Marine biology is the study of animal and plant life within saltwater ecosystems. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology deals with those species in which life is spent only (or mainly) in the water, thus its classification is based on the environment rather than on taxonomy.
Marine biology covers a great deal, from the microscopic plankton, including femtoplankton as small as 0.02 micrometers, and together with phytoplankton, hugely important as the primary producers of the sea, to the huge cetaceans (or whales) which reach up to a reported 33 meters (109 feet) in length.
The habitats included by investigation in marine biology include everything from the tiny layers of surface water in which organisms and abiotic items may be trapped in surface tension between the ocean and atmosphere, to the depths of the abyssal trenches, sometimes 10,000 meters or more beneath the surface of the ocean. Along the way, habitats such as coral reefs, kelp forests, tidepools , muddy, sandy, and rocky bottoms, and the open ocean (pelagic) zone, where solid objects are rare and the surface of the water is the only visible boundary.
A large (exactly how large is unknown at this point) proportion of all life on earth is contained in the oceans. While the oceans comprise about 71% of the Earth's surface, due to their depth they encompass about 300 times the habitable volume of the terrestrial habitats on Earth.
Many species are economically important to humans, including the food fishes. It is also becoming understood that the well being of marine organisms and non ocean dwelling organisms is linked in some very fundamental ways. Human understanding of the relationship between life in the sea and important cycles of matter (such as the carbon cycle) and movement of energy through ecosystems is also growing, despite large areas beneath the surface of the ocean remaining effectively unexplored.
The marine ecosystem is large, and thus there are quite a number of subfields in marine biology, mostly involving specializations in particular species and biomes, such as reef habitats, tide pool ecology, and geothermal/hydrothermal vents.
There are also subfields reserved for studying the physical effects of continual immersion in sea water, and the ocean in general, as well as for the ways that various changing oceanic properties affect marine life. Not being our realm, scientists study how various organisms have adapted to this salty environment. In addition, a subfield of marine biology studies the relationships between oceans and ocean life, and global weather and environmental issues (such as carbon dioxide displacement).
Recent marine biotechnology has focused largely on marine biomolecules, especially proteins, that may have uses in medicine or engineering. An interesting branch of marine biology is aquaculture. Marine environments are the home to many exotic biological materials that may inspire biomimetic materials .
Marine biology is closely linked to both oceanography and biology. It also encompasses ideas from ecology. Fisheries science can be considered a partial offshoot of marine biology, as can marine conservation .
Microscopic life undersea is incredibly varied and still poorly understood. For example, at one end of the scale, the role of viruses in marine ecosystems is barely being explored even in the beginning of the 21st century.
The role of phytoplankton is better understood due to their critical position as the most numerous primary producers on Earth. Phytoplankton fit into these categories: cyanobacteria (also called blue-green algae/bacteria), various types of algae: red, green, brown, and yellow-green, diatoms, dinoflagellates, euglenoids, coccolithophorids, cryptomonads, crysomonads, chloromonads, prasinomonads, and silicoflagellates.
Zooplankton tend to be somewhat larger, and not all in this list are microscopic. Many Protozoa are zooplankton, including dinoflagellates (yes, some of these are phytoplankton as well: the plant/animal distinction often breaks down in very small organisms), zooflagellates, foraminiferans, and radiolarians. Other zooplankton include cnidarians, ctenophores, chaetognaths, annelids such as polychaetes, molluscs, arthropods, and urochordates. Even many larger animals begin their life in the zooplankton before they become large enough to take their familiar forms. Fish larvae generally begin life as zooplankton and sea stars (a.k.a. starfish) do too, just as two examples of the many.
Plant life is relatively rare undersea, most of the niche occupied by plants on land is actually occupied by macroscopic algae in the ocean, such as Sargassum and kelp. The plants that do survive in the sea are often found in shallow waters, such as the seagrasses (of which eelgrass, Zostera, and turtlegrass, Thalassia are examples). The intertidal zone is also a good place to find plant life in the sea, where mangroves or cordgrass might grow.
Cnidarias such as Jellyfish and sea anemone, Ctenophoras, sea worms including phylums: Plathyhelminthes, Nemertea, Annelida, Sipuncula, Echiura, and the Phoronidas; Mollusca including shellfish and squid and octopus, Crustaceans, Poriferas including sponges, Bryozoa, Echinoderms including starfish, Urochordata - sea squirts or tunicates.
Main article: Fish
Fishes inhabit the largest, (by volume) biome on planet earth and since they exist in a watery environment it means that very different biological functions have evolved. Fish anatomy includes two chamber heart , operculum, secretory cells that produce mucous, swim bladder, scales, fins, gills, lips and eyes. Fish breathe under water by extracting oxygen from sea water through their gills. Fins are used to propel and stabilize them in their watery environment.
There are five main types of marine mammals.
Reefs comprise some of the densest bio habitats in the world as far as number of species within a given area. They can be incredibly diverse. Even cold water reefs. Tropical reefs are known best though, and exist in most tropical waters. Reefs are built up by coral and other calcined deposits, usually on top of a rocky outcrop on the ocean floor. Reefs can evolve on other things too though, which has given rise to the human ability to create artificial reefs.
Currently much attention in marine biology is focused on coral reefs and the El Nio weather phenomenon. In the 1998 coral reefs experienced a "once in a thousand years" bleaching event, which killed off vast expanses of reefs across the globe, due to sea surface temperatures rising well above normal. Some reefs are recovering, but scientists say that 58% of the world's coral reefs are now endangered and predict that global warming could exacerbate this trend.
The ocean is deep, very deep in some places. The deepest recorded measure to date is the Mariana Trench, near the Philippines, in the Pacific at 10,924 m (35,838 ft). Water pressure at these depths is extreme and there is no light from above, but some life still exists here. Small flounder (family Soleidae) fish and shrimp were seen by the American crew of the bathyscaphe Trieste when it dived to the bottom in 1960.
Other notable deeps include Monterey Canyon , in the eastern Pacific, the Tonga Trench in the south west at 32,000 feet (9,750 meters), the Philippine trench , the Puerto Rico Trench at 8,605 m (28,232 ft), the Romanche Trench at 7,760 m (24,450 ft), Fram Basin in the Arctic at 4,665 m (15,305 ft), the Java Trench at 7,450 m (24,442 ft), and the South Sandwich Trench at 7,235 m (23,737 ft).
In general the deep sea is considered to start at the photic zone, the point where sunlight loses its power of transference through the water. Many life forms that live at these depths have the ability to create their own light. Much life centers around seamounts that rise from the deeps. Fish and other sea life use these as congregating areas, for spawning, and feeding. Hydrothermal vents in the ocean floor act as oases for life, as do their opposites, cold seeps. These places support unique biomes and many new microbes have been discovered at these places.
An active research topic in marine biology, is discovering and mapping the life cycles of various species and where they spend their time. Marine Biologists study how the ocean currents and tides affect ocean life, and the effect of the multitudes of other oceanic factors on their growth, distribution and well being. This has only recently become technically feasible with the advances in GPS and newer underwater visual devices.
Most ocean life and fish breed in specific places, nest or not in others, spend their time as juveniles in still others, and in maturity in yet others. Scientists were at a loss for quite a while as to the location of many species during different parts of their life cycles. In fact where sea turtles travel is still largely unknown. Tracking devices just don't work for some life forms, and the rigors of the ocean are not friendly to technology. But these factors are being overcome in many instances.
In recent times, marine biologists are trying to complete the mapping of underwater species with the help of modern techniques, which could help in exploring the deepest oceanic depressions in which it is supposed that new species could be found, eventually of potential great interest also for the theories on evolution. Many universities teach courses in marine biology.