The blue whale (Balaenoptera musculus) is a marine mammal belonging to the baleen whales (Mysticeti). At 30 metres (98 ft)[4] in length and 170 tonnes (190 short tons) or more in weight, it is the largest known animal ever to have existed. Long and slender, the blue whale's body can be various shades of bluish-grey dorsally and somewhat lighter underneath. There are at least three distinct subspecies: B. m. musculus of the North Atlantic and North Pacific, B. m. intermedia of the Southern Ocean.
And B. m. brevicauda (also known as the pygmy blue whale) found in the Indian Ocean and South Pacific Ocean. B. m. indica, found in the Indian Ocean, may be another subspecies. As with other baleen whales, its diet consists almost exclusively of small crustaceans known as krill. Blue whales were abundant in nearly all the oceans on Earth until the beginning of the twentieth century. For over a century, they were hunted almost to extinction by whalers until protected by the international community in 1966.
A 2002 report estimated there were 5,000 to 12,000 blue whales worldwide, located in at least five groups. More recent research into the Pygmy subspecies suggests this may be an underestimate. Before whaling, the largest population was in the Antarctic, numbering approximately 239,000 (range 202,000 to 311,000). There remain only much smaller (around 2,000) concentrations in each of the eastern North Pacific, Antarctic, and Indian Ocean groups.
There are two more groups in the North Atlantic, and at least two in the Southern Hemisphere. The blue whale has a long tapering body that appears stretched in comparison with the stockier build of other whales. The head is flat, U-shaped and has a prominent ridge running from the blowhole to the top of the upper lip. The front part of the mouth is thick with baleen plates; around 300 plates (each around one metre (3.2 ft) long) hang from the upper jaw, running 0.5 m (1.6 ft) back into the mouth.
Between 70 and 118 grooves (called ventral pleats) run along the throat parallel to the body length. These pleats assist with evacuating water from the mouth after lunge feeding (see feeding below). The blue whale is the largest animal ever known to have lived. The largest known dinosaur of the Mesozoic Era was Argentinosaurus, which is estimated to have weighed up to 90 metric tons (99 short tons). Blue whales are difficult to weigh because of their size.