Jellyfish belong to the phylum Cnidaria and are the free-swimming medusa stage. Their bodies are simple, composed primarily of a gelatinous substance called mesoglea, which gives them their distinctive umbrella or bell shape. Although commonly perceived as passive drifters, jellyfish are capable of self-propulsion. While ocean currents often limit their movement, their unique anatomy allows them to swim actively, though their speed is relative to their body plan.
Defining Jellyfish Speed
For most species, the actual speed of movement is remarkably slow. The typical swimming speed for common jellyfish, such as the Moon Jelly (Aurelia aurita), is measured in just a few centimeters per second. In laboratory settings, measurements have been recorded as low as 0.007 meters per second.
A more active species, like the barrel jellyfish (Rhizostoma octopus), generally averages around 0.05 meters per second. This translates to a fraction of a mile per hour, confirming that sustained, long-distance travel is not the primary function of their propulsion. Instead, movement is primarily used for positioning in the water column, avoiding obstacles, or maneuvering toward feeding areas. Horizontal travel is largely dictated by the flow of the surrounding water.
The Mechanics of Movement
The propulsion of most jellyfish, known as medusan swimmers, is achieved through a rhythmic muscular process called bell pulsation. This action involves the rapid contraction of the bell margin, which forces a jet of water out from beneath the umbrella-shaped body. This jet propulsion creates an initial forward thrust.
During the contraction phase, the expelled water forms a ring-shaped vortex, known as a starting vortex, in the wake behind the animal. The subsequent phase, where the bell relaxes and returns to its original shape, is where a unique hydrodynamic phenomenon occurs. The elasticity of the bell’s mesoglea causes a passive, mechanical recoil that requires no further muscular energy.
As the bell re-expands, it captures the ring of water it just pushed away, creating a second vortex, or stopping vortex, that rolls under the bell margin. This interaction generates a secondary, entirely passive thrust that accelerates the jellyfish further. This passive energy recapture mechanism is estimated to account for up to 32% of the total distance traveled during a single pulse cycle.
Energy Efficiency vs. Speed
The slow, pulsed swimming style reflects an evolutionary trade-off prioritizing efficiency over speed. Jellyfish have a very low metabolic rate, and their body structure is composed of over 95% water, meaning they possess very little muscle mass. This low muscle mass limits the total force they can produce, restricting them to low velocities.
This simplicity allows them to achieve the lowest cost of transport of any metazoan. They use approximately half the energy of any other marine creature to cover the same distance. The passive energy recapture from the elastic bell rebound significantly reduces the metabolic energy demand on their swimming muscles.
This highly efficient, low-energy movement supports a lifestyle of passive feeding. Most jellyfish are opportunistic predators who encounter food as they slowly pulse or drift. The minimal energy expenditure allows them to dedicate more consumed resources toward growth and reproduction, explaining their ecological success in many ocean environments.
Variations in Speed Among Species
While most jellyfish are slow-moving, drifting organisms, there are notable speed variations across different species and classes. Typical large jellyfish (Scyphozoa) exhibit the slow, low-velocity movement described by the bell pulsation model. For instance, the cannonball jellyfish (Stomolophus meleagris) has been recorded swimming at maximum speeds of about 15 centimeters per second.
The fastest swimmers belong to the class Cubozoa, commonly known as box jellyfish. These species are more active hunters than their scyphozoan relatives, necessitating higher speed and control. Box jellyfish possess a squarish bell shape and a structure called a velarium, which constricts the bell’s aperture to create a more powerful, directed jet of water. This enhanced propulsion allows them to achieve speeds up to 1.5 to 2 meters per second, making them capable of purposeful, directed movement and rapid turning.