What Is the Biggest Animal in the Ocean?

Blue Whale Fluke
A blue whale dives into the waters of Monterey Bay, California. Chase Dekker Wild-Life Images / Getty Images

The biggest animal in the ocean, and in the world, is the blue whale (Balaenoptera musculus), a sleek, light-colored giant.

How Big Is a Blue Whale

Blue whales are thought to be the largest animal ever to live on the Earth. They reach lengths up to about 100 feet and weights of an amazing 100-150 tons.

Blue whales are a type of baleen whale known as a rorqual. Despite their huge size, baleen whales like blue whales feed on small organisms. The blue whale feeds primarily on krill, and may eat 2 to 4 tons of krill per day during their feeding season. Their skin is a beautiful gray-blue color, often with a mottling of light spots.

The second-biggest animal in the ocean is another baleen whale- the fin whale. At an average length of 60-80 feet, the fin whale is still pretty big, but not nearly as big as the blue whale. 

Where to Find Blue Whales

Blue whales are found in all the world's oceans, but their populations are not as large as they used to be due to whaling. After the invention of the grenade-tipped harpoon in the late 1800's, blue whales were subjected to relentless hunting. Blue whale populations had declined so much that the species was given protection from hunting in 1966 by the ​International Whaling Commission. Today, there are an estimated 10,000-15,000 blue whales in the world.

Blue whales are much too large to be kept in captivity. To have a chance of seeing a blue whale in the wild, you could go on a whale watch off the coast of California, Mexico, or Canada.

Other Big Ocean Animals

While the blue whale and fin whale are the largest animals, the ocean has plenty of other large creatures.

The largest fish (and largest shark) is the whale shark, which can grow to about 65 feet and weigh up to about 75,000 pounds. 

The largest jellyfish is the lion's mane jelly. It is possible that this animal could surpass the blue whale in size - some estimates say that the lion's mane jelly's tentacles can be 120 feet long. The Portuguese man o' war is not a jellyfish, but a siphonophore, and this animal also has long tentacles—it is estimated that the man o' war's tentacles can be 50 feet long. 

If you want to get super technical, the largest animal on the planet could be the giant siphonophore, which can grow up to 130 feet long. However, this is not really a single animal, but a colony of jelly-like zooids strung together in a long chain that drifts through the ocean.

Can't get enough of big ocean animals? Watch a slideshow of the largest living sea creatures.

References and Further Information:

  • American Cetacean Society. 2004. “Blue Whale” (Online), American Cetacean Society. Accessed October 31, 2011.
  • Cawardine, M. Whales, Dolphins and Porpoises. Dorling Kindersley. London. 1995.
  • Mead, J.G. Whales and Dolphins In Question. 2002. Smithsonian Institution.
  • Martins, Carol and Craig Knickle. 2009. "Whale Shark" (Online). Florida Museum of Natural History Icthyology Department. Accessed April 7, 2009.
  • NOAA Fisheries Office of Protected Resources. Blue Whale (Balaenoptera musculus). Accessed October 31, 2011.
  • The Marine Mammal Center. Blue Whale. Accessed September 30, 2015.