An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. Some examples of invertebrates are coelenterates. This phylum consists of jellyfish, anemones, corals, and hydras. Another type of invertebrate aquatic animal is the annelids which are segmented worms. There are three different classes they are polychaetes, oligochaetes, and hirudinea. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that live in them can be categorized as aquatic (water) or terrestrial (land). Animals that move readily from water to land and vice versa are referred to as amphibians. When animals live in water, they have special adaptations to help them survive in an aquatic habitat. The more time the animal spends in the water the quicker they adapt to their new habitat. There are numerous ways that an aquatic animal can adapt to their habitat.
The term aquatic can in theory be applied to animals that live in either freshwater (freshwater animals) or saltwater (seawater animals). However, the adjective marine is most commonly used for animals that live in saltwater, i.e. in oceans, seas, etc. Invasive aquatic animals require a water habitat, but do not necessarily have to live entirely in water.
Aquatic animals (especially freshwater animals) are often of special concern to conservationists because of the fragility of their environments. Aquatic animals are subject to pressure from overfishing, destructive fishing, marine pollution and climate change.
Air breathing aquatic animals 
In addition to water breathing animals, e.g., fishes, mollusks etc., the term "aquatic animal" can be applied to air-breathing aquatic or sea mammals such as those in the order Cetacea (whales), which cannot survive on land, as well as four-footed mammals like the river otter (Lontra canadensis) and beavers (family Castoridae).
Aquatic animals include for example the seabirds, such as gulls (family Laridae), pelicans (family Pelecanidae), and albatrosses (family Diomedeidae), and most of the Anseriformes (ducks, swans and geese).
Amphibious and amphibiotic animals, like frogs (the order Anura), while they do require 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.
See also 
- Biology Online Dictionary: "Aquatic"
- "Annelids." Nonindigenous Aquatic Species. Web. 02 May 2012. <http://nas.er.usgs.gov/taxgroup/Annelids/>
- "Invasive Species: Aquatic Species." National Invasive Species Information Center. Web. 02 May 2012. <http://www.invasivespeciesinfo.gov/aquatics/main.shtml>.
"Annelids." Nonindigenous Aquatic Species. Web. 2 May 2012. <http://nas.er.usgs.gov/taxgroup/Annelids/>. " National Invasive Species Information Center. Web. 2 May 2012. <http://www.invasivespeciesinfo.gov/aquatics/main.shtml>.