Jellyfish are translucent, bell-shaped invertebrates belonging to the phylum Cnidaria. These ancient marine organisms have a simple anatomy, composed of three tissue layers and a body mass that is approximately 95% water, held together by the gelatinous substance mesoglea. Seeing these animals washed ashore, often in large numbers, is a common sight for beachgoers. This widespread phenomenon results from their unique biological makeup combined with the powerful forces of the ocean.
The Biological Reason: Passive Swimmers
Jellyfish are susceptible to stranding because they have limited control over their movement in the water column. While they use rhythmic pulsations of their bell for vertical movement or slow locomotion, they are inherently weak swimmers. This simple muscular propulsion is effective for navigating calm waters and feeding, but it cannot overcome significant oceanic forces. Lacking a bony skeleton or powerful fins, jellyfish cannot generate the sustained thrust needed to overcome strong currents or waves. They are categorized as plankton, organisms that drift with the flow of water rather than actively swimming against it. Consequently, when water movement is directed toward the shore, jellyfish are unable to steer away from the coastline.
External Forces: Wind, Tides, and Currents
Powerful external forces dominate the coastal environment and deliver jellyfish onto the sand. Surface ocean currents, driven by global wind patterns, transport marine life across vast distances. When these large currents veer close to a coast, they carry weak-swimming organisms directly into the nearshore environment. Once near land, localized weather conditions, particularly strong onshore winds, become the most immediate cause of stranding. A persistent wind blowing toward the beach piles up surface water, and the animals floating within it, into the surf zone. The combined effect of tidal currents and wind-driven waves ultimately pushes the organisms beyond the water’s edge and onto the beach.
Mass Strandings and Population Blooms
The sight of hundreds or thousands of jellyfish covering a beach often corresponds to a “jellyfish bloom.” A bloom is a substantial, temporary increase in a species’ population, driven by biological and environmental factors. These population increases can occur naturally as part of the organism’s life cycle. Jellyfish have a complex life cycle that includes a bottom-dwelling polyp stage. This stage can bud asexually to rapidly produce numerous juvenile jellyfish, or medusae. Favorable conditions, such as warmer ocean temperatures and excess nutrients from coastal runoff, can trigger the polyps to release a massive pulse of young jellyfish. When these enormous, concentrated groups encounter a persistent onshore current or wind, the resulting mass stranding event can be extensive.
Safety When Encountering Washed-Up Jellyfish
A jellyfish lying on the sand should be treated with caution, as its stinging cells can remain active and potent long after the animal appears lifeless. These specialized cells, called nematocysts or cnidocytes, are microscopic capsules containing a coiled, venom-filled barb. The mechanism for venom injection is triggered by pressure and chemical change, not a live nervous system. This means the cells can still fire even when the jellyfish is dried out.
If a sting occurs, the immediate goal is to prevent the remaining unfired nematocysts from discharging. The affected area should be rinsed thoroughly with saltwater, not fresh water, as the difference in osmotic pressure can cause the stinging cells to fire. Any visible tentacles should be gently removed using an object like a stick or tweezers. Pain can often be relieved by immersing the area in hot water, around 113°F (45°C), for 20 minutes. Medical attention should be sought immediately if the victim experiences severe pain, difficulty breathing, or swelling.