Jellyfish primarily eat plankton, which are organisms that lack the strength and size to swim and therefore drift in marine and freshwater currents. Larger species of jellyfish also eat crustaceans, fish and other jellyfish. Jellyfish are carnivorous and primarily feed on animals. Some jellyfish have tissue containing symbiotic algae, which enables them to receive energy through photosynthesis.
Jellyfish have simple bodies with a mouth in the center and trailing tentacles. They have no bones, brain or heart. Their tentacles are covered by cells that shoot out harpoons that puncture their prey and release a neurotoxin. These dart-like harpoons and stinging venom cause paralysis, which enables the jellyfish to capture their prey and also functions as a defense mechanism.
Despite their name, jellyfish are considered plankton rather than fish. Some jellyfish grow to 7 feet long with 100-foot tentacles, whereas small jellyfish can measure only an inch long. Jellyfish are ancient creatures that have lived in the oceans around the world for at least 650 million years. Most jellyfish are transparent and have pulsating, bell-shaped gelatinous bodies and trailing tentacles. There are thousands of different jellyfish species, most of which are ocean dwelling. While jellyfish can kill small sea creatures, their sting typically causes only temporary pain, fever and muscle cramps in humans.