Marine biology encompasses whole eco-systems over an area that involves about 75 percent of the Earth's surface. Marine biologists study ocean environments -- which includes not only the oceans themselves, but all the various animals and plants that live in them -- from whales down to the smallest diatoms. Of all the countless creatures living in and around oceans, these groups include some of the most commonly studied.
The study of fish, ichthyology, includes both freshwater and marine species. Since at least 25,000 species of fish exist in all their diverse categories -- including cartilaginous species likes sharks and rays, as well as the bony fishes -- this is a broad field for study. The marine biologist would technically study only saltwater fishes, though his special area of study may involve a more specific interest in one or more species and concentrate on a particular aspect within that study -- such as reproduction, migratory behaviors or even fish farming.
One of the most popular categories of animals that marine biologists study is the marine mammals. Though they may look and behave a lot like fish, whales and dolphins are actually mammals called cetaceans. They are warm-blooded, must surface to breathe and they nurse their young. Other marine mammals include the pinnipeds (literally, flipper feet) like walruses, seals and sea lions; the fissipeds (paw-footed) sea otters and polar bears -- the latter has a lifestyle that includes a lot of time in and around the water, where they prey on other marine mammals and marine birds; and the lesser-known manatees and dugongs (order Sirenia) that inhabit primarily coastal waters of the tropical regions.
Penguins are probably the best known of the marine birds, or “seabirds,” as they are more often called. These often comical birds depend upon oceans for their food and use their wings to swim rather than fly. Most penguin species live in the southern hemisphere. Not all marine birds are flightless, but most find their meals and nesting areas in or around the sea. Some notable marine birds include pelicans, albatrosses, petrels, cormorants, gulls, terns and puffins -- among many other species.
Marine Mollusks and Arthropods
Though marine mammals often garner more attention, for sheer flashiness, it's hard to beat the mollusks -- of which there are more than 50,000 species. Right behind arthropods in numbers of species in their phylum, the mollusks include octopuses, squids, many of the shellfish and snails. One of the most amazing of these ocean denizens is the squid, whose high-speed color changes -- due to chromatophores in its skin -- make chameleons look drab by comparison. Arthropods such as shrimp, crabs and lobster are others of the smaller animals studied by marine biologists. All arthropods have exo-skeletons or outer skeletons rather than shells. The horseshoe crab, a species that has existed for many millions of years, is an important species in medical research. Interestingly, of all arthropods -- which make up the largest phylum of animals on Earth, only three major groups are marine animals.
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