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What Influences the Life of a Marine Animal?

The impressive diversity of life on land can seem paltry compared to the infinite variety in the oceans. From corals to cuttlefish, sea cucumbers to sea lions, giant squid to minuscule plankton, organisms in the ocean come in every shape, size and disposition. Despite the dizzying array of lifeforms and the immense dimensions of their habitat, understanding ocean ecology is made easier when you consider a few of the basic constraints and determinants of surviving in this massive watery wilderness.

Salinity
- Penguins possess supraorbital glands that allow them to drink saltwater.
  
  The ocean is a saline environment, so marine creatures must be able to deal with the special challenges that poses. Due to the tendency of water to diffuse through osmosis from areas of lower salt concentration to higher, fish and other marine organisms tend to exist within particular ranges of salinity, or, like anadromous species that migrate between freshwater and saltwater, have the physiological ability to contend with changing salt concentrations.
Finding Food
- The distribution of food sources helps dictate a marine creature's movements.
  
  The availability of food resources dictates much of the wanderings of marine creatures, which can be extensive. For example, many baleen whale species trek thousands of miles to polar waters to feast on plankton in those productive environments, but they journey to tropical and subtropical waters to breed. Some open-ocean species, such as oceanic white-tip sharks, must essentially roam constantly to find enough food.
Pressure
- Elephant seals can dive thousands of feet beneath the ocean's surface to hunt.
  
  Pressure increases enormously as one descends to greater depths of the ocean, essentially because of the overlying weight of all that water. Certain marine mammals like sperm and bottlenose whales and elephant seals can dive miles below the ocean's surface, and must deal with the crushing pressure. Some of these species show adaptations like a reduced heart-rate during dives and greater tolerance of carbon dioxide in the blood that allow them to survive such exposures (see Reference 3, pgs. 15-16).
Breathing
- Many marine organisms have gills or other means of breathing under water.
  
  Fish, cephalopods and a host of other marine creatures have evolved to breathe underwater, just as all life on Earth once did. They do this through gills and other specialized adaptations for extracting oxygen from seawater. Marine mammals, birds and reptiles, however, are still tied to breathing above water, though, depending on the species, they may not need to surface for hours at a time.
Reproduction
- Sea turtles lay their eggs on beaches, but otherwise spend their lives at sea.
  
  Along with finding food, reproduction is the driving force of marine life. Many kinds of ocean creatures lay eggs or give birth at enormous scales so as to better the chances of some of their offspring living. Others, especially the larger organisms, bear fewer young and often care for them for an extended period of time. The pinnipeds -- sea lions, seals and walruses -- must come ashore to give birth, as do egg-laying sea turtles.
Predators
- Orcas are one of the ocean's apex predators.
  
  Food webs are as complex and dramatic in the world's oceans as on its continents. Avoiding being eaten is a major part of most aquatic organisms' day. Shoals of fish are beset by bigger hunting fish like marlin, tuna and sharks, as well as by marine mammals like dolphins and sea lions and seabirds attacking from above. Apex predators in the ocean include orcas, sperm whales and white sharks.

Animals & Science