canadensis) and beavers (family Castoridae), although these are technically amphibious or semiaquatic. There are up to one million types of aquatic animals and aquatic species. Amphibians, like frogs (the order Anura), while requiring water, are separated into their own environmental classification. The majority of amphibians (class Amphibia) have an aquatic larval stage, like a tadpole, but then live as terrestrial adults, and may return to the water to mate. Certain fish also evolved to breathe air to survive oxygen-deprived water, such as Arapaima (family Osteoglossidae) and walking catfish. Most mollusks have gills, while some fresh water ones have a lung instead (e.g. Planorbidae) and some amphibious ones have both (e.g. Ampullariidae). Many species of aquatic animals lack a backbone or are invertebrates. Importance for environment Aquatic animals play an important role for the environment as well as human's daily usage. The importance of aquatic animals comes from the fact that they are organisms that provide humans with sources such as food, medicine, energy shelter, and raw materials that are used for daily life. Each aquatic species plays a different role to help us make every day easier, healthier, and also more productive. They also help with the atmospheric pressure and global climate change. See also Aquatic Aquatic ecosystem Aquatic locomotion Aquatic mammal Aquatic plant Freshwater snail Marine biology Marine invertebrates Marine mammal Terrestrial animal Terrestrial ecosystem Terrestrial locomotion Terrestrial plant Wetland indicator status Zoology References Animals by adaptation