Biodiversity of the oceans

Our oceans comprise almost three quarters of the surface of the earth. "The World Ocean (Oceanosphere) contains on the order of 1,340.7 million km3 of water, making up 1/800th of the total volume of the Earth (1,083.3 billion km3). Alternatively, the volume of fresh water is about 35.8 million km3. If the Oceanosphere was shown in the form of a sphere, its radius would be equal to 690 km, or 0.11 mean radii of the Earth (6,371 km)." - http://www.oceansatlas.org/unatlas/about/physicalandchemicalproperties/background/seemore1.html With an average depth Deepest point on earth is the Marianas Trench with 11.022 metres. Most oceanic waters are less than 1.000 metres or between 3.000 and 4.000 metres. of ~3.700 metres the marine habitats encompass roughly 300 times the habitable volume of the land based (terrestrial) habitats. The oceanic ecosystem is comprised of complex and interlinked systems where each organism relies on its own niche (e.g. habitat) for survival. A change in any biotic Biotic - The living parts of the environment, e.g. plants, animals and fungi, that affect ecological functions. or a-biotic Abiotic - The non-living parts of the environment, e.g. rocks, climate, and pressure, that affect ecological functions. variable, and thus a change or disappearance of a habitat, can have disastrous consequences for the organisms relying on that.

Habitats and ecosystems

The majority of all plant and animal life on earth is found in the oceanic ecosystems. Marine ecosystems can be split up in a number of zones. While a number of classifications exists on this the following is a very practical list: Alexander, D., & Fairbridge, R. W. (1999). Encyclopedia of environmental science. Kluwer Academic encyclopedia of earth sciences series. Dordrecht: Kluwer Academic Publishers.

• Oceanic - relative shallow part of the ocean on top of the continental shelf
• Profundal - deep water below the range of effective light penetration
• Benthic - the seabed, bottom substrates
• Intertidal - area between the high and low tides
• Estuaries - semi-enclosed body of water flowing -e.g. a river- to the sea
• Mangroves - saline coastal swamp in (sub-) tropical region
• Coral reefs - limestone structure constructed by organisms
• Salt marshes - coastal wetland extending landward to the high tide line
• Hydrothermal vents - system driven by chemosynthetic sulphur bacteria

The main oceanic habitats are reefs, deep sea / trenches, open ocean, and intertidal / shore.

Ecology

The oceanic ecosystems are populated with a tremendous amount and diversity of life. Ranging from the smallest krill to the Blue Whale, the largest mammal (up to 40m and 150tonnes in weight) known to men.

• Microscopic plankton
  This is where the oceanic food chain begins. Microscopic free floating plants called phytoplankton are the most abundant primary producers on earth and fully depend on photosynthesis for their existence. This is why they are found close to the surface of the water. (the photic zone) Somewhat larger and feeding on the phytoplankton sits the zooplankton (protozoa), a group of tiny, mostly free floating, animals. These are a mixture of larvae from fish, sea stars, worms and corals (and the likes), but also consist of fully grown adult animals like copepods, krill (euohausiids) and minuscule shrimps.
  
  Primary production by phytoplankton accounts for an estimated 90 percent of the total plant production on earth, and is a major source of oxygen.
  
  
• Plants and algae
  Just as phytoplankton, plant and algae is mostly found in shallow water or in places where light can penetrate the water column to a fair extent. Most real plant organisms are found in intertidal areas (mangroves, beach grass) and in shallow, highly saline, waters (seagrasses such as zostera and thalassia).
  
  Macro-algae (seaweeds) are found everywhere from the intertidal seashore (fucus) to clear, shallow oceans (kelp forests) to free drifting on the oceans currents (sargassum).
  
  No plants -relying on sunlight for their photosynthesis- are found in the deep sea as no light reaches this zone. The only primary producers in the deep sea are bacteria who rely on hydrogen sulphide released by hot water geysers to make their food.
  
  
• Fish
  The majority of the over 22.000 species of fish we've discovered live in the oceans. Fish can be found everywhere in the marine habitats: from the deepest depths of the cold and dark oceans to the coastal intertidal regions we humans inhabit as well. Fish are classified into two groups: fishes with jaws and fishes without jaws (lampreys and hagfish). The jawed fish make up most species and can be divided further into fish with cartilage skeletons (sharks, rays, skates) and fish with bony skeletons (tuna, cod, basically everything else).
  
  Many adaptations can be found among the various species. Whole for example a typical fish is cold blooded some species have adapted themselves and are able to raise their internal body temperature above that of the surrounding water. Examples of this are tuna, swordfish and a variety of shark species. Other species have adapted to an amphibious lifestyle (mudskipper, many adaptations), for great speed (the torpedo shaped rainbow trout), to suck food from the bottom (the sturgeon, equiped with a "vacuum cleaner"), or to lure prey (the anglerfish, equiped with a "light emitting" lure on top of its head).
  
  Small fish mainly feed on plankton (both zooplankton and phytoplankton) and large fish feed on smaller fish. At up to 15 meter the largest fish in the world, the whale shark, is a filter feeder only capable of consuming tiny plankton. Other fish like flounder and plaice feed on molluscs and worms near the bottom.
  
  
• Marine invertebrates
  Invertebrates, those animals without a spinal column or notochord, make up the largest part of the biodiversity of the oceans. A wide variety of species can be found all over the water column, from the benthic substrate, to the sea floor, to the sea surface. Due to lack of a rigid inner system to protect themselves many have shells or hard exoskeletons. Their size ranges from microscopic to a number of metres.
  
  The sheer variety of the group includes corals, worms, shells, sea urchins, sponges, crustaceans (e.g. lobsters, crabs, shrimp, barnacles), molluscs (e.g. squid, octopus, snails, bivalves) and many more species.
  
  
• Marine mammals
  Some of the most loved (if only for their apparent level of cuteness) organisms in the oceans belong to the marine mammals, a group of mammals that evolved on land but migrated back to the sea. These mammals that spend the majority of their lives in or near the water and depend on it for their food can be split up in two: real marine mammals (three groups), and marine mammals still in the process of returning to sea (two groups).
  
  Fully marine mammals (order, species)
  • Sirenia - Manatees and Dugongs
  • Carnivora, family Pinnipedia - Seals, Sea Lions, and Walruses (33 species)
  • Cetacea - Whales, Dolphins, and Porpoises (over 70 species)
  Semi marine mammals (order, species)
  • Carnivora, family Ursidae - Polar bears
  • Carnivora, family Mustelidae - Sea Otters and Marine Otters
  Marine mammals differentiate from other marine life on a number of key points: they breathe air (most other marine animals extract oxygen from the water), they give live birth (one or two calf's at a time), they are warm blooded, have a high internal body temperature and maintain this between narrow limits, and they have hair (cetaceans have little or none).
  
  In order to cope with the marine environmental the mammals have developed a number of adaptations. These vary widely among the animals and their behaviour. Marine mammals have a universal need to keep their inner body warm at all times, most have thick layers of blubber around their body to prevent heat loss. Sea /Marine Otters and Polar Bears are exceptions on this as they rely more on fur and behaviour to deal with this. Other examples of adaptations are the streamlined bodies (where external ears are non-existent and limbs have vanished or changed into flippers), highly efficient livers and kidneys (very good filters which remove excess salts from the blood and prevent dehydration. Marine Mammals get their fresh water by eating fish), and the sonar (echolocation, enabling very good tracking of prey, and communication over thousands of kilometres) toothed cetaceans are equiped with.
  
  
• Sea birds
• Reptiles