Box jellyfish have real eyes, with lenses, and actively hunt fish
Most jellyfish drift. Box jellyfish swim at up to two metres a second, turn 180 degrees in a few pulses, dodge obstacles and hunt small fish. They manage it without a brain in the usual sense, guided by clusters of eyes that come complete with retinas, corneas and lenses, a feature no other jellyfish has.
The name comes from the cube-shaped bell. From each of its four lower corners hangs a stalk carrying long hollow tentacles, around fifteen per corner in some species and up to three metres long. Each tentacle is armed with roughly 500,000 stinging cells that fire microscopic harpoons. A few species, such as Chironex fleckeri and the thumbnail-sized Irukandji, can kill people, while others merely hurt.
Speed comes from a shelf of tissue folded inward around the rim of the bell. It narrows the opening, so each pulse squeezes out a strong jet. A ring nerve coordinates the pulsing, and sensory clubs called rhopalia each hold two image-forming lens eyes. In some species the upper eye gazes straight up through the surface, its field of view matching the circle of sky visible from underwater. The arrangement of nerve cells suggests some visual processing happens right inside these clubs, and researchers describe the animals as behaving rather like fish.
Many feed with a curious manoeuvre: dart upward, flip upside down, stop pulsing and sink slowly until prey blunders into the tentacles, which then fold to deliver it to the mouth. Sea turtles, including hawksbills, seem immune and happily eat them.
Tiny daily growth rings in their balance organs let researchers age them. Chironex fleckeri grows about 3 millimetres a day, and the oldest individual examined was 88 days old. Wild box jellyfish live about three months, though they survive seven or eight in lab tanks. At least 51 species were known by 2018, and a new genus from the Gulf of Thailand, named after Lisa-ann Gershwin, was described in 2023. Because they are translucent and hard to spot, scientists now detect them by filtering seawater for shed DNA.
Source: Box jellyfish
