Fish are animals which live in water, are covered with scales, breathe by taking water in through their mouth by means of gills, and swim by the aid of fins. Some fish have a skeleton of bone, and others with just cartilage. Most are cold-blooded. Interestingly we use the word fish, in the singular, for fishes in general or the whole race. There are over 29,000 species of fish, making them the most diverse group of vertebrates. Fish come in different sizes, from the 51 ft whale shark to a ¼ of an inch, long stout infantfish.
Many types of aquatic animals named "fish", such as jellyfish and cuttlefish, are not true fish. Other sea dwelling creatures, like whales and dolphins, while closely resembling fish in outer appearance, are actually mammals. The term fish has been also extended to other aquatic animals, such as shell-fish, lobsters, etc.
A marine mammal is a mammal that is primarily ocean-dwelling or depends on the ocean for its food. Marine mammals typically swim by moving their spine up and down. By contrast, fish normally swim by moving their spine sideways. For this reason, fish mostly have vertical caudal (tail) fins, while marine mammals have horizontal caudal fins.
Some of the primary differences between marine mammals and other marine life are:
The polar bear spends a large portion of its time in a marine environment, albeit a frozen one. When it does swim in the open sea it is extremely proficient and has been shown to cover 46 miles in one day! For these reasons, some scientists regard it as a marine mammal.
Crustaceans are a large group of arthropods (55,000 species), who live in the sea with a segmented shell and four or more jointed legs, and two antennae. Crustaceans are arthropods that live in the water and breathe by gills, such as lobsters, barnacles, crabs, and shrimps.
The vast majority are aquatic, living in either fresh water or marine environments, but a few groups have adapted to terrestrial life, such as terrestrial crabs, or terrestrial hermit crabs and others. The scientific study of crustaceans is known as carcinology.