Exoskeleton is a hard, external structure that covers, supports, and protects an animal's body, such as the chitinous covering of a crab, the silica shells (frustules) of diatoms, or the calcareous shells, or valves, of bivalve mollusks. The term exoskeleton is used in contrast to the endoskeleton, or internal support structure, that provides body structure and shape to such animals as chordates and echinoderms.
While the term exoskeleton most commonly is used for invertebrates, such as arthropods, it is sometimes extended to such vertebrate structures as the shell of turtles and the hard covering of many groups of fossil fishes (such as the placoderms) (Gilbert et. 2007).
Exoskeletons can play a defensive role in protecting the soft tissues from predators, providing support for those tissues and a framework for attacking musculature, acting as a barrier in terrestrial organisms against desiccation, and even functional roles in feeding, storage, sensing, and movement (Bengtson 2004). For humans, exoskeletons add to the diversity and wonder of nature, such as the diverse sea shells of mollusks, and provide important fossil evidence in understanding the history of life on earth.
Mineralized exoskeletons first appeared in the fossil record about 550 million years ago, and their evolution is considered by some to have played a role in the subsequent Cambrian explosion of animals. The Precambrian-Cambrian transition was a time of a burgeoning of diverse organisms with such exoskeletons (Bengtson 2004).
Types and description
Many taxa produce exoskeletons, which may be composed of a range of materials, including chitin, calcium carbonates, silica, bone, cartilage, and dentine. Organisms range from the microscopic diatoms and radiolaria to the innumerable species of arthropods, to vertebrates such as turtles. Exoskeletons appear to have arisen independently many times, with eighteen lineages involving calcified exoskeletons alone (Porter 2007).
The tough or resistant exoskeleton of arthropods (insects, crustaceans, and so on) typically is constructed of the tough polymer of chitin. A typical arthropod exoskeleton is a multi-layered structure with four functional regions: Epicuticle, procuticle, epidermis, and basement membrane (Meyer 2006). Of these, the epicuticle is a multi-layered external barrier that, especially in terrestrial arthropods, acts as a barrier against dessication. The strength of the exoskeleton is provided by the underlying procuticle, which is in turn secreted by the epidermis.
Arthropod cuticle is a biological composite material, consisting of two main portions: Fibrous chains of alpha-chitin within a matrix of silk-like and globular proteins, of which the most well-known is the rubbery protein called resilin. The relative abundance of these two main components varies from approximately 50/50 to 70/30 protein/chitin, with softer parts of the exoskeleton having a higher proportion of chitin. Although the cuticle is relatively soft when first secreted, it soon hardens in a poorly-understood process that involves dehydration and/or tanning mediated by hydrophobic chemicals called phenolics. Different types of interaction between the proteins and chitin leads to varying mechanical properties of the exoskeleton.
In addition to the chitino-proteinaceous composite of the cuticle, many crustaceans, some myriapods, and the extinct trilobites further impregnate the cuticle with mineral salts, above all calcium carbonate, which can make up up to 40 percent of the cuticle. This can lead to great mechanical strength.
The shell of mollusks is a usually calcareous exoskeleton enclosing, supporting, and protecting the organism. Bivalves also move their two valves for swimming. The majority of shell-forming mollusks belong to two classes: Gastropoda (univalves, or snails) and Bivalvia (bivalves or clams, oysters, scallops, and so on). There are, in addition, three other classes of mollusks which routinely create a shell, and those are Scaphopoda (tusk shells), Polyplacophora (chitons, which have eight articulating shelly plates), and Monoplacophora (single-shelled chiton-like animals which live in very deep water, and which superficially resemble minute limpets.) Nautiluses are the only extant cephalopods which have an external shell.
Mollusk shells are composite materials of calcium carbonate (found either as calcite or aragonite) and organic macromolecules (mainly proteins and polysaccharides). Shells can have numerous ultrastructural motifs, the most common being crossed-lamellar (aragonite), prismatic (aragonite or calcite), homogeneous (aragonite), foliated (aragonite), and nacre (aragonite). Shells of the class Polyplacophora are made of aragonite.
In those mollusks which have a shell, the shell grows gradually over the lifetime of the mollusk by the addition of calcium carbonate to the leading edge or opening, and thus the shell gradually becomes longer and wider, such as in an increasing spiral shape, to better accommodate the growing animal inside. The animal also thickens the shell as it grows, so that the shell stays proportionately strong for its size.
Mollusk 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). They fossilize easily, and fossil mollusk shells date all the way back to the Cambrian period. Large amounts of shells sometimes form sediment, and over geological time spans can become compressed into limestone deposits.
Other non-vertebrate exoskeletons
Calcium carbonates also are used for the exoskeleton in brachiopods and some polychaete worms. Silica is used for the exoskeleton in the microscopic diatoms and radiolaria. Some fungi and bacteria likewise have mineral exoskeletons. Some organisms, such as some formanifera, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms do not possess an exoskeleton, as their test is always contained within a layer of living tissue.
Bone, cartliage, and dentine are used for the exoskeleton in vertebrates such as the Ostracoderm fish and turtles. Turtles, for example, have a special bony or cartilaginous shell. While some consider the turtle shell not to be an exoskeleton, on the basis of it being a modified ribcage and part of the vertebral column (Ardastra 2008; Martinelli 2007), others are specific that the turtle shell and the covering of fossil fishes (particularly placoderms) are indeed an exoskeleton (Smith and Hall 1993; Gilbert et al. 2007). Indeed, Gilbert et al. (2007) includes the cranial and facial dermal bones as part of the vertebrate exoskeleton, and attribute their being derived from the neural crest cells.
Furthermore, other lineages have produced tough outer coatings analogous to an exoskeleton, such as some mammals—constructed from bone in the armadillo, and hair in the pangolin—and reptiles such as crocodiles with their bony scutes and horny scales.
Growth in an exoskeleton
Since exoskeletons are rigid, they present some limits to growth. Some organisms, such as mollusks, can grow by adding new material to the aperture of their shell. In those gastropods with shells, for example, the shell is in one piece and typically coiled or spiraled, and the organism can grow by adding calcium carbonate such that the shell becomes longer, wider, and increasingly spiraled.
However, in arthropods, the animal must molt their shell when they outgrow it, producing a replacement. Growth is periodic and concentrated into a period of time when the exoskeleton is shed. The molting, or ecdysis, is under the control of a hormone called ecdysone. Molting is a complex process that is invariably dangerous for the arthropod involved. Before the old exoskeleton is shed, the cuticle separates from the epidermis through a process called apolysis. New cuticle is excreted by the underlying epidermis, and mineral salts are usually withdrawn from the old cuticle for re-use. After the old cuticle is shed, the arthropod typically pumps up its body (for example, by air or water intake) to allow the new cuticle to expand to a larger size: The process of hardening by dehydration of the cuticle then takes place. Newly molted arthropods typically appear pale or white, and darken as the cuticle hardens.
Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually decompose before they can be fossilized. Mineralized exoskeletons can be preserved, such as with shell fragments. The possession of an exoskeleton also permits other routes to fossilization. For instance, the tough layer can resist compaction, allowing a mold of the organism to be formed underneath the skeleton (Fedonkin et al. 2007). Alternatively, exceptional preservation may result in chitin being mineralized, as in the Burgess shale (Butterfield 2003), or transformed to the resistant polymer keratin, which can resist decay and be recovered.
However, relying on fossilized skeletons also significantly limits and skews an understanding of evolution. Only the parts of organisms that were already mineralized are usually preserved, such as the shells of mollusks. It does help that exoskeletons often contain "muscle scars," marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone (Fedonkin et al. 2007). However, although there are 30-plus phyla of living animals, two-thirds have never been found as fossils since most animal species are soft-bodied and decay before they can become fossilized (Cowen 2005).
Mineralized skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago. The evolution of a mineralized exoskeleton is seen by some as a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells (Fedonkin et al. 2007), while others, such as Cloudina, had a calcified exoskeleton (Hua et al. 2003). Some Cloudina shells even show evidence of predation, in the form of borings (Hua et al. 2003).
On the whole, the fossil record contains mineralized exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started out with a non-mineralized exoskeleton, which they later mineralized, this makes it difficult to comment on the very early evolution of each lineage's exoskeleton. It is known that in a very short course of time just before the Cambrian period, exoskeletons made of various materials—silica, calcium phosphate, calcite, aragonite, and even glued-together mineral flakes—sprang up in a range of different environments (Dzik 2007).
While some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells and others a calcified exoskeleton, mineralized skeletons did not become common until the beginning of the Cambrian period, with the rise of the "small shelly fauna." Used as the title of a paper by Crosbie Matthews and Vladimir Missarzhevsky in 1975, this term denoted fossils of the earliest skeletal animals, although they were not always small and not always shelly (Bengtson 2004). Just after the base of the Cambrian, these fossils become diverse and abundant—this abruptness may be an illusion, since the chemical conditions that preserved the small shellies appeared at the same time (Dzik 1994).
Most shell forming organisms appear during the Cambrian period, with the bryozoans being the only calclfying phylum to appear later, in the Ordovician. The sudden appearance of shells has been linked to a change in ocean chemistry, which made the calcium compounds of which the shells are constructed stable enough to be precipitated into a shell. However, this is unlikely to be a sufficient cause, as the main construction cost of shells is in creating the proteins and polysaccharides required for the shell's composite structure, not in the collection of the mineral components (Bengtson 2004). Skeletonization also appeared at almost exactly the same time that animals started burrowing to avoid predation, and one of the earliest exoskeletons was made of glued-together mineral flakes, suggesting that skeletonization was likewise a response to increased pressure from predators (Dzik 2007).
Ocean chemistry may also control of what mineral shells are constructed. Calcium carbonate has two forms, the stable calcite, and the metastable aragonite, which is stable within a reasonable range of chemical environments, but rapidly becomes unstable outside this range. When the oceans contain a relatively high proportion of magnesium compared to calcium, aragonite is more stable, but as the magnesium concentration drops, it becomes less stable, hence harder to incorporate into an exoskeleton, as it will tend to dissolve.
With the exception of the mollusks, whose shells often comprise both forms, most lineages use just one form of the mineral. The form used appears to reflect the seawater chemistry—thus which form was more easily precipitated&mdas;at the time that the lineage first evolved a calcified skeleton, and does not change thereafter (Porter 2007). However, the relative abundance of calcite—and aragonite—using lineages does not reflect subsequent seawater chemistry; the magnesium/calcium ratio of the oceans appears to have a negligible impact on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions (Kiessling et al. 2008).
A recently-discovered modern gastropod that lives near deep-sea hydrothermal vents illustrates the influence of both ancient and modern local chemical environments: Its shell is made of aragonite, which is found in some of the earliest fossil mollusks; but it also has armor plates on the sides of its foot, and these are mineralized with the iron sulfides pyrite and greigite, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents (Bengtson 2004).
Humans have long used armor as an “artificial exoskeleton” for protection, especially in combat. Exoskeletal machines (also called powered exoskeletons) are also starting to be used for medical and industrial purposes, while powered human exoskeletons are a feature of science fiction writing, but are currently moving into prototype stage.
Orthoses are a limited, medical form of exoskeleton. An orthosis (plural orthoses) is a device that attaches to a limb, or the torso, to support the function or correct the shape of that limb or the spine. Orthotics is the field dealing with orthoses, their use, and their manufacture. An orthotist is a person who designs and fits orthoses.
A limb prosthesis (plural prostheses) is a device that substitutes for a missing part of a limb. If the prosthesis is a hollow shell and self-carrying, it is exoskeletal. If internal tubes are used in the device and the cover (cosmesis) to create the outside shape is made of a soft, non-carrying material, it is endoskeletal. Prosthetics is the field that deals with prostheses, use, and their manufacture. A prosthetist is a person who designs and fits prostheses.
Shells as decorative items in human culture
Throughout the history of humanity, shells of many types and from many different kinds of animals have been popular as human adornments.
Seashells are often used whole and drilled so that they can be threaded like a bead, or cut into pieces of various shapes. Shells have been formed or incorporated into pendants, beads, buttons, brooches, rings, and hair combs, among other uses. Tortoiseshell has been used for jewelry and hair combs, and for many other items as varied as inkwells, sunglasses, guitar picks, and knitting needles.
The Moche culture of ancient Peru worshipped animals and the sea and often depicted shells in their art (Berrin and Larco 1997). Some tribes of the indigenous peoples of the Americas used shells for wampum and hair pipes (Ewers 1957).
Small pieces of colored and iridescent shell have been used to create mosaics and inlays, which have been used to decorate walls, furniture, and boxes. Large numbers of whole seashells, arranged to form patterns, have been used to decorate mirror frames, furniture and human-made grottos.
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