The class Polyplacophora, commonly known as chitons or polyplacophorans, comprises a group of marine molluscs mainly characterized by having a shell composed of eight articulated plates. The name Polyplacophora comes from the Greek ‘poly‘ (many), ‘plakos‘ (plate), and ‘phoros‘ (bearer), directly referring to this distinctive feature. Chitons are benthic animals that live attached to rocks and hard substrates in littoral and sublittoral zones, from intertidal areas to abyssal depths. They are widely distributed across all oceans, although they are most common in temperate and tropical regions, especially along wave-exposed rocky coasts.
Chitons have a flattened, oval body, dorsally covered by eight overlapping calcareous plates that form an articulated shell. This shell provides some flexibility, allowing adaptation to irregular surfaces and protection against predators. The body margin, called the girdle or perinotum, consists of soft tissue with spicules or scales that vary in shape and texture among species. On the ventral side, chitons have a broad, muscular foot used for locomotion and firm attachment to the substrate. Size of this species ranges from just a few millimeters to over 30 cm in species such as Cryptochiton stelleri, known as the “giant chiton.” They lack a distinct head and obvious eyes, though some species possess specialized sensory structures called ‘aesthetes‘ embedded in their plates.
Chitons are mostly herbivorous, grazing on microscopic algae, diatoms, and biofilm scraped from the substrate using a well-developed radula reinforced with magnetite, one of the hardest known biological structures. Some species may supplement their diet with small invertebrates or decomposing organic matter. Their feeding activity occurs mainly at night, when they slowly move over rocks in search of food. During the day, they remain tightly attached to surfaces to avoid desiccation and predation.
There are currently over 900 described species within the class Polyplacophora, distributed among several families, including Chitonidae, Acanthochitonidae, and Ischnochitonidae. Despite their primitive appearance, polyplacophorans possess a relatively complex nervous system and a serial arrangement of gills in the pallial cavity. A notable curiosity is their ability to return to the same resting spot after nightly foraging, a behavior known as “homing.” Additionally, some species have light-sensitive structures similar to eyes on their plates, allowing them to detect changes in light. From an evolutionary standpoint, chitons represent a basal branch of the phylum Mollusca, making them a key group for studying molluscan evolution and ancestral morphology.
