Gastropods are a diverse and widespread group of invertebrates belonging to the phylum Mollusca. This expansive class includes familiar creatures like snails and slugs. With an estimated 65,000 to 80,000 living species, gastropods are the most diverse class within mollusks, second only to insects. They are found globally, inhabiting a wide range of environments.
Key Characteristics
Gastropods are characterized by several distinct features. Their muscular foot, a flat, glandular organ on their underside, enables locomotion. This foot secretes mucus, allowing the animal to glide over surfaces through muscular contractions that ripple along its length.
Most gastropods possess a single, usually spirally coiled shell, serving as an exoskeleton for protection against predators, mechanical damage, and dehydration. The shell is composed primarily of calcium carbonate. While many species can fully withdraw into their shells, others, like slugs, have shells that are significantly reduced or entirely absent. Some shelled gastropods also feature an operculum, a trapdoor-like structure attached to the foot, which seals the shell’s opening when the animal retracts, providing further defense.
Torsion, a defining developmental process, occurs during their larval stage. This unique event involves a 180-degree rotation of the visceral mass, mantle, and shell relative to the head and foot. Torsion repositions the mantle cavity, including the anus and gills, to an anterior position above the head, resulting in an asymmetrical adult body plan.
The radula, a chitinous, ribbon-like structure in the mouth, is covered with numerous tiny teeth. It functions like a rasp or scraper, used to scrape or cut food particles. The shape and arrangement of these teeth vary, reflecting diverse feeding habits.
Variety in Form and Environment
Gastropods exhibit a wide range of forms and have adapted to nearly every environment on Earth. This class includes land snails and slugs, which thrive in terrestrial habitats from lush rainforests to arid deserts and high mountains. They are the only mollusks to have successfully colonized land.
Aquatic gastropods are found in marine and freshwater ecosystems. Marine species, like abalone, conchs, limpets, whelks, and colorful sea slugs (nudibranchs), inhabit environments from shallow coastal waters to abyssal ocean depths, including hydrothermal vents. Freshwater snails and limpets are found in rivers, lakes, and ponds.
Form diversity extends beyond shell presence or absence. Some marine gastropods, like sea hares, have a reduced internal shell. Limpets possess a conical, cap-shaped shell, rather than the typical coil. This array of body plans and adaptations allows gastropods to occupy diverse ecological niches globally.
Diet and Feeding Mechanisms
Gastropods display varied feeding habits, with most species using their radula. Their digestive system adapts to diverse diets, ranging from herbivory to carnivory and detritivory. Many are herbivores, using their radula to graze on algae or consume plant material like leaves, bark, and fruit.
Other gastropods are carnivores, preying on invertebrates. Some predatory marine snails use their radula along with acidic secretions to drill holes through mollusk shells. Cone snails have a specialized radular tooth that functions like a harpoon, injecting venom into prey like worms or fish.
Some gastropods are scavengers, feeding on dead plant or animal matter; others are detritivores, consuming decomposed organic material. Filter feeders capture small food particles from water currents using their gills or mantle lining. A few gastropod species are parasites, feeding on the bodily fluids of invertebrates.