The largest mammal in the ocean is the blue whale (Balaenoptera musculus). Blue whales can reach lengths of up to 100 feet and weigh as much as 200 tons, making them not only the largest mammals but also the largest animals ever known to have existed on Earth. These majestic creatures primarily feed on tiny shrimp-like animals called krill, consuming up to 4 tons of them per day during feeding seasons. Blue whales are known for their deep, resonating calls that can travel thousands of miles underwater, making them fascinating subjects of study in marine biology and conservation.