Last weekend and the one before that I tried to swim at the beach. However, upon seeing a few jellyfish I ended up deciding to spend time reading in the sand. The worst of the afternoon was not that. I found more annoying a few teenagers playing soccer a few meters from my towel. Jellyfish, after all, are in their habitat. But it is true that I had never seen in my entire life jellyfish in the month of May. I did some research and discovered that in recent years their arrival in the Mediterranean at this point in spring has become more and more frequent. They are even starting to appear in other waters in which they are not normally so abundant.
Logically, the first thought that came to mind was that is related to global warming. The temperature of the Mediterranean has risen at a dizzying rate in recent years. However, I had the feeling that there must be something more. After all, the water has been warming for many years, but this boom in jellyfish populations (known as bloom, by the way) seems more recent to me. To answer my questions, I have contacted Jose Carlos Báez, Chief Program Researcher at the Spanish Institute of Oceanography, CSIC. As I feared, warming water plays a role, but there are even more factors that affect this uncontrolled proliferation that is becoming more and more noticeable.
Three phases to give rise to adult jellyfish
Although there are small variations between some species and others, in general the reproductive cycle of jellyfish consists of three phases. On the one hand there are the larvae, which float in the water until they find a place to cling to on the seabed. When they achieve this, they move on to the polyp phase, which can last up to a year. When conditions are favorable, the polyp fragments, releasing the ephyras, which are small immature jellyfish that, over time, become the adult jellyfish.
The transition from polyp to jellyfish It is known as strobilation and depends on factors such as the temperature of the water, the oxygen dissolved in it or the availability of food. Jellyfish are only released into the water if they are going to be able to live in it. The surface temperature of the water is a determining factor. In fact, it has been observed that with an increase of 1.7°C The rate of asexual reproduction in the polyps of some species is accelerated by 20%. Therefore, strobilation normally occurs at the beginning of summer. It may vary between species. In some it occurs at the end of spring, but it is more common for it to take place from June onwards.
According to José Carlos Báez, this is causing “a dilation of the reproductive period“, so we are seeing more generations of jellyfish in a single season. They arrive earlier and leave later.
Not everything is going to be global warming
The massive proliferations that we are seeing with increasing frequency on beaches are known as blooms. As we have seen, global warming is causing us to start seeing jellyfish earlier and stop seeing them later, but it does not seem to be the cause of the blooms.
“It is difficult to affirm with complete certainty that the total biomass of jellyfish in the Mediterranean has increased due to climate change, mainly because we do not have sufficiently long and homogeneous historical series that allow us to compare the current situation with that of past decades,” says Báez. “However, there is evidence that jellyfish blooms, as well as the arrival of large swarms in coastal areas, appear to be increasingly frequent and prolonged.”
The problem of overfishing
“In a healthy ecosystem, teleost fish eat especially zooplankton, in which ephyras are found,” explains Báez. Among those fish that ephyras eat, sardines stand out, for example. On the other hand, adult jellyfish are typically preyed upon by turtles, but also by large fish such as tunas, to which tuna belongs. All of this, taken together, helps keep jellyfish populations more or less stable.
Because of overfishingthere are fewer and fewer predators for jellyfish. There are, for example, fewer sardines being eaten in their ephyra phase and fewer tuna eating adult jellyfish. If we add to all this that more generations of jellyfish are born in a season due to warming water, we have the perfect cocktail for the appearance of blooms.
The whiting that bites its tail (pun intended)
In 2022, José Carlos Báez’s team published a study in which another less known relationship was described between the populations of jellyfish and sardines or anchovies. We have already seen that fish feed on the zooplankton in which ephyras are found, so they can help regulate jellyfish populations.
However, what happens next is not so well known. Adult jellyfish can also feed on the eggs of sardines and anchovies. Therefore, if there are too many jellyfish, they can deplete the sardine population, so there will be fewer of these adult fish to continue feeding on the ephyras. As a result, there are even more jellyfish and we start again. The balance between one predator and another is broken and clearly leans towards the proliferation of jellyfish.
Furthermore, in that study a relationship was also found between the proliferation of jellyfish and the decrease in weight of adult sardines. And, in turn, adult jellyfish also feed on zooplankton, which is why they compete with sardines and anchovies for food. If there are many, they do not allow them to feed properly.
Not everything is jellyfish in the gelatinization of water
With the proliferation of jellyfish, something known as water gelatinization is occurring. Logically, these animals, with their gelatinous appearance, have a great influence. But they are not the only ones who favor that aspect. Other gelatinous animals also proliferate, such as ctenophores. In addition, the water looks cloudier due to excess algae. This is because great eutrophication is occurring in the Mediterranean. That is, the accumulation of excess nutrients such as phosphorus and nitrogen, which often reach the sea through the runoff from farms in which they are used as fertilizers.
This increase in nutrients causes, in turn, a increased algae bloomsthat they cloud the water and make it difficult for sunlight to reach the seabed. If seafloor algae cannot carry out photosynthesis due to lack of light, there will not be enough oxygen dissolved in the water either.
Also, back to the jellyfish, there are studies which show that when they die and are decomposed by microorganisms on the seabed, this decomposition process can alter the diffusion of oxygen into the sediments. If there is a reasonable amount of jellyfish, this is expected, but when there are too many, the balance that normally exists is broken and other organisms can be affected.
In short, we have cloudy water with remains of algae, jellyfish and other gelatinous organisms. There may even be some loose stinging cells in the water, so that when we bathe we feel the itch without there being any jellyfish. To make matters worse, there is such an imbalance in the ecosystem that the organisms that are responsible for fixing the sand on the seabed are also affected. The beaches have more stones than sand and are no longer as comfortable. Also, if you want eat a skewer at the beach barpossibly less meaty than usual.
The solution is not what you think
The last thing we should do is make the mistake of blaming jellyfish for all these problems. They are in their habitat. They are not an invasive species which must be sacrificed or sterilized to prevent it from reproducing. In this case, in Báez’s opinion, the solution lies above all in control fishing.
Without overfishing, the natural mechanism that helps keep jellyfish in balance would be restored. It is true that we would see them on our beaches for longer due to warming water, but they would not be found in such high quantities. The most serious problem, today, is not that there are jellyfish on our beaches for longer. They are found in disproportionate quantities. But that’s not his fault. As almost always, it’s our fault.
Images | andre oortgijs |Wan et al.
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