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(Synonyms: Loricata Schumacher, 1817; Polyplaxiphora Ducrotay-Blainville, 1819; Placophora Ihering, 1876)

Introduction

Two grey chitons (Lepidochitona cinerea): Pontevedra, Gali- cia, Spain. Photo: Jonatan Antunez (iNaturalist).

	Class Name Number of Species Percent % Gastropoda ca. 65.000 - 80.000 ~ 76% Bivalvia ca. 20.000 ~ 21% Cephalopoda ca. 900 ~ 1% Scaphopoda ca. 900 ~ 1% Monoplacophora ca. 25 < 1% Polyplacophora ca. 1.000 ~ 1% Solenogastres ca. 300 < 1% Caudofoveata ca. 150 < 1% Mollusca ca. 85.000 - 100.000		Diagram: Enlarge Diagram!
	Number of species in Mollusca, displayed by classes, including percentage. Sources: WoRMS: MolluscaBase eds. (2025): Mollusca LINNAEUS, 1758.

Chitons predominantly can be observed the the tidal zones of oceans. There are about 1,000 species of this old Mollusc class known today. The largest species is the American Cryptochiton stelleri, 33 cm in size, from the American northwestern coast.

The grey chiton (Lepidochitona cinerea) on the other hand, about 2 cm in size, is found on the coasts of the western Mediterranean, the eastern North Atlantic and the North Sea, from the English Channel as far as western Norway and the western Baltic Sea (to the island of Rügen). In its distribution area the grey chiton usually is the most frequent chiton species.

The chitons live on hard ground (stones, mussel shells, wood), where they graze algae, usually on flat sea coasts (usually below the low water line and in the sublittoral zone, but also in the littoral zone, where they can be found in tidepools during low tide.

Ian Frank Smith: Lepidochitona cinerea, Identification and Biology. (Link).

In German, chitons are called Käferschnecken, but they are, of course, neither beetles nor snails. But, beetles being the best armoured of all insects, chitons might also be imagined as armoured molluscs, compared to their relatives in the Solenogastres and Caudofoveata, who lack shell plates and only are protected by calcareous spikes. In English, chitons are also referred to as coat-of-mail shells. Like a medieval knight's armoured gauntlet, chitons are protected by several shell plates, so the meaning of the expression is clear.

Morphology

Girdle and shell plates of a chiton (Mopalia muscosa): Monte- rey County, California, USA. Photo: Max Erickson (iNaturalist).

In contrast to more highly developed molluscs, such as snails, bivalves, and tusk shells, chitons do not possess a calcareous shell. Instead, their dorsal side, like in most primitive molluscs, is protected by the hard cuticle of the mantle. Unlike in their molluscan ancestors, however, this cuticle in most chitons extends only around the outer edge of the back, and is therefore called the girdle (perinotum). The back itself is protected by eight interlocking calcareous shell plates, which give chitons their characteristic appearance, reminiscent of a suit of armour or a gauntlet. In some chitons the girdle expands over the back so that the shell plates are partly covered. Unlike the hard calcareous plates, the girdle is flexible. The species Cryptochiton stelleri is accordingly known in English as the gumboot chiton. In English usage the word chiton generally refers to any member of this group, though scientifically it is only one of many genus names.

Head of a chiton (Lepidochitona cine- rea): La Rocque, Jersey, Great Britain. Photo: Chris Isaacs (iNaturalist).

Ventral and dorsal side of a chiton (Polyplacophora). Light grey: Girdle; Dark grey: Shell plates; Light pink: Mantle; Dark pink: Foot; Red: Gills. Source: Livingstone, Biodidac. Adaptation: R. Nordsieck.

The body of a chiton is highly adapted to life in the intertidal zone. Looking at the underside of a chiton (no easy task, since these small molluscs cling firmly to the substrate), one can see the strong foot and, at the anterior end of the animal, the mouth. Unlike snails, chitons do not possess a distinct head. A chemical sensory organ beneath the radula, the subradular organ, informs the chiton about edible material within immediate reach.

As in other molluscs, the protective dorsal epidermis is called the mantle. Unlike the more advanced molluscs, however, chitons lack a mantle cavity. Instead, between the girdle on the outside and the foot lies the mantle groove, which runs in a U-shape around the posterior part of the body. Within the mantle groove are arranged numerous gills in series: the typical molluscan ctenidia, or comb gills.

Locomotion

Curled-up chiton (Mopalia lignosa): British Columbia, Canada. Photo:  Marcie Callewaert John/a> (iNaturalist).

TThe vast majority of chitons are herbivores, grazing algae on stones and rocks, but also on mussel shells in the surf zone. The small molluscs cling firmly to the substrate with their strong foot, supported by the girdle, which is also pressed tightly against the surface. As a result, their locomotion is adequately slow. Once they have grazed all available food within reach, they move on with a slow gliding motion of the foot (comparable to that of some snails) until sufficient food is available again. Thanks to their shell plates and a longitudinal muscle unusual among molluscs, the so-called "coiling muscle", chitons can curl up like a isopod when they are removed from the substrate.

Nutrition

Not only the shell of chitons is hard. Like snails, they also possess a radula with which they scrape food from the substrate, unless, like Placiphorella rufa (see below), they belong to the rare carnivorous members of the class. The chiton Chaetopleura apiculata, which occurs in the Gulf of Mexico, the north-western Atlantic, as well as the eastern Pacific and off Colombia, has the hardest known teeth in nature. Chaetopleura often scrapes rocks particularly intensively in order to reach food hidden in cracks and crevices.

Materials science institutes are studying how this extremely hard, yet, interestingly, not brittle, material is formed through the interaction of organic and inorganic components, and how it might possibly be replicated.

• Nina Weber: Hart, härter, Weichtier (Spiegel Online, abgerufen 28.09.2025).
• Stephanie Baum: Nature's hardest teeth: Chitons offer blueprint for advanced dental and industrial materials. Phys.org (07.08.2025).
• Gordon, L. M.; Joester, D. (2011): "Nanoscale chemical tomography of buried organic–inorganic interfaces in the chiton tooth". Nature 469: 194-197.

Anterior view of the predatory chiton Placiphorella rufa: Sitka, Alaska, USA. Bild: Paul Norwood (iNaturalist)

A few chiton species (Placiphorella) have adopted a predatory lifestyle. To do so, they lift the anterior part of the girdle away from the substrate and lie in wait for prey, mostly small crustaceans. Small tentacle-like extensions of the mantle serve as sensory organs. When a prey animal touches these tentacles, the girdle is suddenly pressed against the substrate and the prey is trapped and consumed.

Sense Organs

Chiton (Tonicella lineata) on the skeleton of a sea urchin. Photo: Jerry Kirkhart (Source).

The sensory organs of chitons are a special adaptation to their way of life: Within the dorsal shell plates the so-called aesthetes are present. These are mechanoreceptors capable of detecting water movements. They are usually branched into several micro-aesthetes. In addition, some aesthetes may contain shell eyes.

In chitons there are two types of shell eyes: Intrapigmentary shell eyes are situated within the aesthetes and consist of a small pigment cup with sensory cells, covered by a lens. Extrapigmentary shell eyes, on the other hand, have the pigment cup located outside the aesthetes in the outer layer (tegumentum), while the tip of the aesthete is transformed into a lens.

These shell eyes enable the chiton to distinguish between light and shadow across its back. In this way, the animal can perceive approaching threats without having to lift the protective girdle from the substrate. In Acanthopleura species, several thousand shell eyes may be distributed across the dorsal surface.

Jan Osterkamp: Käferschnecken balancieren Panzerung und Aufklärung. a href="https://www.spektrum.de" target="_blank">Spektrum.de (20.11.2015).

Reproduction

Chitons, for the most part, are dioecious, having two diferent sexes. The gonads, originally paired, usually are grown together as a single unpaired organ with the exiting ducts leading into the pallial groove. Fertlization takes place openly in the water. The Development of the juveniles takes place via a Trochophora-like larval stadium, which, after a metamorphosis, changes into a juvenile chiton, resembling the adult specimens.

Cryptochiton stelleri/em> during spawning (Photos: Robin Gwen Agarwal, iNaturalist/a>)).