The sight of gelatinous, often colorful, organisms scattered across a sandy beach is a common phenomenon. These invertebrates are composed of over 95% water, giving them their characteristic translucent appearance and soft structure. While they are a natural part of the marine ecosystem, their sudden appearance in large numbers, known as mass strandings, can be visually shocking. Understanding why these creatures wash up requires examining both the physical forces of the ocean and their biological rhythms.
External Forces Driving Mass Strandings
The primary reason jellyfish are unable to avoid the shore is their status as a type of plankton. They are largely passive swimmers with limited control over horizontal movement. Their bell pulsations are primarily used to maintain vertical position in the water column, not to make substantial progress against strong currents. This lack of navigational strength leaves their fate entirely dependent on the ocean’s physical dynamics.
Sustained, strong onshore winds are powerful abiotic factors driving jellyfish toward the beach. These winds create surface friction that pushes the water, and organisms floating within it, directly toward the coast. When combined with specific coastal currents, such as longshore currents, the jellyfish are effectively corralled into specific beach areas.
High tides further compound stranding events by carrying the organisms far up the beach face, depositing them beyond the reach of receding waves. Once the tide retreats, the jellyfish are left stranded, unable to re-enter the water due to gravity and physical limitations. Their large size and high water content mean they rapidly lose structural integrity and desiccate once exposed to air and sunlight.
Natural Population Booms and Life Cycles
Physical forces explain how jellyfish reach the shore, but biological factors explain why so many are available to be stranded. This mass availability is often the result of a “jellyfish bloom,” a rapid increase in population size. Blooms are frequently driven by favorable environmental conditions, such as warming ocean waters, which accelerate the life cycles and reproductive success of many species.
Elevated temperatures can decrease the time it takes for jellyfish polyps—the stationary stage of the life cycle—to undergo strobilation, the process of budding off young, free-swimming medusae. Nutrient runoff (eutrophication) also contributes to blooms by fueling plankton growth, providing an abundant food source. Furthermore, the overfishing of predatory fish and sea turtles may reduce natural pressures, allowing jellyfish numbers to swell unchecked.
Another significant factor is the natural conclusion of the jellyfish life cycle, which for many species is quite short, often lasting only about a year. Many jellyfish reproduce en masse in a spawning event and then die shortly thereafter. The resulting large number of deceased individuals floating near the surface are then easily swept toward the shore by winds and currents, contributing to the number of organisms observed during a stranding event.
Identifying Common Stranded Species
Identifying the species found on the beach provides context for local ecological conditions and potential safety concerns.
Moon Jelly
The Moon Jelly (Aurelia aurita) is one of the most common species found stranded globally. It is easily recognized by its translucent bell and four horseshoe-shaped gonads visible through the top. This species has short, delicate tentacles and possesses a very mild sting that is often imperceptible to humans.
Cannonball Jellyfish
Another frequent sight is the Cannonball Jellyfish (Stomolophus meleagris), also known as the cabbagehead jellyfish. It is named for its firm, dome-shaped bell that lacks long, trailing tentacles. These jellies are known for their mild venom used to stun small prey, but their sting is not considered harmful to humans.
Portuguese Man o’ War
Beachgoers may also encounter the Portuguese Man o’ War (Physalia physalis). Despite its appearance, it is not a true jellyfish but a siphonophore—a colony of specialized organisms working as one unit. The Man o’ War is identifiable by its distinctive gas-filled, purplish-blue float or “sail” and its extremely long, blue-tinged tentacles that can reach lengths of 30 feet or more. Unlike true jellyfish, the Man o’ War is entirely wind-driven, making it highly prone to mass strandings. Its presence requires caution due to its potent venom.
Handling Dead Jellyfish Safely
A common misconception is that a jellyfish is harmless once it has washed ashore and died, but the stinging apparatus can remain active. The stinging cells, called nematocysts, are microscopic capsules loaded with venom and a coiled, barbed thread. These cells can be triggered by touch, even in a dead organism, sometimes for hours or days after the creature has been separated from the water.
It is advisable to never handle a dead jellyfish with bare hands and to avoid walking barefoot through a mass stranding event. If a sting occurs, the immediate goal is to prevent any remaining nematocysts from discharging venom into the skin. Rinsing the area with fresh water should be avoided, as the change in osmotic pressure can cause the stinging cells to fire.
For many species, applying vinegar to the area for at least 30 seconds can help neutralize undischarged nematocysts. After neutralizing the cells, the venom can be addressed by applying heat. Immerse the affected area in hot water (not scalding) for 20 to 45 minutes. The heat helps to break down the venom proteins and reduce pain.