the morphology of the shell also undergoes a pronounced modification at metamorphosis.
Thus the shell gradually becomes longer and wider, in an increasing spiral shape, to better accommodate the growing animal inside.
[4] It may be possible to use shell protein information in gastropod systematics, e.g. to discriminate species level diversity, but methods need further development.
The periostracum and prismatic layer are secreted by a marginal band of cells, so that the shell grows at its outer edge.
[13] Shells are composite materials of calcium carbonate (found either as calcite or aragonite) and organic macromolecules (mainly proteins and polysaccharides).
Semi-slugs are pulmonate slugs with a greatly reduced external shell which is in some cases partly covered by the mantle.
The simplification of a shell form is thought to be relatively easily evolved, and many gastropod lineages have independently lost the complex coiled shape.
Despite this, it can still be accomplished; it is known from one lineage that was uncoiled for at least 20 million years, before modifying its developmental timing to restore the coiled morphology.
[40] The nacreous layer of shells is a complex structure, but rather than being difficult to evolve, it has in fact arisen many times convergently.
[8] Mollusc shells (especially those formed by marine species) are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time (sometimes thousands of years even without being fossilized).
Large amounts of shell sometimes forms sediment, and over a geological time span can become compressed into limestone deposits.
[41] Aragonite can be protected from recrystalization if water is kept away by carbonaceous material, but this did not accumulate in sufficient quantity until the Carboniferous; consequently aragonite older than the Carboniferous is practically unknown: but the original crystal structure can sometimes be deduced in fortunate circumstances, such as if an alga closely encrusts the surface of a shell, or if a phosphatic mould quickly forms during diagenesis.
Detorsion of gastropods results in an internal shell, and can be triggered by relatively minor developmental modifications such as those induced by exposure to high platinum concentrations.
This foliated aragonite is presumed to have evolved from the nacreous layer, with which it has historically been confused, but represents a novelty within the molluscs.
In some marine genera, during the course of normal growth the animal undergoes periodic resting stages where the shell does not increase in overall size, but a greatly thickened and strengthened lip is produced instead.
Females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they partially reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal.
As a structure made primarily of calcium carbonate, mollusc shells are vulnerable to attack by acidic fumes.
