Over 38,000 crustacean species are known, most of which are marine, although some are found in fresh water and some are terrestrial. The majority of planktonic animals throughout the world are crustaceans, but the group is also well represented in bottom fauna from coastal waters to abyssal depths. For marine species, there is currently little evidence that any species are threatened with extinction, but there are ample data indicating overfishing of food species. Freshwater and terrestrial crustaceans often have restricted ranges and are subject to habitat loss and pollution.
Crustaceans are a vital link in the food chains of aquatic ecosystems. Crayfish, for example, are often major croppers of aquatic vegetation and their disappearance could cause premature aging of lakes. Although many freshwater and terrestrial crustaceans are of interest to biologists on account of their relict distributions and evolutionary history, the main value of crustaceans to man is as a food resource; and they are a major element in both commercial and subsistence fisheries. Prawn and shrimp fisheries are of greatest economic importance, lobster is the most popular crustacean, and crayfish are considered a luxury food in Europe, where they are eaten in great quantities.