Francis Horne, a biologist who studies shell formation at Texas State University, offers this answer.
The exoskeletons of snails and clams, or their shells in common parlance, differ from the endoskeletons of turtles in several ways. Seashells are the exoskeletons of mollusks such as snails, clams, oysters and many others. Such shells have three distinct layers and are composed mostly of calcium carbonate with only a small quantity of protein--no more than 2 percent. These shells, unlike typical animal structures, are not made up of cells. Mantle tissue that is located under and in contact with the shell secretes proteins and mineral extracellularly to form the shell. Think of laying down steel (protein) and pouring concrete (mineral) over it. Thus, seashells grow from the bottom up, or by adding material at the margins. Since their exoskeleton is not shed, molluscan shells must enlarge to accommodate body growth. This pattern of growth results in three distinct shell layers: an outer proteinaceous periosteum (uncalcified), a prismatic layer (calcified) and an inner pearly layer of nacre (calcified).
In comparison, turtle shells are part of the vertebrate animal's so-called endoskeleton, or skeleton from within the body. Surface scutes are epidermal structures, like our fingernails, made of the tough protein keratin. Underneath these scutes are the dermal tissue and calcified shell, or carapace, which is actually formed by fusion of vertebrae and ribs during development. By weight, such bone consists of about 33 percent protein and 66 percent hydroxyapatite, a mineral composed largely of calcium phosphate with only some calcium carbonate. Why exoskeletons of snails and clams are calcium carbonate while the endoskeleton of vertebrates like turtles are primarily calcium phosphate is not known. Both shells are strong, allow for protection, attachment of muscles and resist dissolution in water. Evolution works in mysterious ways.
Unlike seashells, turtle shells have living cells, blood vessels and nerves, including a large number of cells on the calcareous shell surface and scattered throughout its interior. Bone cells that cover the surface and are dispersed throughout the shell secrete protein and mineral and more or less entomb themselves. The bone can grow and reshape continuously. And when a bone breaks, cells are activated to repair the damage. In fact, turtle shell grows from within just like leg bones in humans. Nutrients such as protein and calcium are supplied by blood vessels within the bone, not from outside of the bone tissue. Damaged seashells, on the other hand, use secretions of proteins and calcium from the mantle cells underneath the shell for repair.
Construction of both turtle shells and seashells share some fundamental mechanical properties. The currently accepted understanding of how shell forms is that the protein matrix of bone and seashell is secreted out of the cells. These proteins tend to bind calcium ions while guiding and directing calcification. Binding of calcium ions to the protein matrix enhances crystal formation according to precise hierarchical arrangements. Exact details of this mechanism remain unclear in both turtles and seashells, but many proteins have been isolated that are known to play a role in shell formation. Whether the calcium carbonate crystal is calcite, as in the prismatic layer, or aragonite, as in the nacre of a seashell, seems to be protein-determined. Secretion of different kinds of proteins at different times and places in the seashells seems to direct the type of calcium carbonate crystal formed. Calcified bone or shell of turtles, on the other hand, does not readily form different crystals.
Whereas turtles grow their bones like humans or other land animals and thus make more room for themselves, snails and clams have to gradually enlarge and extend their shells by adding new organic matrix and mineral to the outer margins of the shell. The newest part of the snail shell, for example, is located around the opening where the animal pokes out. The outer edge of its mantle continuously adds new shell at this opening. First, an uncalcified layer of conchiolin--protein and chitin, a strengthening, naturally produced polymer--is formed. Then comes the highly calcified prismatic layer that is followed by the final pearly layer, or nacre. The iridescence of the nacre occurs, incidentally, because crystal aragonite platelets function like a diffraction grating in dispersing visible light. Sadly, turtles lack this mechanism, which keeps their shells more dull, but their shells are perfect for hiding in the undergrowth or murky waters. Clearly, not all shells are the same.
See what we're tweeting about
10 Comments
Add CommentNice to see this account of how shells are formed. Apart from its intrinsic interest, there is also the biomimicry aspect of this topic. Mollusks have perfected a means of constructing dwellings using calcium carbonate over billions of years, getting their supply of carbon dioxide from the atmosphere. In effect they create a vast carbon sequestration mechanism. By contrast, we build our structures using Portland cement, a process that actually emits vast quantities of carbon dioxide, adding to the global warming problem.
Reply | Report Abuse | Link to thisIsn't if logical to turn things around, so that buildings could be structured with a biocement modeled on shells. In that way, as construction proceeds (as it is in China and India at a vast rate) it will act as a carbon sequestration device, helping to solve the problem of global warming rather than making it worse.
The Tasmanian company TecEco has some ideas on this score. Isn't it worth investigating with some real R&D muscle power?
Truly impressed. I havent much to contribute but I am very encouraged that there are companies that are looking into making construction more eco-friendly.
Reply | Report Abuse | Link to thisWhat is the seaweed like structure baby seashells come/ hatch out of called? In Florida (on the gulf side) they hatch in the spring.
Reply | Report Abuse | Link to thisI would like to differ. Put simply seashells are formed due to sea current and years and years of erosion. Take the mollusk for instance, the only animal born with a shell. The mollusk travels from coral to coral picking up metals such as calcium, sodium etc. Over the years of these atomic changes form bonds and the mollusk has a big pretty seashell. The End.
Reply | Report Abuse | Link to thisso which came first, the seashell or the creature inside? This gives me the impression that all seashells are formed by the creatures who inhabit them. If so, why do some creatures need to leave their shell to find a larger shell to dwell in as they grow? And is there a period of time when this creature exists before it is able to create his protective shell? This has confused me for some time.
Reply | Report Abuse | Link to thisHow are sand dollars formed & do they have 2 separate halves?
Reply | Report Abuse | Link to thisHow do shells make colours & patterns in their shells
You said that, "Mollusks have perfected a means of constructing dwellings using calcium carbonate over billions of years." Are you sure its billions of years and not millions or thousands? You may want to consider making intellectually dishonest and unscientific claims in a non-scientific magazine. Your global warming statements also lack credibility. I suggest you share your unscientific and PC assumptions in a fiction or theory forum, not in a science forum.
Reply | Report Abuse | Link to thisYour hilarious explanation belongs in a Joke magazine, not in a scientific forum. Things as intricate, patterned and detailed as sea shells don't just happen by erosion. It's too bad that silly assumptions like yours do nothing more than take up space in a scientific magazine.
Reply | Report Abuse | Link to thisI have also wondered about how quickly the baby mollusk begins forming it's shell, but it makes sense to me that it would begin immediately. I do know that, unlike the hermit crab, the mollusk remains in it's shell, enlarging or adding to its beautiful underwater home as it grows. I would love to know if there is time lapse photography of a mollusk making and enlarging it's shell. I would love to see it someday.
Reply | Report Abuse | Link to thisAre you aware of any time-lapse photography of a mollusk forming and enlarging it's shell?
Reply | Report Abuse | Link to this