Creature Feature: Jellyfish

The pink circles are the reproductive organs of this moon jelly

Moon jelly pulsing - see the fringe of tentacles around the bell?

Jellyfish. What can you say – they’re beautiful balls of mucous. Living on a boat and snorkeling, as we do, you see a lot of jellyfish. As cnidarians, these critters are related to sea anemones and corals, but instead of settling down as a sessile polyp, they float free in the ocean. Some species, like the moon jelly pictured here, do swim in a rudimentary way by flexing their bell. Other species, such as the by-the-wind sailor Velella velella (don’t you love it when scientific names are so pretty!) or Portugese man o’ war, float on the surface and are pushed about by the wind. These latter two species aren’t true jellyfish (single organisms with a row of tentacles around the bell), but actually colonies of many hydroids specialized for feeding, reproduction, etc. Upside-down jellyfish are cool; they lie upside-down on the bottom, their tentacles facing up. All jellies use tentacles with nematocysts to capture prey. Nematocysts are like little harpoons inside the tentacular tissue; when something brushes against the tentacle, the nematocysts shoot out into it. With many species, you don’t even notice this, but with others, such as the Portugese man o’ war, you get a horrendous sting from the poison injected. And the sting of a few species, such as the box jellyfish, can be fatal. Jellyfish exhibits are popular in aquariums, usually in dark tanks with colorful lights illuminating the beautiful undulations of the creatures.

Upside-down jellyfish laying on the sand

Banners for the "Jellies" exhibit at the Shedd Aquarium last summer