Jellyfish are gelatinous marine animals characterized by an umbrella-shaped bell and trailing tentacles. While they are certainly influenced by the ocean’s flow, the common misconception that jellyfish are purely passive drifters is inaccurate. Jellyfish actively swim, employing a unique and highly energy-efficient method of propulsion that allows them to move purposefully through the water.
The Mechanics of Propulsion
Jellyfish locomotion relies on a rhythmic cycle of muscle contraction and passive elastic recoil of the bell-shaped body. The animal contracts its coronal muscles, forcefully ejecting water from the concave underside of the bell, which generates a forward thrust, a process known as jet propulsion. This initial contraction creates a fluid structure called a starting vortex ring in the wake behind the animal, which pushes the jellyfish forward.
Following the initial muscle contraction, the bell returns to its original shape through the elasticity of the mesoglea, its thick, jelly-like tissue. As the bell relaxes and expands, it pulls water back underneath, creating a second, less obvious fluid structure called a stopping vortex. Research has shown that the jellyfish harnesses the pressure from this stopping vortex to gain a secondary boost in velocity.
This mechanism is referred to as “passive energy recapture” because the animal gains forward momentum during the relaxation phase without expending any additional muscle energy. By exploiting the hydrodynamics of both the contraction and the subsequent passive expansion, the jellyfish is one of the most energetically efficient propulsors in the animal kingdom. This ability to travel 30% further per swimming cycle significantly reduces their metabolic energy demand, allowing them to thrive despite having a muscle mass that is often less than one percent of their total body weight.
The Distinction Between Swimming and Drifting
The question of whether a jellyfish is swimming or drifting is a behavioral choice influenced by their environment. Swimming implies a directed movement, often used to hunt or escape a threat, while drifting means allowing themselves to be carried by the momentum of the surrounding water. Jellyfish possess sensory structures called rhopalia, which house light-sensitive ocelli and pacemaker neurons that control their bell-pulsation rate and orientation.
These sensory organs allow them to detect changes in light, water-borne vibrations, and orientation, providing the necessary input for directed movement. For instance, studies have shown that some species actively swim against a current to prevent stranding on the shore. By orienting their bell pulsations toward the current, they can counteract drift, a behavior that is crucial for maintaining large coastal aggregations or “blooms.”
The decision to switch from active swimming to energy-saving drifting is influenced by external factors like the strength of the current, the location of prey, and water temperature. When currents are too powerful, active swimming is futile, and they conserve energy by drifting. Conversely, when currents are weak or when a food source is detected, they engage in directed swimming to maximize their chances of survival and feeding.
Navigating the Water Column
One of the most striking examples of purposeful jellyfish swimming is their participation in Diel Vertical Migration (DVM). This is considered the largest synchronized animal migration on the planet by biomass. This daily cycle involves moving hundreds of meters between the deep ocean and the surface waters. For many species, this migration is a functional necessity, demonstrating the large-scale application of their propulsion ability.
Jellyfish spend their days in the deeper, darker waters, often in the mesopelagic zone, which can extend down to 1,000 meters. This downward movement provides safety by helping them avoid visual predators that hunt in the sunlit surface layer. It also protects them from damaging ultraviolet radiation. At dusk, they begin their ascent, swimming toward the food-rich surface waters under the cover of darkness.
They spend the night near the surface feeding on zooplankton and other small organisms before descending again before dawn. This massive, coordinated movement requires sustained energy expenditure, confirming that their bell contractions are not merely random pulsations. The ability to execute this demanding vertical journey highlights the effectiveness of their low-energy, jet-propulsion system.