Jellyfish : A clever hunter for a creature with no brain

 Although they look harmless, jellyfish are remarkably efficient predators, able to stun and kill their prey with stinging cells (nematocysts). Each cell contains a tiny harpoon which, when triggered by touch or movement, shoots into the prey and delivers a debilitating toxin.
The potency of this toxin varies among jellyfish, causing reactions in humans ranging from mild rashes to death. Some, like the sea nettle, are simply an annoyance to those who are stung, while others, like the box jellyfish (see A God of Love—Creator of a Killer Jellyfish?) are very dangerous and sometimes cause death.

Of course, jellyfish don’t set out to hurt humans—their preferred food ranges from microscopic animals and fish larvae to other jellyfish. Usually people are stung when jellyfish are carried close to shore, where swimmers inadvertently brush against them.

Floating along in the water, jellyfish can be both predator and prey. Because most jellyfish are almost transparent, they are able to camouflage themselves quite well against their watery background, which is important because, while they can use jet propulsion to move about in the water, they are also at the mercy of the current, and in the open sea, there is nowhere to hide.