Imagine a creature that has no heart, no blood, no brain, and is 95 per cent water, writes columnist James Williams. Meet the jellyfish, one of the oldest and most ubiquitous animals on our planet. These gelatinous creatures have been swimming Earth’s oceans for over 500 million years, far longer than we humans have walked the land.

While a jellyfish may appear simple, they lead complex and fascinating lives. They rhythmically pulse their bell-shaped bodies to propel themselves through the water, often trailing tentacle-like structures with venomous stinging cells.

The lifecycle of a jellyfish is nothing short of a metamorphic wonder. Jellyfish begin as tiny larvae, settling on the seafloor and developing into attached polyps. These small polyps resemble mini sea anemones, extending tentacles into the water to capture food. As the polyp grows, it reproduces asexually by cloning itself and forming a colony of genetically identical polyps. When conditions are right, each polyp then transforms into a free-swimming jellyfish, detaching from its static life passively drifting along the currents seeking food as their tentacles brush against small fish, shrimps or, for the larger species, crabs, even other jellyfish.

Jellyfish display a stunning diversity of sizes and shapes. The smallest are just a few millimetres wide, while the lion’s mane jellyfish can reach diameters over 2m, with trailing tentacles over 30m long. Some jellyfish take on intricate multi-coloured hues, while others are transparent. The moon jelly’s delicate translucent bell pulses iridescent blues and purples, while the fried egg jellyfish resembles its name with its bulbous yellow bell surrounded by fluffy white tentacles.

Ranging from the surface down to the inky depths, different jellyfish inhabit shallow coastal waters and the open ocean. The box jellyfish prefers to hunt close to shore. Its cube-shaped bell and fearsome sting make it a peril for swimmers. Out in the open seas, nomadic jellyfish drift on ocean currents, surviving months or years without food. Species like the violet sea nettle migrate on an annual loop from the coastal Pacific Northwest down to Southern California and back.

Jellyfish don’t have what we would consider to be a brain, but they do have sophisticated ways of sensing and reacting to their environment. Light-detecting organs help jellyfish maintain their vertical position in the water. Touch-sensitive nerve cells along the tentacles detect potential prey, triggering the tentacles to fire stinging cells that paralyse and capture the unlucky victim. Jellyfish have no ears or vocal cords, yet some species can “hear” low-frequency vibrations and communicate through the rhythmic pulsing of their bells.

While jellyfish are beautiful to behold, their stings are not to be messed with. The box jellyfish delivers one of Earth’s most potent venoms. Fortunately, most jellyfish stings just cause mild discomfort, though they can be lethal to small fish or crustaceans. And jellyfish don’t deliberately attack humans, they drift passively and only sting if a swimmer makes contact.

Jellyfish may appear primitive and primordial but they can teach us a lot about advanced science and technology. Researchers are studying jellyfish propulsion to design new energy-efficient underwater vehicles. The aequorea victoria jellyfish produces a green fluorescent protein that is revolutionising biomedical imaging. It allows scientists to tag a gene and if the gene flashes a colour, then we know it’s active. Analysing jellyfish nervous systems gives insights into how simple networks can process information and produce complex behaviours.

The UK coastline has seen an increase in the number of jellyfish and the range of species we find. It’s thought a few factors are contributing to this, such as overfishing, especially of species known to eat jellyfish. The gradual average increase in sea temperature also means species normally only found in warmer waters are now being found in our waters.

If you come across a stranded jellyfish, take care even if it is dead, its stinging cells on the tentacles can still be active and cause discomfort and pain. There is also the old myth that urinating on a jellyfish sting will lessen or stop the pain. It won’t. laboratory controlled tests show this is false. Urine is slightly acidic and has no effect on the sting but it could lead to an infection around the sting site.

From their unique biology to their uncanny ability to thrive where fish struggle, jellyfish continue to fascinate scientists and beachgoers alike. So, keep an eye out for those pulsing bells the next time you are strolling along the shore. Jellyfish will likely continue enthralling future generations with their hypnotic beauty.

Dr James Williams is a senior lecturer in education at the University of Sussex