Box jellyfish, the Cubozoa, were moved out of the true jellyfish in 1973 because they are active hunters rather than drifters. A shelf of tissue called the velarium narrows the bell opening like a thumb over a hose, driving them at up to two meters per second with 180 degree turns in a couple of pulses. They rise into schools of fish and prawns, flip, sink with tentacles spread, and pass prey to the mouth with the pedalia. They range from the thumbnail sized Irukandji to animals of two kilograms with three meter tentacles. Their 24 eyes sit in four clusters called rhopalia and include true eyes with lenses, retinas, and corneas, processed locally with no central brain. A heavy statolith keeps the upper eyes pointing up through Snell’s window, so mangrove species can steer by the tree canopy. Pantyhose protects swimmers because the stinging cells fire on chemical cues from human skin.

They grow fast, some by three millimeters a day, and live about three months in the wild. Some species mate through a wedding dance with internal fertilization, and a July 2023 study of Chiropsalmus quadrumanus found eggs at several stages in one female, suggesting repeated breeding. Researchers now map them with environmental DNA. Each tentacle carries around 500,000 cnidocytes loaded with more than 170 toxic proteins that punch holes in red blood cells, flooding the blood with potassium and stopping the heart within minutes. Australia has recorded about 80 deaths in a century, while the Philippines loses an estimated 20 to 40 people a year. Fresh water and urine trigger unfired cells, and vinegar remains the first aid standard after 2016 and 2017 studies overturned a 2014 lab result.

• An adult can carry up to 15 tentacles on each of the four corners of its bell.
• Only box jellyfish among cnidarians are known to perform a courtship wedding dance.
• eDNA sampling filters seawater through cellulose nitrate membranes and matches the DNA with PCR.
• A CRISPR screen found the venom depends on the calcium pump ATP2B1, and cholesterol drugs blocked it in mice within 15 minutes.
• The box jellyfish genome is arranged on eight linear chromosomes.