10 of the Biggest Sea Creatures

It makes sense that the largest living creatures in the world call the sea their home, and that the largest creature to have ever lived on the planet currently resides in the ocean. There is more room to grow in the open (and unexplored) ocean, and to hide. In fact, some of these animals remain elusive and wildly mysterious.

And that's also why it's been especially difficult to nail down the size of certain sea animals. However, a group of scientific researchers embarked on a comprehensive survey and review of past studies for the largest known marine species. We reveal their top 10 below, based on length, starting with the tentacled Lion's Mane Jellyfish down to the leggy Japanese Spider Crab.

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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 largest sea creature, the lion's mane jellyfish is at the top of the list of the longest ocean animals. 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. That said, their long main of poison-equipped tentacles makes an excellent trap for prey.

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

Aerial shot of a blue whale in the ocean — Carl Finkbeiner / Getty Images

Most of us have seen photos of a glorious, gigantic blue whale, but without something to show scale, it's hard to fathom how tremendous their size is. The blue whale is the largest animal known to have existed — even out-sizing dinosaurs. They weigh up to 441,000 pounds. Their hearts are the size of a car; their heartbeats 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, there are fewer than 25,000 individuals left. These animals remain endangered and face several 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 is the largest of the toothed whales and the largest toothed predator of all. If you placed 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, 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 — a waxy substance found in cavities in the whale's head — for candles, soap, cosmetics, lamp oil, and many other commercial applications. Before whaling, there were an estimated 1.1 million sperm whales. Today, there are several hundred thousand sperm whales — 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 friendly 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 because people still hunt them 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

To our knowledge, The basking shark is the second largest fish in the modern ocean. The largest of these sharks on record measured at more than 40 feet — about the length of a school bus. Even more impressive, basking sharks can weigh in the range of 8,500 pounds. This shark is often seen with its enormous snout open wide near the water’s surface. But don't worry if you come across one while taking a dip in the ocean; they're 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

The giant squid takes the prize for being the longest cephalopod. Scientists have had few opportunities to observe the incredibly elusive animals in their 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've learned about these enormous cephalopods is that they have quite a reach. Their feeding tentacles can catch prey at distances of more than 30 feet. The giant squid is also legendary in the realm of sea monster tales where it is 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 called 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 open ocean's deep, dark water columns 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, or ribbonfish, are long — 26 feet — and do not have scales. They're also known for their large eyes, which are all the better for seeing 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 found in groups when cleaning. Ocean sunfish have a diet consisting mainly of jellyfish and zooplankton. Their predators include 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 more than 12 feet, the Japanese spider crab is an arthropod from the same phylum that includes crustaceans, spiders, and insects. It's not only the largest crab or crustacean in the family but also the largest living arthropod. As the Japanese spider crab ages, its legs grow while its carapace remains the same size. Juvenile Japanese spider crabs are known to decorate their shells for camouflage.

Why This Matters to Treehugger

Understanding our fellow creatures is key to protecting biodiversity and habitat conservation. We hope that the more we learn about amazing species like the ones on this list, the more motivated we’ll all be to help protect our oceans.

10 of the Largest Living Sea Creatures

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

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

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

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

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

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

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

Oarfish | Total Length: 26.25 Feet (8 Meters)

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

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

Why This Matters to Treehugger