Chitons (class Polyplacophora) are marine mollusks recognized by their unique shell structure. Their body is protected by eight overlapping, calcareous plates, allowing flexibility and protection against waves and predators. These animals primarily cling to rocks in the harsh intertidal zone, though some species inhabit deeper waters. They obtain nutrition by scraping food from hard surfaces and relying on chemical signals.
The Primary Diet of Chitons
Most chiton species are herbivores, grazing on the microscopic film covering rocks and other hard substrates. Their diet consists mainly of minute organisms and biological matter known as biofilm, including microscopic algae (like diatoms), cyanobacteria, and detritus.
The abundance of nutrient-rich biofilm provides a constant food source for these slow-moving grazers. Chitons consume this material by rasping it directly from the rock face, ingesting organic matter and some inorganic sediment. Their consumption patterns control the composition of local algal communities.
While most chitons are grazers, a few specialized species are carnivorous, such as the deep-sea genus Placiphorella. These predatory chitons use an enlarged anterior mantle girdle, which they lift to form a trap. They wait for small invertebrates, like crustaceans or worms, to seek shelter, then clamp down quickly to capture the prey.
The Specialized Radula: A Feeding Tool
The chiton scrapes food from rough rock surfaces using a highly adapted feeding apparatus called the radula. This ribbon-like structure is lined with numerous rows of minute, backward-pointing teeth. The radula acts like a flexible rasp, extended from the mouth and pulled across the substrate to abrade adhered food particles.
The teeth on the chiton radula are among the hardest biominerals known in nature, necessary for continually grinding against rock. This hardness comes from biomineralization, where the teeth are coated with iron compounds. The cusps of the major lateral teeth are specifically tipped with magnetite, a hard ferric/ferrous oxide mineral.
The iron-reinforced teeth are highly resistant to wear, allowing the chiton to effectively remove tough, encrusted layers of algae and biofilm. During feeding, the radula is pushed out and retracted back into the mouth, sweeping the scraped food material into the digestive tract. Complex muscles and cartilage support this mechanical action.
Sensing the Environment to Locate Food
Chitons rely on chemical sensing (chemoreception) to locate food sources on the rocky substrate. They navigate by responding to chemical gradients left by microscopic organisms, allowing them to detect fresh biofilm or algae.
A key sensory structure is the subradular organ, located near the mouth. This protrusible organ allows the chiton to physically sample and test the chemical composition of the substrate immediately in front of it. This sampling ensures a viable food supply before grazing.
Chitons also possess numerous tactile nerve endings within the muscular girdle that encircles their body. These nerve endings provide sensory input regarding the texture and chemical nature of the rock surface. Their slow movement means they respond to localized chemical cues that confirm the quality of the immediate grazing area.