Some have spread to humid areas near water. They use gills for respiration. Their excretory organs are modified nephridia, as a rule producing a dilute urine that contains a great deal of ammonia. Crustaceans are herbivores, carnivores, or scavengers and are often vital elements of the food chain. Some, such as lobsters, shrimp, and crayfish, are important economically as edible shellfish.
Barnacles are notorious as fouling organisms of ship bottoms and harbor installations.
Some crustaceans are significant parasites of other aquatic organisms. As a rule they pass through a complex set of molts during development, involving a series of larval stages. The characteristic larva is called a nauplius, with three pairs of appendages. More appendages are added as the organism passes through its developmental molts. The cuticle of crustaceans, unlike that of other arthropods, contains calcium deposits.