How do jellyfish move?

Jellyfish, as gelatinous marine creatures, possess a unique locomotion mechanism known as jet propulsion. They rely on the coordinated contraction and relaxation of their bell-shaped bodies to propel themselves through the water. Here's a step-by-step explanation of how jellyfish move:

1. Bell Structure: Jellyfish have a bell-shaped body structure, with the opening of the bell facing downwards. The bell is made of a jelly-like substance called mesoglea, sandwiched between two layers of cells.

2. Muscle Contraction: The inner surface of the bell contains circular and radial muscles. When the circular muscles contract, they cause the bell to shrink in diameter. This contraction reduces the volume of water inside the bell, creating a high-pressure zone.

3. Water Expulsion: The contraction of the circular muscles also closes the opening of the bell, preventing water from escaping. As a result, the high-pressure water inside the bell is forced out through the bell's margin, which is the rim of the bell.

4. Jet Propulsion: The expulsion of water from the bell creates a powerful jet of water. This jet propulsion pushes the jellyfish forward, propelling it through the water. The direction of movement is determined by the orientation of the bell's opening.

5. Relaxation and Recoil: After the contraction, the circular muscles relax, allowing the bell to expand to its original shape. This relaxation creates a low-pressure zone inside the bell, which draws water back in through the bell's opening.

6. Buoyancy: Jellyfish have a low density, thanks to their gelatinous composition. This low density provides them with buoyancy, enabling them to float in the water.

7. Tentacles and Bell Margin: Many jellyfish species also have tentacles around the bell's margin. These tentacles help in capturing prey and defending against predators. The bell margin itself can be modified into complex structures, enhancing the efficiency of jet propulsion.

By repeating the cycle of contraction and relaxation, jellyfish can generate a series of pulsed jets that propel them through the water. While this locomotion method may seem simple, it is remarkably effective and allows jellyfish to navigate their marine environment with surprising agility.