How are marine organisms classified?
Marine organisms are classified according to their mode of life as planktonic, nektonic or benthic. Planktonic organisms, usually very small or microscopic, have little or no power of locomotion and merely drift or float in the water (e.g. phytoplankton and zooplankton). Nektonic animals are those that swim and migrate freely (e.g., fishes, whales and squid). Benthic organisms live on the surface and inside 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 ).