study of ocean plants and animals and their ecological relationships. Marine organisms may be classified (according to their mode of life) as nektonic, planktonic, or benthic. Nektonic animals are those that swim and migrate freely, e.g., adult fishes , whales , and squid . Planktonic organisms, usually very small or microscopic, have little or no power of locomotion and merely drift or float in the water. Benthic organisms live on the sea bottom and include sessile forms (e.g., sponges , oysters , and corals ), creeping organisms (e.g., crabs and snails ), and burrowing animals (e.g., many clams and worms ). Seafloor areas called hydrothermal vents , with giant tube worms and many other unusual life forms, have been intensively studied by marine biologists in recent years. The distribution of marine organisms depends on the chemical and physical properties of seawater (temperature, salinity, and dissolved nutrients), on ocean currents (which carry oxygen to subsurface waters and…