Invertebrates
Jellyfish
Medusozoa · also called jellies, sea jelly
A jellyfish is proof of how little an animal needs in order to hunt. It is about 95 per cent water, with no brain, no heart, no blood, and no bones. What it does have is a bell that pulses, a nerve net that steers, and tentacles carrying millions of venom-tipped harpoons that fire in less than a thousandth of a second.
Overview
Jellyfish are the medusa forms of Medusozoa, around 4,000 species, in every ocean from the surface to the deep sea and pole to pole. The group includes the true jellyfish (Scyphozoa), the deadly box jellyfish (Cubozoa), and many small hydrozoan jellies. All are carnivores, catching drifting prey on stinging tentacles.
A stripped-down body
A jellyfish is mostly water around a jelly middle layer. It has no centralised organs — gases and waste pass straight through its thin tissue, and a distributed nerve net with simple light and balance sensors round the bell rim is enough to coordinate swimming. It moves by squeezing water from the bell in pulses, one of the most energy-efficient ways to travel known.
Harpoons
The tentacles carry cnidocytes, each holding a coiled, venom-tipped thread under pressure that fires in under a millisecond when touched — among the fastest movements in biology. Feeding needs no aim: anything that brushes the tentacle curtain is stung automatically, then curled to the mouth by the oral arms.
Two lives, and one that loops
Most jellyfish alternate a seabed polyp, which buds off clones for years, with the swimming medusa, the sexual stage — so one larva can seed a whole swarm. The hydrozoan Turritopsis dohrnii can revert from adult back to polyp when starved or hurt, the “immortal jellyfish”.
Blooms
Jellyfish numbers can explode into blooms of millions that clog power-station intakes, burst fish-farm nets, and strip the plankton from a bay. Warming water, overfishing of their competitors, nutrient pollution, and new hard surfaces for polyps may all favour them, though whether global numbers are truly rising is still debated.
Status
Jellyfish as a group are Not Evaluated and are generally abundant. A few restricted lake species, such as the golden jellyfish of Palau’s marine lakes, are vulnerable to drought and El Nino swings.
Jellyfish: common questions
Is a jellyfish an animal if it has no brain or heart?
Yes, firmly. A jellyfish is a multicellular animal that grows from an embryo, feeds on other organisms, and has specialised tissues, muscle, nerve, digestive, and reproductive, arranged in a definite body plan. What it lacks is centralised organs. Instead of a brain it has a distributed nerve net, with small clusters of nerve cells and simple light and balance sensors around the bell rim that are enough to coordinate swimming pulses and steer toward or away from light. Instead of a heart and blood it relies on its thinness, letting oxygen and waste diffuse directly through tissue that is never far from seawater. It is a demonstration that an animal can hunt and thrive with a very stripped-down design.
What should you do about a jellyfish sting?
For most stings, rinse the area with sea water, not fresh water, since fresh water can make undischarged stinging cells fire. Remove any tentacle pieces with tweezers or a gloved hand. Douse the area with vinegar for 30 seconds where box jellyfish or Irukandji are a risk, as vinegar disables their stinging cells, though for some other species vinegar is unhelpful, so local advice matters. Then soak in hot water, around 45 degrees Celsius, or as hot as is bearable, for 20 to 45 minutes to break down the venom, and take painkillers as needed. Seek emergency help for difficulty breathing, chest pain, severe whole-body symptoms, a large area stung, or any sting from a box jellyfish.
Are jellyfish really becoming more common?
In some places, though the global picture is debated because long-term jellyfish data are patchy. There are well-documented large blooms in areas such as the Black Sea, the Sea of Japan, the Mediterranean, and off Namibia, and plausible reasons why humans might favour jellyfish. Overfishing removes the small fish that both eat jellyfish and compete with them for plankton. Warmer water speeds up jellyfish growth and breeding. Nutrient pollution and low-oxygen dead zones suit jellyfish better than fish. And piers, pilings, and platforms give the seabed polyp stage far more hard surface to colonise. Whether worldwide jellyfish numbers are truly rising or simply fluctuating in long natural cycles is still an open scientific question.
How does a jellyfish move and catch food without eyes or a brain?
It swims by contracting a ring of muscle around the bell, squeezing water out and pushing itself forward in gentle pulses, with the jelly acting as a spring that helps the bell rebound. This is one of the most energy-efficient ways to move known in the animal kingdom. Steering is crude, using light and gravity sensors around the rim, so much jellyfish travel is really controlled drifting on currents. Feeding does not need targeting at all. The tentacles hang like a curtain, and any small animal that touches them triggers thousands of stinging cells automatically, after which the oral arms curl the paralysed prey up to the mouth in the centre of the bell.