Mollusca is one of the largest and most varied animal phyla, encompassing creatures like snails, slugs, clams, and octopuses. With over 85,000 recognized living species, mollusks have successfully colonized nearly every environment on Earth, from the deep ocean floor and freshwater rivers to terrestrial habitats. This widespread distribution is mirrored by an equally diverse set of feeding strategies, adapting specialized methods to consume every available food source. Their diets range from microscopic algae and suspended particles to active predation on fish and shelled invertebrates.
Molluscan Grazers and Herbivores
The majority of mollusks, particularly the largest class, Gastropoda (snails and slugs), are primarily grazers that subsist on plant matter, algae, and microbial biofilms. The defining anatomical feature enabling this diet is the radula, a ribbon-like structure found in all mollusk classes except bivalves. This chitinous ribbon is covered in hundreds of microscopic, hardened teeth, functioning like a flexible file or rasping tongue.
The radula is supported by the odontophore, a cartilage structure, which allows the mollusk to protrude the ribbon and scrape food off hard surfaces. Marine grazers like limpets and chitons use this action to detach microscopic algae, diatoms, and cyanobacteria from rocks. The teeth are often reinforced with iron-containing minerals, which increases their hardness and reduces wear from constant abrasion.
Terrestrial snails and slugs also employ the radula to consume plant tissue, carving crescent-shaped feeding traces into leaves and stems. As the front teeth become worn down, new sections are continuously produced at the back of the ribbon to replace them, ensuring a sharp feeding surface. The specific shape and arrangement of these teeth vary significantly, reflecting adaptations to different food types, such as tearing soft leaves or scraping hard algae.
Suspension and Deposit Feeders
Bivalves, such as clams, oysters, and mussels, have lost the radula entirely and rely on passive methods of collecting food. These mollusks are primarily suspension feeders, capturing microscopic food particles suspended in the water column. They draw water in through a specialized tube, the inhalant siphon, by generating currents with the beating of cilia on their gills.
Inside the shell, the gills (ctenidia) are covered in mucus that filters and traps particles like phytoplankton and detritus. The cilia transport this mucus-bound food material along grooves toward the mouth, where labial palps sort the particles. This sorting mechanism is selective, allowing the bivalve to optimize energy intake by rejecting low-quality material, which is then expelled as pseudofeces.
The intensive filtering activity of bivalve populations affects aquatic ecosystems, as they constantly process large volumes of water and transfer nutrients from the water column to the sea floor. Deposit feeders consume organic matter settled onto the sediment surface. Some bivalves exhibit a dual feeding strategy, switching between suspension and deposit feeding by extending their siphons to draw water or vacuum up the detritus.
Specialized Predators and Hunters
The most active feeding strategies belong to the carnivores, which actively hunt and consume other animals. Cephalopods, including octopuses, squid, and cuttlefish, are advanced predators that use acute vision and intelligence to locate prey like fish and crustaceans. Once captured by their arms or tentacles, the prey is subdued by a powerful, parrot-like beak used to bite and tear flesh.
Many octopuses inject a potent venom into their prey, sometimes after drilling a small hole into the shell of a crab or bivalve using a specialized tooth or acidic secretion. The venom paralyzes the victim and begins external digestion, allowing the octopus to consume the liquefied contents. Squids and cuttlefish are fast-swimming hunters that often use camouflage and sudden bursts of speed, propelled by water jets, to ambush targets.
Predatory behavior is not limited to cephalopods; many gastropods have also evolved specialized hunting methods. Drilling snails (e.g., Muricidae and Naticidae) prey on shelled mollusks by using their radula and chemical secretions to bore a precise, smooth hole through the victim’s shell. Cone Snails (Conidae) are an extreme adaptation, possessing a modified, harpoon-like radular tooth injected into fish or worms to deliver a complex cocktail of neurotoxins, instantly immobilizing the prey.