The marine environment is relatively uniform and stable when compared with fresh water and with land. Oxygen is generally available, and the salinity is relatively constant, ranging from 34 to 36 parts per thousand, depending on the latitude. Light and temperature however, vary greatly, depending principally on the depth. Wave action, tides, and vertical and horizontal ocean currents produce an incessant mixing of sea water thereby ensuring a medium in which the concentration of dissolved gases and salts fluctuates relatively little.
Life is not uniformly distributed throughout the depth and breadth of the oceans. Vertical distribution of marine organisms is largely controlled by the depth of light penetration. Sufficient light for photosynthesis penetrates only a short distance below the surface to a depth of around 200 m, depending on the turbidity of the water. Transparent worms, shrimps and jellyfish live in the twilight zone but at greater depths it becomes a world of darkness ruled by prehistoric-looking fish with knife-sharp teeth and cavernous mouths patrolling the waters.