Chitons are flattened, bilaterally symmetrical mollusks armored with eight calcified plates on their shell. They constitute the class Polyplacophora and are hence known as polyplacophorans. Sometimes, they are also called by the names sea cradles, coat-of-mail shells, suck-rocks, and loricates.
These mollusks are exclusively marine and typically inhabit intertidal or subtidal zones of the oceans. They scrape off microscopic algae from substrates using their feeding organ, radula, though some chitons consume small invertebrates, too.
Most chitons measure around 2 in (5 cm) in length, though Cryptochiton stelleri can grow up to 16.9 in (43 cm).
Their bodies are elongated-oval, bilaterally symmetrical, and flattened, encased in a hard dorsal shell comprising eight plates or valves. These plates are embedded in a round muscular skirt surrounding the shell called the girdle.
The girdle is often decorated with various structures such as spicules, bristles, fine tufts, spikes, or scales that resemble snakeskin. From the top, the body mainly appears as a large, foot-like mass similar to a snail, with no head or other soft tissues visible beyond the girdle. Being a mollusk, they possess a characteristic mantle cavity that exists as a narrow channel between the body and the girdle on each side.
Additionally, there are openings on either side of the mouth. One acts as the port through which water enters the chiton’s body, and the other (which is close to the anus) helps to expel them.
Unlike single or two-part shells of most mollusks, the eight-plated shells in chitons allow them to roll into a protective ball when needed. These plates are composed of aragonite, a crystal form of calcium carbonate.
The anterior-most plate of the shell is referred to as the cephalic or head plate, whereas the posterior-most is called the anal or tail plate. Six intermediate plates are sandwiched between the cephalic and the anal plates. The inner layer of these intermediate plates may either extend forward as an articulating flange (articulamentum) or extend laterally as notched insertion plates that aid in attachment.
After a chiton’s death, its shell plates fall apart as they lose their connection to the articulating girdle. The individual cell plates are sometimes called ‘butterfly shells.’
The girdle is often ornamented with sharp scales or spicules composed of aragonite, just like the shell. The spicules may exist in paired tufts, such as those in members of the genus Acanthochitona.
These elements possess a bulky matrix of polysaccharides and often bear parallel striations. They play an important role in camouflage and defense, especially in species like West Indian green chiton (Chiton tuberculatus) and Acanthochitona crinita.
They have an open circulatory system characterized by a three-chambered heart located at the hind end of the body. The heart comprises two auricles that receive blood from the gills and a single muscular ventricle that pumps blood throughout the body with the help of the aorta.
A number of gills hang down into the mantle cavity, aiding in respiration. Each gill comprises a central axis along which numerous flattened filaments are arranged. It is through these filaments that the gaseous diffusion occurs.
Their mouth, located on the underside of the body, comprises a tongue-like rasping organ called the radula (a characteristic of all mollusks). The radula contains multiple rows of teeth (17 in each row) coated with magnetite, which helps scrape algae off the substratum. Additionally, at the back of the mouth lies a sensory subradular organ, which helps taste the ingested food.
The mouth is followed by a pair of large pharyngeal glands and an esophagus, which, in turn, opens into a stomach. A highly coiled intestine (divided into two parts by a sphincter muscle) follows the stomach.
Behind the mouth lies a pair of pharyngeal glands and an esophagus, which leads into the stomach. A tightly coiled intestine follows the stomach, divided into two sections by a sphincter muscle.
A pair of excretory organs called nephridia help expel waste through an excretory pore close to the rear end of the mantle cavity. These nephridia are also connected to the pericardial cavity around the heart.
Waste is expelled through the anus, lying just behind the foot.
These mollusks have a ladder-like simple nervous system comprising two primary pairs of lateral nerve cords: the pedal and the palliovisceral cords. While the pedal cords innervate the foot, the palliovisceral cords innervate the mantle and other internal organs.
A ring of neural tissue surrounds the esophagus, from which nerves spread out to the mouth and the subradula.
Apart from the subradular organ, chitons have multiple light-sensitive cells called aesthetes lying under the shell. These cells usually do not facilitate vision, but in some species, like the West Indian fuzzy chiton (Acanthopleura granulata), they are modified into simple eyes or ocelli. These ocelli possess minute, aragonite-based lenses (hence, also called shell eyes) that form clear images and aid in vision in its true sense.
They also have numerous tactile nerve endings within the mantle cavity, as well as on the girdle.
Both males and females possess a single gonad (testis or ovary) in front of the heart. The gametes produced by the gonad are released through a pair of gonopores located right in front of the excretory pore.
The term ‘chiton’ has its roots in the Ancient Greek word khitōn, which means tunic. Similarly, the word’ Polyplacophora,’ coined by de Blainville in 1816, is also derived from the Greek words poly (many), plako (tablet), and phoros (bearing), referring to the eight plates in the shell of chitons. They were first studied and described by Carl Linnaeus in 1758, and ever since then, these animals have been variously classified by scientists.
In 2006, the most recent classification was proposed by Sirenko for both extinct and extant taxa, based on features other than morphology, like girdle, radula, gills, glands, aesthetes, egg hull projections, and spermatozoids. In his classification, around 940 extant species are classified under the subclass Loricata, which has 2 orders: Lepidopleurida and Chitonida.
Fossil evidence of chitons is abundant and dates back to the Cambrian Period. Members of the genus Preacanthochiton, discovered from Late Cambrian deposits in Missouri, are considered the oldest known chitons.
On the basis of these fossil specimens, scientists hypothesized that chitons probably evolved from a monoplacophoran (class Monoplacophora), which had multiple centers of calcification instead of a single, continuous shell. Modern-day chitons evolved from the stem group Multiplacophora in the Palaeozoic Era.
Around 400 million years ago, during the Early Devonian Period, aesthetes first appeared in these mollusks.
Chitons are distributed worldwide, ranging from tropical to polar waters. They are exclusively marine and typically inhabit subtidal and intertidal zones, living on hard surfaces, like rocks, or within their crevices. Although they do not usually live beyond the photic zone, some chitons have been spotted at depths of about 20,000 ft.
They are typically herbivorous but become opportunistic carnivores or omnivores, depending on the availability of prey. Their diet usually includes algae, diatoms, and bacteria.
In contrast, some species, like Placiphorella velata, are predatory and feed on small invertebrates, such as shrimp, bryozoans, sponges, and barnacles, as well as small fish.
These mollusks generally use their radula to scrape food from surfaces. Cilia within their mouths then move the food along a mucous pathway toward the esophagus, where enzymes from pharyngeal glands begin breaking down food particles. In the stomach, additional enzymes from digestive glands continue the chemical breakdown before the food passes into the intestine for nutrient absorption. After the nutrients are absorbed, waste is compacted into small fecal pellets and expelled through the anus.
They are slow movers that creep on the ocean floor by undulating (rather, contracting) their muscular foot in a wave-like motion. Such a locomotory pattern allows for movement in both forward and backward directions. However, during low tide or when they are disturbed, chitons cling tightly to the surface of rocks.
Some species of chitons, like Chiton granosus, exhibit homing behavior, in which they set off to forage from a certain location during high tide and return to the same in low tide after seeking food. Although they are believed to have a topographical profile of the region they inhabit, some species may leave chemical cues on their way, which they detect on their way home.
They can also sense and respond to the polarity of the Earth’s magnetic field (magnetoreception) owing to the iron crystals found in the magnetite of their teeth.
Chitons are dioecious, with separate male and female sexes. Both males and females release their gametes, the sperm and eggs, in the surrounding water, and fertilization is usually external. The females may release their eggs either singly or in gelatinous strings. Internal fertilization is observed in some brooding species, like Callistochiton viviparus, which retain the eggs within the ovary and later give birth to live young (ovoviviparity).
The fertilized eggs have a tough, spiny coat. They hatch into a free-swimming trochophore larva, characteristic of many other mollusk groups. In some cases, the trochophore is lecithotrophic and remains within the egg, deriving nutrition from the yolk.
The trochophore develops directly into a miniature adult or juvenile, bypassing the veliger larval stage (unlike many mollusks). During this developmental process, a shell gland forms on one side, while a foot forms on the opposite end. The shell gland slowly starts secreting the shell while the larva increases in length. It typically loses its pair of simple eyes before developing into an adult.
They are usually preyed upon by sea stars, crabs, lobsters, fish, and seagulls. Moreover, chitons are also consumed by humans in many parts of the world, such as the Pacific coasts of North and South America, The Bahamas, Trinidad, Tobago, and the Caribbean.
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