The blue whale is a whale.
The blue whale ( Balaenoptera musculus ) is a marine mammal belonging to the baleen whales ( Mysticeti ) . At up to 29.9 m in length and with a maximum recorded weight of 173 tonne and probably reaching over 181 tonnes ( 200 short tons ) , it is the largest animal known to have ever existed . As with other baleen whales , its diet consists almost exclusively of small crustaceans known as krill .