are they under any threat?
Blue whales were nearly hunted to extinction in the 19th and 20th centuries by commercial whalers until the International Whaling Commission banned the taking of blue whales in 1967. The International Union for Conservation of Nature has listed blue whales as "endangered".
Blue whale
Blue whales were nearly hunted to extinction in the 19th and 20th centuries by commercial whalers until the International Whaling Commission banned the taking of blue whales in 1967. 
what characteristics do all the organisms in the animal kingdom share Animals are eukaryotic, multicellular, heterotrophic, respire aerobically what is meant by the third feature? A heterotroph  is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. can you give example of such species? All animals and fungi, ome bacteria and protists, and many parasitic plants. regarding the first one; how are they classified? Insecta, Pisces, Amphibia, Aves, and Mammalia. which is the largest of it? The blue whale and the smallest? Myxobolus shekel what is the size of the largest one? length of 29.9 meters where are they found? In all major ocean basins, except the Bering Sea and the Arctic Ocean how fast do they swim? 20 to 48 km/h
wiki:1554483
Blue whale [SEP] Globally
Blue whales were nearly hunted to extinction in the 19th and 20th centuries by commercial whalers until the International Whaling Commission banned the taking of blue whales in 1967. The International Whaling Commission catch database estimates that 382,595 blue whales were caught between 1868 and 1978, including 7,973 in the North Pacific (2.09%), 10,442 in the North Atlantic (2.73%), 5,383 in the South Pacific (1.41%), and in the Southern Ocean, 13,022 pygmy blue whales (3.40%) and 345,775 Antarctic blue whales (90.40%). The Soviet Union continued to illegally hunt blue whales in the Northern and Southern Hemisphere through to 1973, and Spanish vessels caught occasional blue whales up until 1978.