Box jellies are found off the west coast of South Africa and are often encountered in swarms by scuba divers. Box jellies are known to have very well-developed eyes and other sensory organs that help them distinguish between shapes, obstacles and movements. Due to their box shape, they can propel themselves in any direction, actively hunting through schools of fish and shrimp. These jellies have strong tentacles armed with thousands of stinging cells called “nematocysts”. Nematocysts are used to stun and kill prey, which is pulled into the mouth by the jelly’s tentacles. Box jellies are related to the deadly sea wasps found off the coast of Australia. Although not as venomous as sea wasps, the sting of a box jelly is very painful. Reports of cardiac arrest have been noted but no one along the South African coast has died from the sting of this jelly.
Box jellies can sting with their tentacles.
They often occur in swarms together.
A swarm of jellies is called a "smack".