A jellyfish's meal begins with a poisonous sting

"Why do jellyfish sting, and how do they eat?" asked a young patron from the Grayslake Area Public Library District.

Jellyfish are really simple creatures. They don't have brains, stomachs, or eyes.

Because they are so simple, body openings multi-task and become the point of entry for food intake and the exit hatch for reproductive output and waste.

There are two main jellyfish shapes - one a bell-shape with tentacles hanging down, the other a polyp with tentacles sprouting up.

Brookfield Zoo's Living Coast exhibit includes fringed jellies, the polyp-upside-down kind, and soon will add bell-shaped moon jellies.

Andy Snider, curator of herps and aquatics at Brookfield Zoo, explains that all jellies have tentacles with stinging cells called nematocysts.

"Some can cause a rash or sting, depending on how susceptible you are to the venom," Snider said.

The tentacles can give rise to a venomous ouch, shooting tiny harpoons of poison into anything trapped in the stringy tentacle net. The animal's automatic response drags prey trapped in the tentacles to its mouth.

The type of food jellies eat depends on the type of jelly.

"Some eat crustaceans and fish. The majority eat very small things like plankton," Snider said.

Some have a symbiotic relationship with algae that create a home in between a jelly's two skin layers. The zoo's fringed jellies have a green, pink or purple cast because algae thrive inside of them. The tiny scavengers create energy for the jelly through photosynthesis.

The zoo is welcoming new jellies at its Living Coast exhibit - the luminescent, bell-shaped moon jellies, which can grow to the size of dinner plates.

Snider said jellies live in oceans around the world. Some that are native to China's Yangtze River adapted to the freshwater Great Lakes, making them an invasive species.