9 Remarkably Giant Sea Creatures Ever Discovered

Think you know what hides in the depths of the ocean? With nearly 95% of our oceans still unexplored, the mysteries of the deep are vast and intriguing. However, thanks to intrepid explorers, advanced deep-sea cameras, and dedicated researchers, we have discovered some truly remarkable sea creatures, both from ancient times and today.

From the 9-foot (2.7-meter) spider crab to the 60-foot (18-meter) prehistoric Megalodon, the ocean is home to some of the largest and most fascinating creatures. Let’s take a closer look at these sea giants renowned for their impressive size.

Can you guess which living whale species holds the title for the largest? While the orca, also known as the killer whale, is impressive, it’s not the biggest. Orcas can grow between 23 to 32 feet (7 to 9.8 meters) long, which is roughly the size of a school bus. The narwhal, often called the “unicorn of the sea,” is also not the largest. Narwhals typically reach lengths of 13 to 20 feet (4 to 6 meters), including their long tusk, which adds about 9 feet (2.7 meters) to their length.

So, who is the largest? The blue whale holds the title for the largest whale and indeed the largest animal ever known to have lived on Earth. Blue whales can reach lengths of 82 to 105 feet (25 to 32 meters). They are not just larger than any whale but are also bigger than the prehistoric Megalodon and even the T-Rex!

To give you a sense of scale, if you placed a blue whale next to a school bus, the whale would dwarf the bus. National Geographic notes that a blue whale’s tongue can weigh as much as an elephant, and their heart can be as heavy as a car. It’s no surprise these giants need to consume about 4 tons of krill daily to sustain their enormous size!

While few animals today can rival the blue whale in sheer size, another contender for the title of largest marine creature is not what you might expect: the lion’s mane jellyfish. This jellyfish is the largest of its kind, with some specimens reaching lengths of up to 120 feet (36.6 meters). They can be found in vibrant colors like red, purple, and orange, and their impressive size is matched by their extensive number of tentacles. A lion’s mane jellyfish can have up to 1,200 tentacles, organized into eight groups of 70 to 150.

Another impressive marine giant is the giant oceanic manta ray, the largest species of ray. Manta rays can have a wingspan that exceeds 30 feet (9.1 meters). They also possess the largest brain relative to their body size among all fish. Unlike stingrays, mantas do not have venomous tails.

While the lion’s mane jellyfish and the giant manta ray are among the largest marine creatures, there is another contender for the title of longest marine animal: the Portuguese man o’ war, also known as Physalia physalis. This siphonophore can reach lengths of up to 165 feet (50 meters), including its tentacles. Despite resembling a jellyfish, the Portuguese man o’ war is a colonial organism composed of genetically identical individuals. Its long tentacles are used for capturing prey, and its sting can be dangerous to humans. Remarkably, detached tentacles can float for days before they decompose.

Even detached, the tentacles of a Portuguese man o’ war (Physalia physalis) can still sting. However, the likelihood of a dangerous encounter with one is relatively low. If stung, you might experience welts, stomach cramps, an elevated heart rate, and nausea. While these creatures are best admired from a distance, their vibrant colors are quite striking.

The Shastasaurus holds the title of the largest marine reptile ever known. This enormous predator roamed the oceans during the Late Triassic Period, around 210 million years ago. Shastasaurus could grow up to 69 feet (21 meters) in length and weigh more than 75 tons, comparable to the size of a blue whale. If it were stood upright, it would reach the height of a seven-story building. Despite its impressive size, the Shastasaurus had a relatively narrow ribcage, measuring only 6 feet (2 meters) across.

Unlike what one might expect, the Shastasaurus did not prey on dinosaurs. Instead, its diet consisted of small fish and cephalopods like octopuses and squids.

Another fascinating marine reptile was the Albertonectes, a member of the plesiosaur family. Living around 76 to 70 million years ago in the seas of North America, the Albertonectes had a small head, a remarkably long neck, and large, flipper-like limbs. It could reach lengths of up to 38 feet (11.5 meters), with its neck alone stretching 23 feet (7 meters). This elongated neck contained an unprecedented 76 vertebrae, making it the longest-necked animal known. Scientists speculate that its lengthy neck may have been used to forage for shellfish on the seabed or to capture fish and squids. The Albertonectes also had gastroliths in its stomach, some up to 5.5 inches (14 centimeters) in diameter, which likely helped with digestion.

The Tylosaurus was a formidable marine reptile from the Mosasaur family that lived in the shallow seas of North America around 85 to 80 million years ago. It could grow up to 45 feet (13 meters) in length and was a highly effective predator. The Tylosaurus had a streamlined body with a blunt, powerful head designed for ramming and stunning prey. Its body featured agile flippers and a long tail with a fin, enhancing its maneuverability. The Tylosaurus fed on a diverse diet, including fish, turtles, small sharks, other mosasaurs, plesiosaurs, and even flightless birds.

Another notable prehistoric marine reptile is the Ophthalmosaurus, which lived during the late Jurassic period and inhabited oceans worldwide. This reptile weighed around 6,000 pounds (2,722 kilograms) and reached lengths of approximately 16 feet (4.9 meters), similar in size to a modern beluga whale.

While it’s unfortunate that we can’t observe these ancient creatures in person today, their distinctive wide eyes and dolphin-like features add a fascinating dimension to their appearance. Just to clarify, Ophthalmosaurus did not evolve into ophthalmologists; that’s just a playful twist.

The Mosasaurus was a formidable marine predator that roamed the seas approximately 66 million years ago. Fossil evidence suggests that some individuals could exceed 50 feet (15 meters) in length, making it one of the largest marine carnivores of its era.

This ancient predator had a head resembling that of a crocodile, equipped with numerous razor-sharp teeth arranged in multiple rows along both jaws. The Mosasaurus also had specialized pterygoid teeth located on the roof of its mouth, which aided in grasping and retaining its prey. These adaptations made it highly effective at hunting and securing its meals in the aquatic environment.

The Styxosaurus was a member of the Plesiosaur family, living during the Late Cretaceous Period, around 85 to 70 million years ago. This marine reptile, which could reach lengths of approximately 35 feet (10.6 meters), had an exceptionally long, flexible neck that comprised over 16 feet of its total length. Its relatively small body weighed about 4 tons.

The Styxosaurus had cone-shaped, sharp teeth adapted for catching fish. It did not chew its prey; instead, it likely relied on around 200 gastroliths—small stones found in its stomach—to aid in digestion by grinding up food. Some researchers also suggest that these stones might have helped the Styxosaurus to control its buoyancy, allowing it to dive to the ocean floor in search of specific prey.

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