| Chiton Temporal range: Devonian–Recent[1][2] |
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| Lined chiton, Tonicella lineata. The anterior end of the animal is to the right | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Polyplacophora Blainville, 1816 |
| Subclasses | |
Chitons ( /ˈkaɪtənz/) are small to large, marine molluscs in the class Polyplacophora. There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura.[3]
These molluscs are also sometimes commonly known as sea cradles or "coat-of-mail shells". They are also sometimes referred to more formally as loricates, polyplacophorans, and rarely as polyplacophores.
Chitons have a dorsal shell which is composed of eight separate shell plates or valves. These plates overlap somewhat at the front and back edges, and yet the plates articulate well with one another. Because of this, although the plates provide good protection for impacts from above, they nonetheless permit the chiton to flex upward when needed for locomotion over uneven surfaces, and also the animal can slowly curl up into a ball when it is dislodged from the underlying surface. The shell plates are surrounded by a structure known as a girdle.
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Chitons live worldwide, in cold water and in the tropics. Most of them inhabit intertidal or subtidal zones and do not extend beyond the photic zone.
They live on hard surfaces, such as on or under rocks, or in rock crevices. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. Others live subtidally. A few species live in deep water, as deep as 6,000 metres (20,000 ft).
Chitons are exclusively and fully marine. This is in contrast to the bivalves which were able to adapt to brackish water and freshwater, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments.
All chitons bear a row of aragonitic shells, although in some species they are reduced or covered by the girdle tissue.[4] The calcareous valves that chitons carry dorsally are protective, made wholly of aragonite,[5] and variously colored, patterned, smooth or sculptured. The shell is divided into eight articulating calcareous (aragonite) valves embedded in the tough muscular girdle that surrounds the chiton's body. This arrangement allows chitons to roll into a protective ball when dislodged and to cling tightly to irregular surfaces.
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The most anterior plate is crescent shaped, and is known as the cephalic plate (sometimes called a "head plate", despite the absence of a complete head). The most posterior plate is known as the anal plate (sometimes called the "tail plate", although chitons do not have a tail.)
The front simultaneously, with the rear plate being added later in the developmental process. Growth lines are formed each winter.[6] The inner layer of each of the six intermediate plates is produced anteriorly as an articulating flange. This is called the articulamentum. This inner layer may also be produced laterally in the form of notched insertion plates. These function as an attachment of the valve plates to the soft body. A similar series of insertion plates may be attached to the convex anterior border of the cephalic plate or the convex posterior border of the anal plate.[7]
The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle[7]
After a chiton dies, the individual valves which make up the 8-part shell come apart because the girdle is no longer holding them together, and then the plates sometimes wash up in beach drift. The individual shell plates from a chiton are sometimes known as "butterfly shells" because of their shape.
The girdle may be ornamented with scales or spicules which, like the shell plates, are mineralized with aragonite – although a different mineralization process operates in the spicules to in the teeth or shells (implying an independent evolutionary innovation).[5] This process seems quite simple in comparison to other shelly tissue; the crystal structure of the deposited minerals closely resembles the disordered nature of crystals that form inorganically.[5]
The protein component of the scales and sclerites is minuscule in comparison with other biomineralized structures, whereas the total proportion of matrix is higher than in mollusc shells. This implies that polysaccharides make up the bulk of the matrix.[5] The girdle spines often bear length-parallel striations.[8]
The wide form of girdle ornament suggests that it serves a secondary role; chitons can survive perfectly well without them. Camouflage or defence are two likely functions.[5]
The girdle is often ornamented with spicules, bristles, hairy tufts, spikes, or snake-like scales. The majority of the body is a snail-like foot, but no head or other soft-parts beyond the girdle are visible from the dorsal side. The mantle cavity consists of a narrow channel on each side, lying between the body and the girdle. Water enters the cavity through openings either side of the mouth, then flows along the channel to a second, exhalant, opening close to the anus.[9] Multiple gills hang down into the mantle cavity along part or all of the lateral pallial groove, each consisting of a central axis with a number of flattened filaments through which oxygen can be absorbed.[10]
The heart has three chambers and is located towards the animal's hind end. Each of the two auricles collects blood from the gills on one side, while the muscular ventricle pumps blood through the aorta and round the body.
The excretory system consists of two nephridia, which connect to the pericardial cavity around the heart, and remove excreta through a pore that opens near the rear of the mantle cavity. The single gonad is located in front of the heart, and releases gametes through a pair of pores just in front of those used for excretion.[10]
The mouth is located on the underside of the animal, and contains a tongue-like structure called a radula, which has numerous rows of 17 teeth each. The teeth are coated with magnetite, a hard ferric/ferrous oxide mineral. The radula is used to scrape microscopic algae off the substratum. The mouth cavity itself is lined with chitin and is associated with a pair of salivary glands. Two sacs open from the back of the mouth, one containing the radula, and the other containing a protrusible sensory subradula organ that is pressed against the substratum to taste for food.[10]
Cilia pull the food through the mouth in a stream of mucus and through the oesophagus, where it is partially digested by enzymes from a pair of large pharyngeal glands. The oesophagus in turn opens into a stomach with where enzymes from a digestive gland complete the breakdown of the food. Nutrients are absorbed through the linings of the stomach and the first part of the intestine. The intestine is divided in two by a sphincter, with the latter part being highly coiled and functioning to compact the waste matter into faecal pellets. The anus opens just behind the foot.[10]
Chitons lack a clearly demarcated head; their nervous system resembles a dispersed ladder.[2] There are no true ganglia as there are in other molluscs, although there is a ring of dense neural tissue around the oesophagus. From this ring, nerves branch forwards to innervate the mouth and subradula, while two pairs of main nerve cords run back through the body. One pair, the pedal cords, innervate the foot, while the pallio-visceral cords innervate the mantle and remaining internal organs.[10]
Some species bear an array of tentacles in front of the head.[11]
The primary sense organs of chitons are the subradula organ and a large number of unique organs called aesthetes. The aesthetes consist of light sensitive cells just below the surface of the shell, although they are not capable of true vision. In some cases, however, they are modified to form ocelli, with a cluster of individual photoreceptor cells lying beneath a small lens. An individual chiton may have thousands of such ocelli[10]
There is a relatively good fossil record of chiton shells, but ocelli are only present in those dating to 10 million years ago or younger; this would make the ocelli, whose precise function is unclear, the most recent eyes to evolve.[2]
Although chitons lack osphradia, statocysts, and other sensory organs common to other molluscs, they do have numerous tactile nerve endings, especially on the girdle and within the mantle cavity.
However, chitons lack a cerebral ganglion.[12]
Several species of Chiton are known to exhibit homing behaviours, journeying to feed and then returning to the exact unique spot they previously inhabited.[13] The specific method of how chitons can perform such behaviors has been investigated to some extent, however the reason remains unknown. Suggestions include the chitons remembering the topographic profile of the region, thus being able to guide themselves back to their home scar via a physical knowledge of the rocks and a visual input by their primitive eyes,[14] The gastropod Nerita textilis is known to deposit mucus as it moves, which a chemoreceptive organ is able to detect and guide the snail back to its home site,[15] It is unclear if chitons function in this manner. However, it is theorized that they may leave chemical cues along the rock surface and at the home scar which olfactory senses can detect and home in on. Furthermore, old trails may be detected, providing further stimulus for the Chiton to find its home,[14] Also, chitons have teeth made of magnetite on their radula making them unique among animals. This means they have an exceptionally abrasive tongue with which to scrape food from rocks. These crystals are thought to be involved in magnetoreception, the ability to sense the polarity or the inclination of the Earth's magnetic field, and to be involved in navigation.
Chitons are eaten in many islands in the Caribbean, including Trinidad, Tobago and Barbados. They were also eaten by native Americans of the Pacific coasts of both North and South America. The foot of the chiton is prepared in a manner similar to abalone.
A chiton creeps along slowly on a muscular foot. They have considerable power of adhesion and can cling to rocks very powerfully, like a limpet.
Chitons are herbivorous grazers. They eat algae, bryozoans, diatoms and sometimes bacteria by scraping the rocky substrate with their well-developed radula.
A few species of chitons are predatory, such as the small western Pacific species Placiphorella velata. These predatory chitons have an enlarged anterior girdle. They catch other small invertebrates, such as shrimp and possibly even small fish, by holding the enlarged, hood-like front end of the girdle up off the surface, and then clamping down on unsuspecting, shelter-seeking prey.
Some chitons exhibit homing behavior, returning to the same spot for the daylight hours and roaming around at night to feed.
Chitons have separate sexes, and fertilisation is external. The male releases sperm into the water, while the female releases eggs either individually, or in a long string. In most cases, fertilisation takes place either in the surrounding water, or in the mantle cavity of the female. Some species brood the eggs within the mantle cavity, and the species Callistochiton viviparus even retains them within the ovary and gives birth to live young, an example of ovoviviparity.
The eggs have a tough spiny coat, and usually hatch to release a free-swimming trochophore larva, typical of many other mollusc groups. In a few cases, the trochophore remains within the egg (and is then called lecithotrophic – deriving nutrition from yolk), which hatches to produce a miniature adult. Unlike most other molluscs, there is no intermediate stage, or veliger, between the trochophore and the adult. Instead, a segmented shell gland forms on one side of the larva, and a foot forms on the opposite side. When the larva is ready to become an adult, the body elongates, and the shell gland secretes the plates of the shell. Unlike the fully grown adult, the larva has a pair of simple eyes, although these may remain for some time in the immature adult.[10]
Animals which prey on chitons include humans, seagulls, seastars, crabs, lobster and fish.
The largest chiton (up to 33 cm in length) is the brick-red gumboot chiton of the Pacific Northwest. In this species the valves are completely internal.
Chitons have a relatively good fossil record, stretching back 400 million years[2] to the Devonian. Before this, some organisms have been interpreted (tentatively) as stem-group polyplacophora; the record of polyplacophora stretches back to the Ordovician.[16]
Kimberella and Wiwaxia of the Precambrian and Cambrian may be related to ancestral polyplacophora. Matthevia is a Late Cambrian polyplacophoran preserved as individual pointed valves, and sometimes considered to be a chiton,[1] although it can at best be a stem-group member of the group.[17] Based on this and co-occurring fossils, one plausible hypothesis for the origin of polyplacophora has that they formed when an aberrant monoplacophoran was born with multiple centres of calcification, rather than the usual one. Selection quickly acted on the resultant conical shells to form them to overlap into protective armour; their original cones are homologous to the tips of the plates of modern chitons.[1]
The chitons evolved from multiplacophora during the palaeozoic, with their relatively conserved modern-day body plan being fixed by the mesozoic.[17]
Chitons were first studied by Carolus Linnaeus in 1758. Since his description of the first four species, chitons have been variously classified. They were called Cyclobranchians ("round arm") in the early 19th century, and then grouped with the aplacophorans in the subphylum Amphineura in 1876. The class Polyplacophora was named by J. E. Gray in 1821.
The English name "chiton" originates from the Latin word chitōn, which means "mollusc", and in turn is derived from the Greek word "khitōn", meaning tunic (which also is the source of the word chitin). The Greek word "khitōn" can be traced to the Central Semitic word "*kittan", which is from the Akkadian words "kitû" or "kita’um", meaning flax or linen, and originally the Sumerian word "gada" or "gida".
The Greek-derived name Polyplacophora comes from the words poly- (many), plako- (tablet), and -phoros (bearing), a reference to the chiton's eight shell plates.
Most classification schemes in use today are based, at least in part, on Pilsbry's Manual of Conchology (1892–1894), extended and revised by Kaas and Van Belle (1985–1990).
Since chitons were first described by Linnaeus (1758) there have been extensive taxonomic studies at the species level. However, the taxonomic classification at higher levels in the group has remained somewhat unsettled.
The most recent classification (Sirenko 2006) is based not only on shell morphology, as usual, but also other important features including aesthetes, girdle, radula, gills, glands, egg hull projections and spermatozoids. It includes all the living and extinct genera of chitons.
This system is now generally accepted.
