The term "box jellyfish" actually refers to a sub-group of jellyfish comprising about 20 species from all over the world. They go by this name because of their cube-shaped bells, which are distinct from the round bells of other jellyfish. Most box jellyfish have very potent venom. They can easily kill and digest fish and shrimp half their size. They range in size from the Irukandji jellyfish at 2 cm to the Australian Sea Wasp with tentacles several feet in length. Even though they lack a central nervous system, box jellyfish can swim around obstacles using very simple light-sensing organs. They are also very powerful swimmers. Whereas most jellyfish drift along at the mercy of the ocean currents, the powerful swimming contractions of a box jellyfish propels it at over 4 miles per hour.
Swimmers are rightfully fearful of box jellyfish since their sting is so dangerous. In popular tourist destinations like Australia, some beaches are closed for weeks at a time when box jellyfish are present. Other beaches are enclosed by protective nets that keep swimmers safe from the jellyfish.
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