Chitons are ancient marine mollusks belonging to the class Polyplacophora, meaning “many-plated.” Often called sea cradles or coat-of-mail shells, they are protected by eight overlapping shell plates. Found in all oceans, they inhabit hard substrates from the shallow intertidal zone down to the deep bathyal zone. Their ability to survive in diverse environments is linked to their varied diet and specialized feeding apparatus. This article explores their primary food sources, feeding mechanics, and unique dietary exceptions.
Primary Food Sources
Most chiton species are herbivores and omnivorous grazers, with their diet determined by the rocky surfaces they inhabit. In the intertidal zone, their primary food is the microscopic layer of life, or biofilm, that coats the rocks. This nutrient-rich layer is dominated by microalgae, particularly diatoms, which provide a highly nutritious source of fatty acids.
Chitons also consume detritus, which is non-living organic matter scraped from the substrate. Larger fragments of plant matter, including red calcareous crustose algae, may be ingested non-selectively along with smaller organisms. This generalist approach means many common intertidal species are omnivorous browsers, capable of digesting a mix of plant and small animal matter encountered while feeding.
Specialized Feeding Mechanics
Chitons possess a unique anatomical structure called the radula, the specialized tool they use to scrape food from the rock surface. This ribbon-like organ operates like a flexible rasp. The second lateral teeth are enlarged and mineralized, coated with magnetite, a hard iron oxide. Magnetite is one of the hardest biological materials known, allowing the chiton to effectively graze on abrasive rock.
The grazing action involves sweeping the radula over the substrate to dislodge and collect the biofilm. Constant scraping causes significant wear, necessitating a continuous replacement process that functions like a natural conveyor belt. New teeth are constantly produced in a radular sac and move forward as they are hardened through a complex biomineralization process. The perpetual renewal of this magnetite-capped tooth structure ensures the chiton always has a strong, sharp apparatus ready for feeding.
Dietary Variations Among Species
Although most chitons are grazers, the class Polyplacophora includes species with highly specialized diets that deviate from the typical herbivorous model. Certain species are true predators, actively hunting and consuming small invertebrates. For example, the genus Placiphorella employs a unique “trap-feeding” strategy, lifting its girdle to capture small crustaceans and other motile prey.
Other exceptions include spongivorous species that feed exclusively on sponges. Deep-sea chitons, such as those associated with whale falls, exhibit unusual diets, consuming organic matter found on the bones. Species that feed on wood, like Nierstraszella lineata, may possess specialized wood-digesting bacteria in the gut. These specialized diets are often reflected in a shorter, simpler intestinal tract compared to the long, complex gut required for digesting plant cellulose in common grazing species.