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Culinary and fisheries term for exoskeleton bearing aquatic invertebrates

Shellfish is a colloquial and fisheries term for exoskeleton bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. In addition, a few species of land crabs are eaten, for example Cardisoma guanhumi in the Caribbean. Shellfish are among the most common food allergens. Despite the name, shellfish are not fish. Most shellfish are low on the food chain and eat a diet composed primarily of phytoplankton and zooplankton. Many varieties of shellfish, and crustaceans in particular, are actually closely related to insects and arachnids; crustaceans make up one of the main subphyla of the phylum Arthropoda. Molluscs include cephalopods (squids, octopuses, cuttlefish) and bivalves (clams, oysters), as well as gastropods (aquatic species such as whelks and winkles; land species such as snails and slugs). Molluscs used as a food source by humans include many species of clams, mussels, oysters, winkles, and scallops. Some crustaceans that are commonly eaten are shrimp, lobsters, crayfish, crabs and barnacles. Echinoderms are not as frequently harvested for food as molluscs and crustaceans; however, sea urchin gonads are quite popular in many parts of the world, where the live delicacy is harder to transport. Though some shellfish harvesting has been unsustainable, and shrimp farming has been destructive in some parts of the world, shellfish farming can be important to environmental restoration, by developing reefs, filtering water and eating biomass.

The term "shellfish" is used both broadly and specifically. In common parlance, as in "having shellfish for dinner", it can refer to anything from clams and oysters to lobster and shrimp. For regulatory purposes it is often narrowly defined as filter feeding molluscs such as clams, mussels, and oyster to the exclusion of crustaceans and all else. Although the term is primarily applied to marine species, edible freshwater invertebrates such as crayfish and river mussels are also sometimes grouped under the umbrella term "shellfish". Although their shells may differ, all shellfish are invertebrates. As non mammalian animals that spend their entire lives in water they are "fish" in an informal sense; however, the term ”finfish" is sometimes used to distinguish fish, animals defined by having vertebrae, from shellfish in modern terminology. The word "shellfish" is both singular and plural; the rarely used "shellfishes" is sometimes employed to distinguish among various types of shellfish.

Archaeological finds have shown that humans have been making use of shellfish as a food item for hundreds of thousands of years. In the present, shellfish dishes are a feature of almost all the cuisines of the world, providing an important source of protein in many cuisines around the world, especially in the countries with coastal areas.

In Japanese cuisine, chefs often use shellfish and their roe in different dishes. Sushi (vinegared rice, topped with other ingredients, including shellfish, fish, meat and vegetables) features both raw and cooked shellfish. Sashimi primarily consists of very fresh raw seafood, sliced into thin pieces. Both sushi and sashimi are served with soy sauce and wasabi paste (a Japanese horseradish root, a spice with extremely strong, hot flavor), thinly sliced pickled ginger root, and a simple garnish such as shiso (a kitchen herb, member of the mint family) or finely shredded daikon radish, or both.

The Torah forbids the consumption of shellfish (i. e. the only permitted seafood is fish with fins and scales), in the books of Leviticus and Deuteronomy. Jews (of all religious traditions) who fully observe the dietary laws thus do not eat shellfish, neither do Seventh day Adventists, who also follow Jewish dietary law. Shia Islamic schools of thought vary on whether (and which types of) shellfish may be acceptable. Sunni Muslims view them as halal.

Approximately 1% of the population is estimated to suffer from shellfish allergy, which is more common in teenage and adult life than very early childhood.

Some shellfish, such as whelk, contain arsenic. A sample of whelk was found to have a total content of arsenic at 15.42 mg/kg of which 1% is inorganic arsenic. Shellfish caught in Alaska can cause paralytic shellfish poisoning (PSP). PSP is caused by toxins namely Saxitoxins released by dinoflagellate, a type of protista (also considered algae), which are extremely poisonous (1000 times more potent than cyanide) and can lead to death by paralyzing the breathing muscles. Due to warming oceans, algae blooms have become more widespread, thereby increasing the likelihood of intoxications of various types.

Shellfish of various kinds contribute to the formation of reefs, such as when millions of oysters or mussels aggregate together. Reefs provide habitat for numerous other species, bury carbon, contributing to climate change mitigation, and defend the shore against erosion, floods and waves. Conversely, when they are destroyed or exploited, carbon can be released into the atmosphere, simultaneously increasing the likelihood of severe weather while removing the natural defence against its consequences. In addition, some shellfish are known for filtering water, removing suspended particles and contaminants, which contributes to both quality and clarity. These benefits cascade to other species that are helpful to humankind such as seagrasses.