The animals inhabiting the sea are called marine organisms. Marine organisms may be classified as Nektonic, Planktonic, or Benthic. The distribution of marine organisms depends on the chemical and physical properties of seawater, ocean currents & penetration of light.
Photosynthetic organisms (plants, algae, and cyanobacteria), the primary sources of food, exist only in the photic, or euphotic, zone (to a depth of about 300 ft/90 m), where light is sufficient for photosynthesis.
Very abundant phytoplankton include the diatomsand dinoflagellates. Heterotrophic plankton include such protozoans as the foraminiferans; they are found at all depths but are more numerous near the surface. Bacteria are abundant in upper waters and in bottom deposits.