The phylum Mollusca is one of the largest and most diverse groups of invertebrates, ranging from garden snails to deep-sea giant squid. Mollusks are generally characterized by a soft, unsegmented body often protected by a hard, calcareous shell. They typically possess a muscular foot for locomotion or attachment, and a mantle, which secretes the shell and encloses the mantle cavity. Most mollusks also possess a radula, a rasping, tongue-like organ covered in tiny teeth used for feeding. These features have allowed mollusks to successfully colonize marine, freshwater, and terrestrial environments globally.
The Current Count and Why It Is Variable
The precise number of mollusk species is a dynamic estimate that changes with ongoing research and discovery. Scientific databases currently recognize approximately 86,600 valid, extant species, making Mollusca the second-largest animal phylum after Arthropoda. Estimates for the total number of described living species, however, range widely from 70,000 to over 120,000. This variability illustrates the difficulty in establishing a definitive count and stems from factors inherent to biological classification, or taxonomy.
A major challenge is the constant discovery rate, with an estimated 800 to 1,000 new mollusk species formally described annually. Many new finds come from unexplored habitats, such as the deep sea, or involve micro-mollusks that are easily overlooked.
Taxonomic Challenges
Taxonomic revisions further complicate the count as scientists refine the classification of existing species. This process sometimes involves splitting one recognized species into several distinct ones, or lumping multiple named species into a single one due to synonymy. The number of undescribed species, particularly in tropical or remote regions, is estimated to be very high, suggesting true diversity is much greater than the current official count.
The Four Most Prominent Mollusk Classes
The vast majority of mollusk species are concentrated within four well-known classes, each exhibiting a distinct body plan and lifestyle.
Gastropoda
The class Gastropoda, which includes snails, slugs, and sea slugs, is the largest group of mollusks, accounting for roughly 80% of all known species. Most gastropods are defined by torsion, a developmental process where the visceral mass and mantle cavity rotate up to 180 degrees over the foot during the larval stage. Most species protect their asymmetrical bodies within a single, usually spirally coiled shell, though slugs and sea slugs have significantly reduced or lost this external shell. Gastropods use a large, muscular foot for slow, creeping locomotion, and many possess a prominent head with sensory tentacles and eyes.
Bivalvia
Bivalves (clams, oysters, mussels, and scallops) are recognizable by their shell, composed of two lateral valves joined by a dorsal hinge and ligament. Unlike gastropods, bivalves lack a distinct head and are typically filter feeders. They draw water into their mantle cavity where modified gills capture microscopic food particles. They use a powerful, wedge-shaped muscular foot to burrow into soft substrates, while others attach themselves to hard surfaces using specialized threads or cement. The two-part shell is closed by powerful adductor muscles, providing effective protection.
Cephalopoda
The Cephalopoda class (octopuses, squid, cuttlefish, and nautiluses) represents the most complex and specialized group of mollusks. The name “head-footed” reflects their body plan, where the foot is modified into a ring of prehensile tentacles or arms surrounding the mouth. Most modern cephalopods have lost the external shell, though the nautilus retains a coiled, chambered shell that aids in buoyancy. These marine animals are active, intelligent predators, possessing a concentrated nervous system, large, camera-like eyes, and the ability to move rapidly through jet propulsion.
Polyplacophora
Polyplacophora, or chitons, are characterized by a shell composed of eight overlapping, articulating calcareous plates held together by a tough girdle. This segmented shell allows them to curl into a ball for protection and conform tightly to irregular rock surfaces in their marine intertidal habitat. Chitons possess a large, flat foot that provides strong suction for clinging to rocks, where they graze on algae using a radula reinforced with the hard mineral magnetite. They have a reduced head lacking eyes or tentacles, instead using light-sensing organs, called aesthetes, that pierce the shell plates.
The Lesser-Known Mollusk Classes
Beyond the four major groups, three other classes, collectively less numerous, contribute to the phylum’s overall diversity. These classes often live in obscure or deep-sea habitats.
Scaphopoda
Scaphopoda, or tusk shells, are exclusively marine mollusks named for their slender, tubular shell open at both ends. These animals are infaunal, meaning they live buried vertically in soft sediments on the seafloor, ranging from the subtidal zone to abyssal depths. They lack gills, instead using the mantle lining for gas exchange, and possess specialized, thread-like tentacles called captaculae to capture microscopic food from the sediment.
Monoplacophora
Monoplacophorans, often called “living fossils,” were only known from the fossil record until living specimens were discovered in the 1950s. They possess a single, low, cap-like shell, giving them a superficial resemblance to limpets. Their distinguishing feature is the serial repetition of internal organs, such as multiple pairs of gills, nephridia, and foot retractor muscles. This repetition led early researchers to mistakenly link them to segmented worms. These small mollusks inhabit the deep-sea floor, grazing on microorganisms.
Aplacophora
The class Aplacophora consists of small, worm-like marine mollusks that lack a shell entirely and a distinct head. Instead of a shell, their body is covered in a cuticle embedded with calcareous spines or scales, giving them a bristly texture. This class is sometimes divided into two subclasses, Caudofoveata and Solenogastres, both of which are exclusively marine and live in or on the seabed. They are typically small, ranging from a few millimeters to a few centimeters in length, and feed either on detritus or act as predators on small invertebrates.