Jellyfish, with their translucent bells and trailing tentacles, are ocean inhabitants. Often perceived as passive drifters, these ancient marine creatures exhibit a more complex and purposeful form of movement than their appearance might suggest. While they may not be the fastest swimmers, their unique locomotion mechanisms allow them to navigate their environment effectively. This article explores how jellyfish move, their swimming speeds, and influencing factors.
The Mechanics of Jellyfish Movement
Jellyfish propel themselves through water using jet propulsion. This involves rhythmic contraction and relaxation of their umbrella-shaped bell. When the bell contracts, it expels water from underneath, generating a force that pushes them in the opposite direction. This muscle-powered swimming is highly efficient.
As the bell contracts, it creates a doughnut-shaped vortex, similar to a smoke ring, in the water behind it. As the bell then relaxes and re-expands, a second vortex forms, spinning in the opposite direction. This interaction between the two opposing vortices creates a “virtual wall” or “ground effect,” providing an additional boost of thrust without requiring extra energy. This passive energy recapture allows jellyfish to move further with less metabolic effort, making them efficient swimmers.
Actual Swimming Speeds and Their Purpose
While energy-efficient, jellyfish are generally not fast swimmers compared to many other marine animals. Most species move at slow speeds, often measured in centimeters per second. For instance, a study observed jellyfish swimming at approximately 10 centimeters per second. Moon jellyfish (Aurelia aurita) can be particularly slow, with lab measurements indicating speeds as low as 0.7 cm/s.
The exception is the box jellyfish, an active hunter. Unlike most jellyfish that passively encounter prey, box jellyfish can achieve burst speeds of up to 1.5 to 2 meters per second (7.4 km/h or 4.6 mph) by forcefully ejecting water from their bell. This speed allows them to pursue prey and actively avoid threats. For most jellyfish, movement serves purposes like maintaining position, slowly pursuing planktonic prey, or making small adjustments to avoid immediate dangers. Much of their larger-scale travel relies on ocean currents, which carry them over significant distances.
Factors Influencing Speed and Overall Mobility
Several variables influence jellyfish swimming speeds and overall mobility. Species plays a role, as different body shapes and bell sizes affect propulsion efficiency. For example, a box jellyfish’s stiff bell allows for more rapid swimming compared to other species’ more flexible bells. Smaller jellyfish also tend to swim faster relative to their bell diameter than larger individuals.
Water conditions, such as currents, significantly influence a jellyfish’s displacement, as they often drift with the flow. Despite this, some jellyfish can detect and swim against currents, indicating directional control that challenges the notion of them being mere drifters. Temperature can also affect their metabolic rate and pulsation frequency, impacting swimming activity. Ultimately, jellyfish are efficient movers, often pulsing just enough to stay afloat or make minor adjustments, conserving energy for growth and reproduction.