Marine life is full of strange, beautiful, and often misunderstood creatures, but few are as bizarre and fascinating as the jellyfish. While they may look like simple blobs floating aimlessly through the sea, jellyfish are ancient survivors. And their unique adaptations have helped them persist for over 600 million years — from long before the dinosaurs appeared.

More than just a blob 

Jellyfish are brainless, boneless, eyeless creatures that are 95% water. Despite this simplicity, however, they’ve withstood five mass extinction events and continue to thrive in oceans around the world. With bell-shaped bodies and stinging tentacles, they’re members of the phylum Cnidaria, related to sea anemones and corals.

They have radial symmetry, which means they can detect and respond to stimuli from all directions. Their central mouth serves both as an entry point for prey (like shrimp and small fish) and an exit for waste. And because too much undigested food would weigh them down, they digest quickly to stay nimble.

These simple creatures, have several remarkable adaptations that have enabled them to thrive over hundreds of millions of years. For instance, one secret to jellyfish survival is in their mouth, which is situated at the core of their body (it’s a bit like you having a mouth where your belly button is). Having their mouth at the center of their body allows jellyfish to efficiently capture, ingest, and expel food and waste while maintaining balance and mobility in the water — crucial for their survival as slow, drifting predators.

They’re also good at using physics to their advantage. A study led by Brad Gemmell at the University of South Florida revealed that jellyfish move by rhythmically pulsing their bells, creating vortex pairs that push them forward. This method is not only energy efficient but could also inspire the next generation of underwater vehicles.

They’re also a very varied group. Jellyfish come in all shapes and sizes as well as display cool capabilities such as bioluminescence and apparently, immortality. Here, we enumerate seven enthralling such jellyfish and dish out their unique characteristics.  

1. Moon Jellyfish (Aurelia aurita)

One of the most common and well-studied jellyfish species is the moon jellyfish, which is also featured in various aquariums. Moon jellies are almost transparent with a saucer-shaped bell that can reach up to 12 inches (30cm) in diameter. Found mostly near the coast and in upwelling areas, this sea jelly possesses numerous, short, and fine tentacles which they use to capture and sting prey. 

Back in 1991, approximately 2,500 moon jelly polyps and ephyrae, representing two early phases in the jellyfish life cycle, were sent into orbit aboard the space shuttle Columbia. Scientists wanted to observe what would happen when a living thing was reared exclusively in microgravity. So, you might ask why jellyfish? Their sensory structures bear striking similarities to those of humans in determining orientation in space. The outcomes of the experiment were unfavorable. After developing in space, the moon jellies struggled to distinguish between up and down, experiencing impaired movement that rendered them unsuitable for life on Earth. 

But, left on Earth, they’re very well adapted. They’ve been around for over 500 million years and show no signs of slowing down.

2. Immortal jellyfish (Turritopsis dohrnii)

This tiny, unassuming jellyfish — no bigger than a pinky nail — harbors one of nature’s greatest secrets: biological immortality. When facing stress, disease, or aging, it can transform its mature body back into a juvenile polyp stage through a process called transdifferentiation.

It’s like a butterfly turning back into a caterpillar. Scientists are fascinated by this ability, which could offer insight into regenerative medicine and anti-aging research. It’s not invincible (it can still be eaten), but theoretically, it could live forever by continually resetting its life cycle.

3. Portuguese man o’ war (Physalia physalis)

You got me. We cheated. Distributed across the Pacific Ocean, Indian Ocean and Gulf of Mexico, the Portuguese man o’ war is not a true jellyfish. Instead, it’s actually a siphonophore — a colonial organism consisting of specialized individual animals known as zooids that collaborate to function as a single entity.

Its vibrant blue-purple gas-filled float acts like a sail, allowing wind to carry it across the ocean’s surface. Beneath, its long, venomous tentacles dangle like underwater tripwires, delivering paralyzing stings to prey — and painful welts to unsuspecting swimmers. While rarely fatal to humans, encounters can be excruciating, and their stings remain potent even after the organism is dead.

4. Box jellyfish (class Cubozoa)

Box jellyfish, as its name suggests, have a cube-shaped or box-like bell, with each side possessing a cluster of long, slender tentacles. Typically found in shallow coastal waters, the highest concentration of box jellyfish is in the Indo-Pacific region but they have also been reported in waters around Thailand, Malaysia, and Northern Australia. Their venom is considered to be among the deadliest in the world, containing toxins that can target the heart, nervous system, and skin cells.

Box jellyfish are highly advanced in comparison to most other jellyfish. For one, they have the ability to move at up to four knots rather than just drift away with the ocean currents. Moreover, they possess a remarkably sophisticated set of 24 eyes grouped into four clusters called rhopalia. A study headed by Anders Garm from Lund University revealed that box jellyfish use their eyes to avoid obstacles and orient themselves toward light shafts. Scientists are still unsure how box jellyfish process what they see because even if they have a nervous system, it is a very simple one. 

5. Lion’s mane jellyfish (Cyanea capillata)

This cold-water colossus is the largest known jellyfish species. Its bell can reach over 7 feet (2 meters) in diameter, and its tentacles — organized into eight distinct clusters — can trail over 100 feet behind it, longer than a blue whale.

Despite its intimidating size, it poses little threat to humans beyond mild stings. Its long, flowing tentacles, give it its mythic presence. At the base of the ocean food chain, it’s a key prey animal of leatherback sea turtles, which are uniquely equipped to feast on jellyfish without harm. Named after its “mane” of long tentacles resembling hairs, this jellyfish is capable of bioluminescence, producing its own light and glowing in the dark.

6. Peach blossom jellyfish (Craspedacusta sowerbii)

Though it looks delicate and dainty, the peach blossom jellyfish is a global hitchhiker, spreading from its native Chinese lakes to freshwater bodies on every continent except Antarctica. Only about the size of a coin, it’s a seasonal bloomer — often appearing only under specific temperature and nutrient conditions.

It’s now classified as invasive in places like Canada and Europe, though its ecological impact is still being studied. Its spread may have been aided by the global trade of ornamental aquatic plants like water hyacinth, inadvertently transporting dormant polyps.

7. Psychedelic medusa (Crossota millsae)

Inhabiting the midnight (bathypelagic) zone (3,300- 10,800 feet or 1,000- 3,300m), the psychedelic medusa is found in waters of the North Pacific Ocean, specifically near California and Hawaii. First described in 2003, this species is easily noticeable due to its stunning ruby or vivid orange color that contrasts the midnight zone. Unlike most sea jellies, males and females of this species can be easily distinguished. Female eggs are generally large and rounded, while male gonads are shaped like sausages. In addition, this iridescent jellyfish reproduces sexually, and is one of the few jellies that bear live young. 

In a blog post by NOAA, scientists observed an interesting feeding pose suggesting that this species may feed by hovering above the seafloor with its sting-loaded tentacles waiting for prey.  Through such recordings, researchers are able to understand more how organisms thrive in their inaccessible habitat.

And so, the jellyfish — those ancient, pulsating sacs of seawater — continue their slow, deliberate dance through the world’s oceans. They are reminders that complexity isn’t always about neurons or limbs, that survival can come from simplicity, elegance, and a billion-year head start. Long before vertebrates tested the waters, before coral reefs blossomed or whales sang, jellyfish were already here, drifting, stinging, feeding, glowing. In their translucent bells and flickering lights, we glimpse a kind of alien intelligence — one tuned not to thought or reason, but to rhythm, pressure, and pulse. The ocean is old. And jellyfish, in their quiet, gelatinous way, are its memory.