Mollusks

Mollusks are a large and diverse phylum of invertebrate animals, comprising more than 80,000 described species. They inhabit marine, freshwater and terrestrial environments, from deep oceans to forests. The phylum includes well-known groups such as gastropods (snails and slugs), bivalves (clams, mussels, oysters), cephalopods (octopuses, squids and cuttlefish), as well as chitons and tusk shells. Most mollusks are soft-bodied and, in many species, a calcium carbonate shell protects the body. The basic body plan includes a muscular ventral foot, a dorsal mantle that secretes the shell in many forms, and a visceral mass that contains most organs. A fold of tissue called the mantle cavity houses gills or lungs. A feeding structure called the radula is present in most classes, though bivalves rely more on filter feeding. Cephalopods have a highly developed nervous system and complex sense organs.

Reproduction and development vary. Many mollusks release sperm and eggs into the water, with free-swimming larval stages such as trochophore and veliger; others have direct development or are hermaphroditic. Mollusks occupy a wide range of ecological roles: grazers, detritivores, filter feeders and active predators. Humans rely on several species for food, shells and pearls, and mollusk shells are significant for geology and industry. Conservation concerns include overharvesting, habitat destruction and ocean acidification, which affects calcifying mollusks and coral-associated communities.