Ask anyone who walks along the coast in Scotland or elsewhere in Britain, and they will tell you the same thing. Some years, the shallow water is almost empty. Other years, thousands of pale jellyfish appear at once, drifting through the water as if they came out of nowhere.
Moon jellyfish do this in many parts of the world, and scientists say there is a reason for these sudden changes.
What people see near the shore is only one part of the jellyfish’s life. The jellyfish people see floating in the water, with its round, clear body, usually lives for several months. It reproduces and then dies. But before it becomes a jellyfish, it lives as a tiny animal called a polyp. The polyp attaches itself to a hard surface underwater, such as a rock, a pier, or the bottom of a boat.
While the jellyfish swims freely through the water, the polyp stays in one place. It can live there for months or even years. When conditions are right, it starts to form small sections, which grow into young jellyfish. This process is called strobilation.
The number of young jellyfish that appear depends partly on what happened months earlier. Water temperature is especially important. The conditions the polyp experiences during winter can affect how many young jellyfish it produces later.
This helps explain why some summers have very few jellyfish while others bring thousands. A beach with no jellyfish in August does not mean they have disappeared for good. Tiny polyps may still be living underwater. They may simply not have had the right conditions to produce young jellyfish that year.
These changes also happen at different times in different places. A beach can remain quiet for a long time, then suddenly fill with jellyfish. To people walking along the shore, it may seem as if the animals appeared overnight.
People have also changed the places where jellyfish live. Harbours, marinas, piers, and other coastal structures give tiny polyps more surfaces to attach to. In some places, these structures may help jellyfish populations grow.
Scientists are also studying how warmer seas and changes in water quality may affect jellyfish numbers. These factors do not explain every bloom, and the reasons can be different from one place to another. Near Edinburgh, in Scotland, the Firth of Forth is one area where jellyfish numbers can change greatly from year to year.
The moon jellyfish has no brain and little control over where it goes. Yet its life cycle makes its numbers surprisingly hard to predict. The jellyfish people see in summer may be the result of changes that began months earlier, hidden underwater where almost nobody notices them.