Giant Jellyfish and Colossal Squid: Discover 6 Real Sea Monsters

It's easy to picture the ocean as the home of make-believe monsters - but these disturbing or deadly creatures are quite real. Let's take a look at...

• The Serpentine Oarfish
• The Giant Pacific Octopus
• The Deadliest Dolphin
• The Lion's Mane Jellyfish
• The Giant and Colossal Squid
• The Sperm Whale and the Livyatan

The Serpentine Oarfish

The oarfish is an incredibly strange creature from the deep sea. They have a long (they can reach 17m) tapering body with a dorsal fin running along its length. Added to this is a set of a trailing pelvic fins that look like oars (hence the name) and a habit of swimming by undulating the dorsal fin while the body remains still. The fish has even been seen swimming vertically in the water column, holding position while feeding on krill with their toothless mouths. To cap off the alien image, they don't actually have scales - instead, they have skin covered in silvery guanine that contrasts with the deep red of their fins.

It's thought that these strange fish and their swimming habits may have inspired legends of sea serpents. In addition, the oarfish only approaches the surface when sick or dying. Perhaps that's why Japanese folklore paints them as harbingers of doom - known as ryūgū-no-tsukai (or the sea god’s palace messenger) their presence was said to foretell tsunamis, earthquakes or other natural disasters!

The Giant Pacific Octopus

It may not be quite the size of the legendary kraken, but the giant Pacific octopus is still impressively large - this cephalopod's body can be 60cm across, with arms that often span more than 4m. The creature can reach sizes comparable with a human, while the heaviest known specimen weighed in at around 200 pounds!

Though hardly diminutive, the octopus doesn't have to rely on its bulk. Like many cephalopods they are smart enough to solve simple puzzles or open jars, capable of changing color to camouflage themselves and possess an incredible grip - a result of around 2,000 suction cups on their arms.

The cephalopod relies on these features to keep itself fed, hunting shellfish, crustaceans, sea urchins... and even grabbing seabirds for a bit of variety!

The Deadliest Dolphin

Orcas may seem like a strange addition to this list, but few things in the modern day ocean have the same killing power as these oversized dolphins.

Let me preface this by saying that there have been almost no attacks by wild orcas on humans. Hans Kretschmer is one of the few known people to have been bitten by a killer whale (which possibly mistook him for a tasty sea lion) but the creature subsequently released the surfer from its jaws and allowed him to swim to shore. On the other hand, wild orcas have been known to wreck boats and captive individuals have been known to kill trainers and trespassers.

It seems that wild killer whales prefer to hunt far more impressive game than us. Pods of orca have been documented methodically wearing down and killing massive blue whales. They also make a habit of hunting great white sharks, often securing victory by flipping them upside-down to cause tonic immobility - a poorly understood state similar to paralysis. With the shark left helpless, the orca can rip out the liver for a rich meal.

Generally it's juvenile sharks that fall prey to these attacks, but not for the reason you'd think. Adult great whites learn to spot orcas on the prowl and flee their hunting grounds rather than risk a confrontation!

The Lion's Mane Jellyfish

The lion's mane is a fairly big jellyfish with tentacles that frequently reach 3m in length. These red-ish appendages are armed with a fairly nasty sting and dangle in a thick plume beneath the golden-brown bell... that can be 50cm across.

While that size is fairly impressive already, the jellyfish has the potential to grow much larger - the biggest example ever found was a colossal 36.5m from bell to tentacle-tip!

The Giant and Colossal Squid

With a pair of 10m long razor-barbed hunting tentacles and eyes the size of dinner plates, the giant squid is clearly a predator... but unfortunately, a lot of what we know about this creature is based on the few partial corpses that have washed ashore. Researchers have estimated that (based on the various parts we've recovered) the giant squid could reach up to 20m in length - easily enough to be considered a sea monster.

The colossal squid is another mysterious cephalopod, though this one seems to have a much stockier body than its cousin. They're thought to reach around 500kg (which makes them the heaviest known invertebrate) but like the giant squid, sightings are incredibly rare. Despite the species being identified around a century ago, the first time one of these creatures was captured on film was in 2025 when a translucent juvenile was recorded in the South Atlantic Ocean!

The Sperm Whale and the Livyatan

Sperm whales are the largest toothed whale alive today, with males often exceeding 45 tons (females tend to be around 15 tons.) They are powerful hunters, and researchers have found the remains of deep-sea fish, sharks and even giant squid in their stomachs.

These whales are named for their spermaceti, a valuable oil used by the whales to focus sound... and by humans for oil-lamps, lubricants, candles and perfumes. This bounty made sperm whales frequent (but risky) targets of the whaling industry - and encounters with real sperm whales are thought to have inspired "Moby Dick."

Though an apex predator, the sperm whale isn't as terrifying as its extinct ancestor Livyatan melvillei, a name referencing both the author of Moby Dick and the biblical Leviathan. This monster of the depths matched the sperm whale for size, but boasted a mouth full of vicious teeth that could reach a foot long and four inches wide. A robust and shorter jaw would allow it to use a brutal, crushing bite on prey... which was probably other whales!

Thanks for reading - perhaps you'd also like...

• Toxic Turtles and a Man-Killing Frog: 5 Damp and Deadly Natural Poisoners
• Big Eater? These 8 Animals Hunt Prey Larger Than They Are!
• Hell Pigs and the Abnormal Shrimp: 6 Nightmarish Predators From Prehistoric Earth

Sources and Further Info:

• Oarfish
• Three-metre giant oarfish, ‘palace messenger’ of doom, washes up on Tasmanian beach
• Giant Pacific octopus
• Orcas found to kill blue whales, the largest animals on Earth, for first time
• Orcas filmed paralysing and eating great white sharks in the Gulf of California
• Lion's mane jellyfish
• Extreme Jellyfish
• Giant Squid
• Colossal squid filmed alive in deep ocean for the first time ever
• Sperm Whale
• Behold Livyatan: the sperm whale that killed other whales