Do jellyfish have a heart?

Jellyfish do not possess a heart. Nor do they have blood, a circulatory system, or lungs. Rather, they absorb oxygen from water directly through their epidermis (outer skin layer) in a process known as diffusion.

Jellyfish are marine invertebrates with radial symmetry, in which their body parts are arranged uniformly around a central axis, and a simple two-layered gelatinous tissue structure. The simplicity of this organization, owing to several key features, eliminates the need for a heart or other complex organs and body systems.

Of particular importance concerning the circulation of oxygen—a function performed by the heart in vertebrates—are the layers of the jellyfish body, which consist of an outer epidermis and an inner gastrodermis, separated by a thick, non-living gel layer known as the mesoglea. The epidermis is essential for the rapid absorption of oxygen from the water. Oxygen taken up through epidermal cells is immediately used for metabolism and the generation of energy. Small quantities of oxygen are stored in the mesoglea, allowing for survival in water with low oxygen levels.

Jellyfish, owing to their size or other factors, also appear to require more than simple diffusion to circulate oxygen through their bodies. Studies have shown, for example, that some species rely on bell pulsing. This phenomenon is characterized by the gentle, rhythmic beat of the jellyfish’s bell (umbrella-shaped body), which is thought to assist in the spread of nutrients and gases throughout tissues.