ASK A SCIENTIST
Question: How do jellyfish eat?
Jellyfish can be six feet across, with tentacles longer than a whale, but their construction is simple. They are like a folded balloon made from two sheets of cells with jelly-like stuff between. The layer to the outside is the ectodermis (outer skin); the layer to the inside, the gastrodermis (digestive skin), breaks down and absorbs food.
The typical jellyfish is shaped like a bell, so think of a balloon pushed in so that the nozzle-the mouth-is at the center of the inner surface, where a bell's clapper would be attached. Food passes through the mouth to the inside of the balloon, where it is broken down. Waste goes out the same opening.
But how does a jellyfish get food into its mouth? The rim of the bell is drawn out to form tentacles and there may be flaps, called oral arms, around the mouth. The trailing tentacles come in contact with food, mostly small fish and crustaceans (shrimp-like animals). You probably have seen the gentle, pulsing movement of jellyfish in an aquarium or zoo. This movement brings more animals in contact with the tentacles. The tentacles are loaded with cells called cnidocytes (said with a silent 'c', long 'i'). (ny – doe' – cytes) All animals that have such cells are called cnidarians, (ny – dare' – e – ans) and these also include corals and sea anemones. Each cnidocyte contains a tightly coiled structure that can shoot out like a microscopic harpoon. Even harmless jellyfish use harpoons, but they are too small to hurt you. Many jellyfish also produce toxic chemicals that enter the skin already damaged by the spears and can paralyze the prey. These chemicals are the main cause of the burning sensation that we call the jellyfish's sting. As with a bee sting, an allergic reaction can be more dangerous than the chemical itself. After the snared food item is under control, it is moved to the mouth by the tentacles or oral arms and taken in for digestion.
A remarkable thing about some jellyfish is that, like their cousins the corals, they get an important part of their energy from single-celled algae originally taken into their gastrodermis cells as food but then not digested. Only certain species belonging to an algal group called the dinoflagellates are spared and can live and divide within the animal's cells. The greenish-brown color of most corals and some jellyfish is due entirely to the pigments of their algal partners. The benefit for the animal is that some of the energy-rich chemicals made by algal photosynthesis leak out, feeding the host cell. A striking example of this partnership is the upside-down jellyfish, which is more a farmer than a hunter. With bell down, it lays on the bottom of shallow lagoons where the algae packed into cells of the oral arms can get lots of light. Imagine how your family could save on food bills if you could make sugars and fats just by laying in the sun!
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail email@example.com. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).