Explanation:

Ganges shark:

• Sharks are the most important predators found in oceans across the world. But it is a poorly known fact that sharks are also found in freshwater habitats in some countries.
• There are six species of river sharks found in the world, out of which the Ganges shark (Glyphis gangeticus) is endemic to India.
• It inhabits the River Hooghly in West Bengal, as well as the rivers Ganges, Brahmaputra, Mahanadi in the states of Bihar, Assam and Orissa.
• While some of the other river sharks are also known to inhabit saltwater, the Ganges shark is only found in rivers and possibly estuaries, with no confirmed records from oceans or seas.
• Its population has been steadily decreasing due to over fishing, habitat degradation, increasing river utilisation, and building of dams.
• Recognising the dangers faced by this species, it is protected under Schedule I, Part II A of the Wildlife (Protection) Act of India, 1972. Ganges River dolphin:
• The Ganges River dolphin or Susu, lives in one of the most densely populated regions of the world.
• One of the main threats to the species is loss of habitat due in large part to the creation of dams and irrigation projects.

Dugongs:

• Dugongs are cousins of manatees and share a similar plump appearance, but have a dolphin fluke-like tail.
• Unlike manatees, which use freshwater areas, the dugong is strictly a marine mammal.
• Commonly known as "sea cows," dugongs graze peacefully on sea grasses in shallow coastal waters of the Indian and western Pacific Oceans.