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Food and Phytoplankton
The diets of all marine animals depend on phytoplankton. Phytoplankton are microscopic plants that live in the ocean. Phytoplankton uses chlorophyll to turn sunlight into energy, and like land plants, they consume carbon dioxide and release oxygen, which is necessary for all marine animals. Phytoplankton are at the bottom of the aquatic food chain. Phytoplankton is eaten by zooplankton, microscopic animals, which are eaten by a crustacean like krill, which can be eaten by a fish, which can be eaten by a seal, which can be eaten by a top-of-the-line predator like a killer whale.
Finding Oxygen
Marine animals metabolize oxygen, which is dissolved in sea water. Deep sea water is well oxygenated. Fish take in oxygen through their gills, and animals like sea anemones and sponges simply take in oxygen from the water via diffusion. Marine mammals like seals, sea otters, whales and walruses must reach the surface of the water and breathe air.
Salinity and Water Pressure
Salinity affects the life forms found in the ocean. Coral reefs can't grow where the salinity is too high, but oysters can't tolerate water when the salinity is low. Marine fish excrete excess salt from ocean water from their gills. When the ocean reaches depths of 650 feet or more the salinity is constant. Water pressure increases at lower depths. Marine animals who live at such depths can tolerate the pressures as long as they don't have gas-filled body cavities. The sperm whale, who dives to great depths for squid, protects its lungs by allowing them to collapse under great pressure. Still, the bones of some elderly sperm whales have been found to be riddled with tiny holes, a sign of decompression sickness.
Tides and Currents
Marine animals have also adapted to the changing tides and currents caused by wind or the pull of the moon. Sea anemones have a basal disk that holds them to a substrate and keeps them from being dislodged by strong currents, though free-swimming sea anemones and jellyfish are blown around by ocean currents. Fish, whales and pinnipeds have fins and/or flukes that help them power through the water. Animals that are subject to dramatically changing tides and currents like those who live near the coast and in reefs adapt by growing horizontally instead of vertically, like the corals, or shutting their shells during times of low tide and opening them up to feed when the tide rushes in with nutrients, like mussels and barnacles. Shutting the shells also allows the animal to retain water and avoid drying out.
Changing Temperature
Marine animals also must adapt to temperature. Fish in the waters off the Antarctic have pale blood that lack hemoglobin and blood cells. This keeps their blood thin and doesn't require them to spend too much energy on circulation. Their livers also produce a natural antifreeze. These fish also don't have scales or swim bladders. Their skeletons are made out of cartilage and they store fat. This anatomy is designed to allow them to swim without expending too much energy. Deep-diving whales and most other marine mammals have a high internal body temperature as well as an insulating layer of blubber to keep them from getting too cold as they search for prey.