Jellyfish Facts & Trivia: 505 Million Years Old

• About 95% water. Only around five percent of a jellyfish is solid matter.
• No bones, brain, heart, teeth or blood. What it has instead is a decentralised nerve net — with roughly 1,000 neurons active at any moment in a box jellyfish, one of its four rhopalia taking a turn as its nervous system.
• One opening does both jobs. Food goes in and waste comes out of the same hole.
• The oldest known free-swimming jellyfish is 505 million years old. Burgessomedusa phasmiformis predates the first dinosaurs by roughly a quarter of a billion years.
• A jellyfish protein rebuilt modern biology. Green fluorescent protein was first seen in Aequorea victoria in 1962 and won the 2008 Nobel Prize in Chemistry; it is now standard equipment in labs worldwide.
• They sleep without a brain. The upside-down jellyfish Cassiopea drops from about 58 bell pulses a minute in daytime to about 39 at night — the most evolutionarily ancient animals known to sleep.
• The moon jellyfish is among the most efficient swimmers alive. A trick called passive energy recapture cuts its cost of transport by 48%.
• Box jellyfish have 24 eyes in four clusters, and use them to steer around mangrove roots.
• The "immortal jellyfish" rewinds itself. A stressed or ageing Turritopsis dohrnii medusa settles, forms a cyst, and within 24 to 72 hours is a juvenile polyp again. This is a stress response rather than everyday behaviour, and it does not make individuals unkillable.
• Probably the heaviest jellyfish — though few medusae have ever been weighed — is Nomura's jellyfish, about 2 m across the bell and around 200 kg, roughly the mass of a large male lion.
• The Australian box jellyfish carries up to 60 tentacles, each up to 3 m long, in four clumps at the corners of a bell up to 30 cm across.
• The Irukandji is the size of a fingernail. Its bell is 1–2 cm across, with tentacles up to a metre long.
• People have fished and eaten jellyfish in China for roughly 1,700 years.
• The golden jellyfish of Palau's Jellyfish Lake commute daily, tracking the sun so the algae living in their tissues can photosynthesise.

"Someone should pee on it." Popularised by a 1997 episode of Friends and repeated on beaches ever since. Urine has never been shown to work, and fresh water and ice may make envenomation worse: stinging capsules left on the skin fire in response to changes in the surrounding salt concentration, which is exactly what rinsing with anything unsalted does. Household vinegar, lidocaine and non-scalding hot water are what the clinical advice actually names.

"The Portuguese man o' war is a big jellyfish." It is not a jellyfish, and it is not even one animal. It is a siphonophore — a floating colony of genetically identical zooids, each specialised for floating, hunting, feeding or reproducing. Its trailing tentacles average around 10 metres.

"A jellyfish on the sand is harmless, and so is a loose bit of tentacle." Neither is safe. Stinging capsules are self-contained pressurised cells that fire on contact. They do not need the animal to be alive, or even attached to it.

"Jellyfish are taking over the oceans." The first global analysis of the data found no such trend. Numbers swing on a roughly 20-year worldwide cycle, and the 1990s happened to be a rising phase — which is precisely when the takeover story took hold. Some regions genuinely have seen increases; the planet-wide version does not survive the data.

SpongeBob SquarePants made jellyfishing a national pastime for a generation of children: the episode "Jellyfishing" aired on 31 July 1999, three months after the show's Nickelodeon premiere on 1 May that year. Jellyfish anchor the aquarium experience in most major cities — Monterey Bay Aquarium's 1992 Planet of the Jellies exhibit is often credited with starting the trend of displaying them as living art. In Finding Nemo (2003) a jellyfish bloom nearly kills Dory. And the immortal jellyfish, Turritopsis dohrnii, has become a fixture of internet science writing, usually with its actual biology mangled.

Aequorea victoria is the most consequential jellyfish that has ever lived. Osamu Shimomura isolated green fluorescent protein from it in 1962; the protein is now used to make the inner workings of cells visible, and Shimomura shared the 2008 Nobel Prize in Chemistry for it.

Turritopsis dohrnii, the immortal jellyfish, earns its reputation honestly if not literally: a stressed or ageing medusa can settle, form a cyst, and become a juvenile polyp again within 24 to 72 hours.

Burgessomedusa phasmiformis, described from the Burgess Shale, is the oldest free-swimming jellyfish known — 505 million years old, and swimming a quarter of a billion years before the first dinosaur.