Tomorrow morning, The Grandma has to work in Castelldefels, so they will set off on the return journey early. They will stop at Port Ginesta, and while The Grandma attends her training session, Claire will enjoy the beaches of the neighbouring town -this time, in the Baix Llobregat area, before returning to Barcelona.
During the trip, they have enjoyed the stunning spectacle of the Barcelona coastline as seen from the sea and they have been accompanied by some very special travel companions: dozens of jellyfish. Taking advantage of the high temperatures and sea currents, these creatures have been appearing in large numbers recently along the beaches of el Barcelonès, el Baix Llobregat, and el Garraf areas.
Every summer, thousands of people visiting the Mediterranean coast are surprised -or alarmed- by the appearance of jellyfish drifting close to the shore. Beaches may be temporarily closed, warning flags raised, and swimmers advised to stay out of the water. Yet jellyfish are not newcomers to the Mediterranean. They have inhabited these waters for millions of years and play an important role in marine ecosystems.
So why do they suddenly appear in such large numbers along our beaches? Are there really more jellyfish than in the past, or are we simply noticing them more? The answer lies in a fascinating combination of ocean currents, weather conditions, marine ecology, climate change, and human activity.
Jellyfish are among the oldest animals on Earth. Their ancestors appeared more than 500 million years ago, long before dinosaurs walked the planet. Despite their simple appearance, jellyfish are highly successful marine organisms that have survived multiple mass extinctions.
The Mediterranean Sea hosts dozens of jellyfish species, although only a handful are commonly encountered by beachgoers. Along the Catalan coast and much of the western Mediterranean, the most familiar species include the mauve stinger (Pelagia noctiluca), the fried-egg jellyfish (Cotylorhiza tuberculata), the barrel jellyfish (Rhizostoma pulmo), and the compass jellyfish (Chrysaora hysoscella). Each species has different habits, preferred temperatures, and ecological roles. Some possess powerful stings, while others are almost harmless to humans.
To understand why jellyfish suddenly appear in large numbers, it is important to understand their life cycle. Most jellyfish alternate between two completely different forms. One stage is a tiny polyp attached to rocks, harbour walls, shells, or other hard surfaces. The other is the familiar free-swimming jellyfish, known as the medusa.
When environmental conditions become favourable -particularly water temperature and food availability- the polyps reproduce, releasing many young jellyfish into the sea. Within weeks or months, these tiny organisms grow into adult jellyfish capable of reproducing themselves. This means that what people observe on beaches is only the final stage of a much longer biological process that has often begun months earlier.
Jellyfish are surprisingly poor swimmers. Although they can pulsate their umbrella-shaped bodies to move vertically or make small adjustments, they cannot swim against strong currents. In practice, they are largely transported by the sea itself.
The Mediterranean is a dynamic environment where surface currents constantly move water masses across large distances. These currents can carry jellyfish dozens or even hundreds of kilometres from where they originated.
Along the Catalan coast, prevailing winds often determine whether jellyfish remain offshore or are pushed towards beaches. When persistent onshore winds blow for several days, surface waters carrying jellyfish gradually move landward. As a result, beaches that were completely free of jellyfish one day may experience large numbers only a few days later. Conversely, offshore winds may quickly remove them from coastal waters.
Weather conditions also play an important role. Strong storms may break up offshore jellyfish aggregations and redistribute them across coastal waters. Changes in atmospheric pressure, wave action, and wind direction can all influence where jellyfish eventually appear. Interestingly, many large beach strandings occur after periods of calm weather followed by changing winds. This explains why jellyfish appearances can seem unpredictable even though they often follow well-understood oceanographic processes.
Climate change is one of the factors receiving the greatest scientific attention. The Mediterranean is warming faster than many other seas around the world. Sea surface temperatures have increased significantly during recent decades, and marine heatwaves are becoming increasingly frequent.
Warmer waters can benefit certain jellyfish species in several ways:
-Longer reproductive seasons.
-Faster growth rates.
-Higher survival of juvenile stages.
-Expansion into areas that were previously too cold.
However, scientists emphasize that climate change alone does not explain every jellyfish bloom. It is one contributing factor among several interacting processes.
Jellyfish feed mainly on zooplankton, fish eggs, fish larvae, and tiny crustaceans. When plankton populations increase, jellyfish have abundant food available, allowing them to grow rapidly and reproduce successfully. Nutrient-rich waters -sometimes enriched by river runoff or human activities- can stimulate plankton growth, indirectly benefiting jellyfish populations. This ecological chain illustrates how changes at the microscopic level may eventually become visible to beach visitors.
Another important factor may be overfishing. Many commercial fish species consume plankton or compete directly with jellyfish for food. Others prey on juvenile jellyfish. When fish populations decline due to intensive fishing, competition decreases and jellyfish may gain an ecological advantage. Although the relationship is complex and varies between regions, many marine biologists consider overfishing one of the possible drivers behind increasing jellyfish blooms in some Mediterranean areas.
Modern coastlines contain many artificial structures such as marinas, breakwaters, piers, and harbour walls. These hard surfaces provide ideal places for jellyfish polyps to attach and reproduce. As coastal development expands, the number of suitable habitats for polyps also increases. This hidden stage of the life cycle often goes unnoticed because polyps measure only a few millimetres and resemble tiny sea anemones.
No two summers are exactly alike. A year with abundant jellyfish usually results from several favourable conditions occurring simultaneously:
-Successful reproduction during spring.
-Warm sea temperatures.
-Plenty of plankton.
-Persistent winds carrying jellyfish towards the coast.
-Ocean currents concentrating them near beaches.
If one or more of these factors are absent, jellyfish numbers may remain relatively low. This is why one summer may experience widespread blooms while the following year has very few jellyfish despite similar temperatures.
This remains one of marine biology's most debated questions: Are jellyfish increasing?
Some long-term studies suggest that jellyfish blooms have become more frequent in certain regions of the Mediterranean. Other researchers argue that natural population cycles have always existed and that improved monitoring, satellite observations, and increased public reporting make blooms appear more common than they once seemed. Most scientists agree on one point: local increases are real, but there is no universal trend affecting every part of the world's oceans equally.
Despite their reputation, jellyfish are valuable members of marine ecosystems. They serve as food for loggerhead sea turtles, ocean sunfish, some seabirds, and numerous fish species. Young fish often shelter among their tentacles for protection from predators. When jellyfish die, their bodies sink to the seabed, transporting organic matter that supports deep-sea communities. Rather than being merely a nuisance for swimmers, jellyfish contribute to the health and complexity of marine food webs.
Complete elimination of jellyfish from Mediterranean beaches is neither possible nor desirable. Instead, scientists focus on monitoring environmental conditions and predicting blooms using satellite observations, oceanographic models, weather forecasts, and citizen science initiatives.
In Catalonia, organizations monitor jellyfish throughout the bathing season, helping local authorities provide timely information to the public. By understanding how currents, winds, climate, ecosystems, and human activities interact, we gain a clearer picture of why jellyfish appear on our shores.
Far from being mysterious invaders, jellyfish are natural inhabitants of the Mediterranean, carried by the sea and responding to environmental changes. Their seasonal arrival reminds us that even seemingly simple marine creatures are part of an intricate ecological system whose balance increasingly depends on how humans care for the oceans.
More information: Orca
begins to look like a heart beating.
It doesn't matter what kind...
like a ghost heart
-a heart you can see right through,
right into some other world.
Ali Benjamin
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