Jellyfish are pretty simple creatures. They lack a heart and a brain. Their body is primarily water (estimated at 95%) and a jelly-like, elastic substance called mesoglea.  The bell of the jellyfish is mostly mesoglea. Some jellyfish have a small flap of skin along the margins of the bell called the velum which they can undulate and swim – but they are not strong swimmers. If the tide is going out, swim as they may, they are heading out also. On the bottom of the bell is a single opening that is the mouth as well as the anus – pretty simple creatures.

The common Moon Jellyfish has interesting markings within the bell that are horseshoe-shaped reproductive organs.

Jellyfish are carnivorous animals, but most do not seek their prey. It finds them. Hanging from their bell are the tools for finding a meal, the tentacles. Some can extend for several feet, others are hardly noticeable. Along the tentacles there are capsules that contain cells known as nematocyst, which contain a coiled dart with a drop of venom. There is a trigger associated with this cell. The jellyfish does not fire it – instead, when contact is made the nematocyst reacts. The drop of venom is injected from hundreds of other nematocysts along the tentacle. This venom paralyzes the prey, other tentacles coil around it firing more nematocysts, and the tentacles retract towards the mouth. The same happens when people make contact with them.  The sting ranges from unpleasant to painful depending on the species – but we are not consumed.

Contact with the Portuguese Man-o-War should be avoided even when it’s washed up on shore. This hydrozoan is not one creature – like a “true” jellyfish – but a large “condo” of many specialized cells used for feeding or reproduction. The blue colored air bag is exposed above the surface and acts like a sail to move the creature around in search of food.  Hanging from the bag are long tentacles which are made up of individuals whose stomachs are all connected. When one group of cells kills prey – they consume it and the food is moved through the Man-o-War to feed the whole colony. To feed a colony, you need a big fish and a strong toxin to kill it.