Jellyfish take in oxygen dissolved in water by diffusion: oxygen passes directly across their thin body tissues, while carbon dioxide diffuses out. They have no lungs, gills, blood, or heart to move respiratory gases around.
How jellyfish get oxygen
Jellyfish do not gulp air or draw water through a set of gills. Instead, dissolved oxygen moves from the surrounding water across cells in the jellyfish’s body. Carbon dioxide, a waste gas, moves in the opposite direction and diffuses into the water.
Diffusion is the movement of molecules from an area where they are more concentrated to an area where they are less concentrated. It lets oxygen and carbon dioxide cross cell surfaces without a lung or a gas-carrying blood supply.
Where gas exchange happens
A jellyfish has two main tissue layers that meet water and can exchange gases:
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- Epidermis: The outer cell layer is in contact with the surrounding water, so oxygen can diffuse across it directly.
- Gastrodermis: The inner cell layer lines the gastrovascular cavity. Water in that cavity provides another surface for gas exchange.
The gastrovascular cavity is a central space involved in digestion, not a lung or blood vessel. OpenStax describes cnidarian gas exchange as diffusion between epidermal cells and surrounding water, and between gastrodermal cells and water in the gastrovascular cavity (OpenStax Biology 2e, “Phylum Cnidaria”).
Why jellyfish do not need lungs or gills
Jellyfish have a simple, thin body plan. Because oxygen can reach cells across the body’s surfaces, they do not need a specialized respiratory organ or a circulatory system to carry gases between that organ and the rest of the body. The National Ocean Service notes that a jellyfish’s body is about 95 percent water, but that figure describes its composition—not how efficiently it exchanges oxygen (NOAA National Ocean Service, “What are jellyfish made of?”).
Their gelatinous middle layer, called the mesoglea, is not a lung or a gas-carrying fluid. In some cnidarians with thicker mesoglea, canals help distribute gases and nutrients; diffusion alone places limits on how thick a body wall can be without internal distribution (OpenStax Biology 2e, “Phylum Cnidaria”).
Does pulsing help jellyfish breathe?
Jellyfish pulsing helps move water around and through parts of the body, supporting fluid exchange. It is not breathing in the lung-based sense: the pulse does not ventilate a specialized respiratory organ. Gas exchange still occurs as oxygen and carbon dioxide diffuse across tissue surfaces (OpenStax Biology 2e, “Overview of the Circulatory System”).
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What counts as a jellyfish?
“Jellyfish” is a common name, not a precise label for every drifting, gelatinous marine animal. True jellyfish are cnidarians, while comb jellies belong to a separate group called ctenophores. The Portuguese man o’ war is a colony of specialized animals in the hydrozoan group, not a typical individual jellyfish. These distinctions matter when comparing anatomy across animals called “jellies” (Natural History Museum, London, “Jellyfish: The smart stinging creatures drifting through our oceans”; Smithsonian Ocean, “Jellyfish and Comb Jellies”).
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