Jellyfish are common inhabitants of the world’s oceans, typically drifting through marine environments. However, they are frequently found washed ashore on beaches. This raises questions about why these creatures end up out of water and what happens to them once stranded.
Oceanic Forces
Jellyfish are passive drifters, largely at the mercy of the ocean’s physical movements. Their primary propulsion methods, like contracting their bell-shaped bodies or using cilia, offer weak resistance against strong water flow. Consequently, oceanic forces such as strong tides, powerful ocean currents, and prevailing winds play a primary role in pushing them toward coastlines. Incoming flood tides can carry jellyfish closer to shore, and coastal currents can sweep them into shallow areas or coves, making escape difficult.
Specific types, like the “by-the-wind sailor” (Velella velella), possess a rigid, sail-like structure that protrudes above the water, enabling them to be propelled by wind. This adaptation aids their widespread distribution but also makes them susceptible to onshore winds that can blow entire colonies onto beaches. Storms and rough weather also contribute, as powerful waves and turbulent waters disorient jellyfish, pushing them onto the shoreline.
Biological and Environmental Factors
Beyond direct oceanic forces, biological and environmental factors contribute to jellyfish strandings. Jellyfish have relatively short lifespans, and as they reach the end of their natural life cycle, they become more susceptible to washing ashore. This can lead to large numbers of individuals weakening and being carried by currents to the coast.
Mass aggregations, known as “blooms,” also increase the likelihood of strandings. These population explosions can occur naturally, but changes in water conditions often influence their development and eventual collapse. For example, warmer sea temperatures can promote jellyfish blooms, and marine heatwaves have been linked to large stranding events. Conversely, jellyfish, being cold-blooded, can lose mobility if water temperatures fall below their optimal range.
Changes in water chemistry, such as shifts in salinity, can also contribute to the collapse of these blooms, leading to die-offs and subsequent strandings. The decomposition of large jellyfish blooms can lead to localized oxygen depletion in the water, creating conditions unfavorable for marine life and causing more jellyfish to wash ashore. Human activities, including pollution and overfishing of jellyfish predators, can contribute to increased bloom frequency and size, thereby increasing stranding events.
The Fate of Stranded Jellyfish
Once jellyfish are washed ashore, their survival time is limited. Composed of approximately 95% water, they rapidly dehydrate and die due to exposure to air and sun. Lacking gills or lungs, they cannot extract oxygen from the air once out of seawater, leading to their demise within hours.
Even when dead, stranded jellyfish can still deliver a sting. Their stinging cells, called nematocysts, are venom-filled capsules that can discharge independently of the organism’s life signs. These cells can remain active for days or even weeks if the tentacles remain damp. Therefore, it is important to avoid touching any jellyfish found on the beach.
Following their death, stranded jellyfish decompose, returning nutrients to the coastal ecosystem. Bacteria and other microorganisms play a role in breaking down their organic matter, recycling carbon and other nutrients back into the food web. While this decomposition is a natural process, large stranding events can lead to localized impacts, such as temporary shifts in oxygen levels and nutrient concentrations in the immediate vicinity of the decaying biomass.