Mollusks represent an incredibly diverse and ancient group of invertebrate animals inhabiting nearly every corner of the planet. These creatures range in size from microscopic organisms to the massive giant squid, showcasing a remarkable array of forms and lifestyles. Despite their variety, all mollusks share fundamental characteristics that link them together within their phylum.
The Vast World of Mollusks
The phylum Mollusca stands as the second-largest animal phylum, surpassed only by arthropods. Estimates for described living mollusk species vary widely, from around 50,000 to 120,000, with recognized figures often near 85,000. These animals are defined by a soft, unsegmented body, featuring a specialized mantle that secretes a shell. Most also possess a muscular foot for locomotion and a unique feeding organ called a radula, though absent in some groups like bivalves. Mollusks are globally distributed, thriving in marine, freshwater, and terrestrial environments.
Key Mollusk Groups
The immense diversity within mollusks is categorized into several major taxonomic classes, each with distinct features. Gastropods, including snails and slugs, form the largest class, accounting for about 80% of all known mollusk species. These animals have a single, often coiled shell, though some, like slugs, have reduced or absent shells. Gastropods possess a distinct head with sensory tentacles and eyes, and they move using a muscular foot. A unique developmental process called torsion rotates their internal organs during development.
Bivalves, such as clams, oysters, mussels, and scallops, are characterized by their two-part, hinged shell, known as valves. Their bodies are laterally compressed, and unlike most other mollusks, they lack a distinct head and a radula. Bivalves are filter feeders, drawing water through their mantle cavity, and use a muscular foot for burrowing into sediment. These aquatic mollusks inhabit both marine and freshwater environments.
Cephalopods are a group of exclusively marine mollusks that includes octopuses, squids, cuttlefish, and nautiluses. They are known for their prominent head, which is surrounded by a ring of arms or tentacles derived from their foot. Cephalopods exhibit highly developed nervous systems and complex eyes, comparable to those of vertebrates. While nautiluses retain an external shell, most modern cephalopods have reduced internal shells or no shell at all. They are also notable for their ability to move rapidly through jet propulsion.
Chitons, belonging to the class Polyplacophora, are marine mollusks with a flattened, oval-shaped body. A defining characteristic is their shell, which is composed of eight overlapping plates or valves. These plates are bordered by a fleshy girdle, which may be covered in scales or spines. Chitons use a broad, muscular foot to cling tightly to hard surfaces, and their radula, hardened with metallic ions, is used to scrape algae from rocks. Most chiton species are found in intertidal zones.
Why the Numbers Change
The exact number of mollusk species consistently changes due to ongoing scientific exploration and taxonomic revisions. Scientists continually discover new species, especially in underexplored habitats like the deep sea, with 800 to 1000 new mollusk species described annually. Reclassification also influences species counts as new genetic and morphological data refine evolutionary relationships, leading to splitting or grouping of species. The vast diversity and often cryptic nature of many mollusk groups pose challenges for accurate counting. Environmental changes and habitat alterations also impact mollusk populations and their distribution.