Exoskeleton

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 nd 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).

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 miscroscopic 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).

Arthropod exoskeleton

The tough or resistant exoskeleton of arthropods (insects, crustaceans, etc.) 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 dessiccation. 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% of the cuticle. This can lead to great mechanical strength.

Mollusk exoskeleton

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, etc). 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.

Vertebrate exoskeletons

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

{{#invoke:Message box|ambox}} Since exoskeletons are rigid, they present some limits to growth. Some organisms grow by adding new material to the aperture of their shell, but many must moult their shell when they outgrow it, producing a replacement.

Palaeontological significance

Borings in exoskeletons can provide evidence of animal behaviour. In this case, boring sponges attacked this hard clam shell after the death of the clam

Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilised. Mineralised exoskeletons can be preserved "as is", as shell fragments, for example. The possession of an exoskeleton also permits a couple of other routes to fossilisation. For instance, the tough layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay. Alternatively, exceptional preservation may result in chitin being mineralised, as in the Burgess shale, or transformed to the resistant polymer keratin, which can resist decay and be recovered.

However our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. It helps 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. The most significant limitation is that, 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 fossilised.

Mineralised skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago. The evolution of a mineralised 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 outer shells, while others, such as Cloudina, had a calcified exoskeleton. Some Cloudina shells even show evidence of predation, in the form of borings.

Evolution

On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton which they later mineralised, this makes it difficult to comment on the very early evolution of each lineage's exoskeleton. We do know 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.

Some Precambrian (Ediacaran) organisms produced tough but non-mineralised outer shells, while others, such as Cloudina, had a calcified exoskeleton, but mineralised skeletons did not become common until the beginning of the Cambrian period, with the rise of the "small shelly fauna". Just after the base of the Cambrian, these miniature fossils become diverse and abundant - this abruptness may be an illusion, since the chemical conditions which preserved the small shellies appeared at the the same time. Most other 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 precipitiation of the mineral components. Skeletonisation 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 skeletonisation was likewise a response to increased pressure from predators.

Ocean chemistry may also control which mineral shells are constructed of. 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 outsite 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 molluscs, 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 - at the time that the lineage first evolved a calcified skeleton, and does not change thereafter. 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. 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 molluscs; but it also has armor plates on the sides of its foot, and these are mineralised 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.

Artificial "exoskeletons"

Humans have long used armour 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 which 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

File:Strombuslarcomuseum.jpg

Moche Shell. 200 C.E. Larco Museum Collection Lima, Peru.

Use of gastropod shells, specifically cowries, in traditional dress of the Kikuyu people of Kenya, Africa.

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. Some tribes of the indigenous peoples of the Americas used shells for wampum and hair pipes.

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 man-made grottos.

References

ISBN links support NWE through referral fees

Bengtson, S. (2004), Early skeletal fossils. In Lipps, J.H., and Waggoner, B.M., "Neoproterozoic- Cambrian Biological Revolutions", Palentological Society Papers 10: 67-78, <http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf>. Retrieved on 18 July 2008

[http://www.cals.ncsu.edu/course/ent425/tutorial/integ.html EXTERNAL ANATOMY THE EXOSKELETON]. NC State University]

2006

John R. Meyer

http://www.ardastra.com/turtleinfo.html Turtles Testudines Ardastra Gardens, Zoo, and Conservation Center

Classification Chelonia mydas http://bioweb.uwlax.edu/bio203/s2007/martinel_meli/Classification.htm Martinelli, M. 2007

.

Smith, M. M. and B. K. Hall. 1993. A developmental model for evolution of the vertebrate exoskeleton and teeth: the role of cranial and trunk neural crest. Evol. Biol 27:387-448.

http://icb.oxfordjournals.org/cgi/content/full/icm020v1 The contribution of neural crest cells to the nuchal bone and plastron of the turtle shell Scott F. Gilbert1,*, Gunes Bender{dagger}, Erin Betters{dagger}, Melinda Yin{ddagger} and Judith A. Cebra-Thomas§ Integrative and Comparative Biology Advance Access published online on June 1, 2007 Integrative and Comparative Biology, doi:10.1093/icb/icm020

See also

• Mechatronics
• Powered exoskeleton
• Spiracle - Small openings in the exoskeleton that allow insects to breathe

External links

Biology

• Geometry and Pattern in Nature 3: The holes in radiolarian and diatom tests

Modern technology

• Bionic Boots
• PowerSkip
• Berkeley Lower Extremity Exoskeleton (BLEEX)
• University of Washington Exo Arm Project
• Cyberdyne's HAL power suit

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