What are molluscs?

The molluscs compose the large phylum of invertebrate animals known as the Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats, underwater animals.

Characteristics of the molluscs

The “generalized” mollusc’s feeding system consists of a rasping “tongue”, the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered “hairs” called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. Thebrain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, underwater animals and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. molluscs express great morphological diversity, so many textbooks base their descriptions on a “hypothetical ancestral mollusc” (see image below). This has a single, “limpet-like” shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface.

The Different species of molluscs

They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates and either the giant squid or the colossal squid is the largest known invertebrate species underwater animals. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total. The scientific study of molluscs is called malacology.

The shell of the mollusc

The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs that consists of mainly chitin and conchiolin (aprotein hardened with calcium carbonate), except the outermost layer in almost all cases is all conchiolin (see periostracum). Molluscs never use phosphate to construct their hard parts, with the questionable exception of Cobcrephora. While most mollusc shells are composed mainly of aragonite, underwater animals, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells.

The shell consists of three layers: the outer layer (the periostracum) made of organic matter, a middle layer made of columnar calcite, and an inner layer consisting of laminated calcite, often nacreous.

Human Relations

Molluscs have been a source of food for humans, as well as important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, and chemical compounds. Their shells have also been used as a form of currency in some preindustrial societies. A number of species of molluscs can bite or sting humans, and some have become agricultural pests.