10 of the Largest Living Creatures in the Sea

A giant Pacific octopus with its tentacles spread wide is taller than the scuba diver standing on the reef next to it.
Giant Pacific octopus.

Alexander Semenov / Getty Images

Meet the incredibly large creatures that have inspired researchers throughout the ages. The largest living things in the world call the sea their home, and in fact, the largest creature to have ever lived on the planet currently resides in the ocean. Some of these creatures remain elusive and wildly mysterious. That's what happens when you live in a place as unexplored as the ocean. And that’s also why it has been especially difficult to nail down the size of certain sea creatures. At least it was until a group of scientific researchers embarked on a comprehensive survey and review of past studies for the largest known marine species.

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Lion’s Mane Jellyfish | Total Length: 120 Feet (36.6 Meters)

An orange lion's mane jellyfish floating with its long white tentacles extended into the water.

Alan Weir / Flickr / CC BY 2.0

While the blue whale is the overall-largest king of the sea, the lion's mane jellyfish goes to the top of the list for being the longest. And as the longest medusozoa of all, these languid beauties have tentacles that reach an astonishing 120 feet in length. It's hard to know why they are graced with such extraordinary appendages. They are said to get tangled in marine debris or with other tentacles, and as they take notably more time to contract, they are more vulnerable to predators with a taste for jellyfish arms. Regardless, they don't seem to mind — and if nothing else, it's earned them a place of distinction with us humans.

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Blue Whale | Total Length: 108.27 Feet (33 Meters)

A silvery gray blue whale diving in the ocean.

Gerard Soury / Getty Images

Most of us have seen photos of a glorious, gigantic blue whale. The blue whale is the largest animal ever known to have existed — even out-sizing dinosaurs. They weigh up to 441,000 pounds. Their hearts are the size of a car; its beat can be detected from two miles away. At birth, they already rank amongst the largest full-grown animals. Because of commercial whaling, the species almost went extinct by the 20th century. Thankfully, it has slowly recovered following the global whaling ban. That said, these animals remain endangered and face a number of serious threats including ship strikes and the impacts of climate change.

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Sperm Whale | Total Length: 78.74 Feet (24 Meters)

A male sperm whale swimming near the ocean's surface.

by wildestanimal / Getty Images

At almost 80 feet in length, the beautiful sperm whale happens to be the largest of the toothed whales and the largest toothed predator of all. If you were to place it on its end and put it on the street, it would be as tall as an eight story building. Its clicking call can be as loud as 230 decibels underwater, equivalent to 170 decibels on land— about the loudness of a rifle shot within a few feet of one's ear. It has the largest brain of any animal on the planet, tipping the scales at around 20 pounds. Unfortunately for the sperm whale, they were fiercely hunted in the 18th, 19th, and 20th centuries; whalers sought the spermaceti, which was used for candles, soap, cosmetics, lamp oil, and many other commercial applications. Before whaling, there was an estimated 1.1 million of them. Today, there are several hundred thousand — which may be a lot compared to other whales in peril, but still disheartening given their once abundant population.

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Whale Shark | Total Length: 61.68 Feet (18.8 Meters)

A white-spotted whale shark swims underwater above a reef.

torstenvelden / Getty Images

Meet the largest fish in the sea, the beautiful whale shark. These majestic giants roam the oceans across the planet, looking for plankton and doing other things that fish do — sometimes even playing with people who love to swim with them. At 60 feet in length, if you run into a whale shark, you’re unlikely to miss this gentle creature. If the shark’s size doesn’t get your attention, the distinct light and dark markings should. More whale than shark, these fish are listed as endangered as they are still hunted in some parts of the world.

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Basking Shark | Total Length: 40.25 Feet (12.27 Meters)

A basking shark underwater with its mouth wide open feeding with two scuba divers nearby.

Chris Gotschalk / Wikimedia Commons / Public Domain

The basking shark is, to the best of our knowledge, the second largest fish in the modern ocean. The largest one on record measured in at over 40 feet — about the length of a school bus. And even more impressively, they can weigh in the range of 8,000 pounds. The basking shark is often seen with its enormous snout open wide near the water’s surface. But not to worry should you come across one while taking a dip in the depths of the ocean; they are gentle giants with a diet of mostly plankton, fish eggs, and larvae.

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Giant Squid | Total Length: 39.37 Feet (12 Meters)

black and white historic photo of Giant squid (Architeuthis sp.) found near Dildo, Newfoundland in December 1933.

A. Proctor / Wikimedia Commons / Public Domain

Taking the prize for being the longest cephalopod, the giant squid. Scientists have had few opportunities to observe the incredibly elusive giant squid in its natural habitat. The first time a giant squid was filmed in its deep-sea home was in 2012 by a group of scientists from Japan's National Science Museum. What we have learned about this enormous cephalopod is that it has quite a reach. Their feeding tentacles can catch prey at distances of over 30 feet. The giant squid is also legendary in the realm of sea monster tales where it has been associated with the sea monster Kraken.

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Giant Pacific Octopus | Radial Spread: 32.15 Feet (9.8 Meters)

A close-up view of a giant Pacific octopus with its white suction cups showing on its orange tentacles.

Damn_unique / Flickr / CC BY-SA 2.0

The aptly named giant Pacific octopus is the biggest cephalopod of all. This oversized octopus has a radial spread of more than 32 feet. Though typically reddish brown, the octopus can change its color when threatened or in need of camouflage. Intelligent by nature, the giant Pacific octopus can open jars, solve mazes, and play with toys. Aquariums often have enrichment activities for the octopuses to engage their brains. In the wild, the giant Pacific octopus is found throughout the Pacific from Alaska to Baja California, and as far northeast as Japan.

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Oarfish | Total Length: 26.25 Feet (8 Meters)

U.S. Navy servicemen holding giant 23 foot long oarfish

U.S. Navy / Wikimedia Commons / Public Domain

The decidedly odd shaped oarfish is often referred to as a sea serpent or dragon. These guys are long — the longest bony fish that we know of — and live at depths of 3,300 feet. Because they reside in the deep dark water columns of the open ocean and rarely come to the surface, they are not often seen alive and healthy. Most of our knowledge comes from specimens that have washed ashore. Oarfish, also known as ribbonfish, are long — 26 feet — and do not have scales. They are also known for their large eyes, all the better to see in their deep, dark habitat.

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Ocean Sunfish | Total Length: 10.82 Feet (3.3 Meters)

A silvery ocean sunfish swimming near a reef with schools of other fish nearby.

LeoPatrizi / Getty Images

Also known as a mola mola, the wonderfully weird ocean sunfish is the heaviest of all bony fish. Affectionately called a "swimming head," the giant fish without a tail has been measured at 10.82 feet and an astonishing 5,070 pounds. And if you're wondering how a fish without a tail swims, it powers itself by its mighty fins. These fins also allow them to swim on their side. Generally a solitary fish, ocean sunfish are sometimes in groups for cleaning. Ocean sunfish have a diet consisting mainly of jellyfish and zooplankton. Their prey includes sharks and sea lions.

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Japanese Spider Crab | Leg Span: 12.14 Feet (3.7 Meters)

A Japanese spider crab with its orange back and long legs walking on a bed of rocks surrounded by more crabs in an aquarium in Japan.

michael0shea / Getty Images

With a leg span of over 12 feet, the Japanese spider crab is an arthropod, from the same phylum that includes crustaceans, spiders, and insects. And it is not only the largest crab or crustacean in the family, but it also holds the title for the largest arthropod of all. As the Japanese spider crab ages, its legs continue to grow while its carapace remains the same size. Juvenile Japanese spider crabs are known to decorate their shells for camouflage.