mollusk

Mollusks are a large and diverse group of predominantly soft-bodied invertebrates within the phylum Mollusca. They occupy a wide range of habitats, from deep oceans to deserts, and include snails, slugs, clams, oysters, squid, and octopuses. There are about 85,000 described species, with many more likely undiscovered, making Mollusca one of the most species-rich animal groups.

A typical mollusk body plan includes three main parts: a head-foot, a visceral mass containing most organs, and a mantle, a fold of tissue that may secrete a calcium carbonate shell. Most have a muscular foot used for locomotion, while a radula—a toothed feeding ribbon—is present in many groups, though it is absent in bivalves. The mantle cavity houses the gills, lungs, or combination, depending on the group.

Mollusks are traditionally divided into several major classes, including Gastropoda (snails and slugs), Bivalvia (clams, mussels, oysters), and Cephalopoda (octopuses, squids, cuttlefish), as well as less diverse groups such as Polyplacophora (chitons) and Scaphopoda (tusk shells). Cephalopods are especially notable for their intelligence and complex nervous systems.

Reproduction and development vary; many species release free-swimming larvae, while others have direct development. Most mollusks possess separate sexes, though some are hermaphroditic. Larval forms such as trochophore and veliger are characteristic of several marine lineages.

Ecologically, mollusks perform important roles: grazers control algae, filter feeders help purify water, and predatory cephalopods regulate prey populations. They are also a key food source for humans and other animals, and their shells and fossils provide insights into environmental history.

Conservation concerns include habitat loss, overharvesting, and ocean acidification that affects calcifying species. Mollusks have a rich fossil record and by studying them scientists learn about past climates and ecosystems.