Why are most jellyfish bodies almost entirely water?

Jellyfish bodies are composed of almost 99 percent water primarily due to the composition of the mesoglea, a thick, gelatinous, and largely acellular layer that separates their two cellular layers (the epidermis and gastrodermis).123

This high water content and gelatinous structure are essential to the cnidarian body plan for several reasons:

• Low Metabolic Demand: Because the mesoglea is mostly acellular, it has very low metabolic requirements, which allows the animal to maintain a large body size despite lacking a complex circulatory system.13
• Structural Support: The jelly provides the necessary bulk and form for the bell-shaped body without requiring the energy-intensive tissues found in more complex organisms.12
• Diffusion and Circulation: Their simple, water-based architecture facilitates the direct diffusion of oxygen from the surrounding environment into their cells. While some species use bell pulsing to assist in the movement of nutrients and gases, their overall low metabolic rate is well-suited to a body composed primarily of water.13