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Jellyfish in winter?
Is it normal to see jellyfich blooms in winter? Is climate change involved?
04/02/2023
Jellyfish usually attract our attention in summer, coinciding with the bathing season, when they cause greater social alarm. However, in recent weeks a large number of jellyfish have been detected on the coasts. Is this normal? Is it influenced by climate change? To answer these questions we need to know more about the life cycle of jellyfish.
Jellyfish are animals that are part of the plankton: the group of aquatic organisms that, although they have some capacity for movement, drift along with the currents. Depending on the habitat, jellyfish can be differentiated into coastal and oceanic species. The coastal species have a seasonal life cycle, with two main forms: the jellyfish, which lives free in the water; and the polyp, which lives attached to the substrate. Polyps form jellyfish normally in late winter or early spring, depending on water temperature and food availability. For this reason, on the western Mediterranean coast it is common to find more jellyfish from spring to early autumn.
In contrast, oceanic species live in open water, rather than on the shoreline. Their life cycle does not include the form of a polyp fixed to the substrate. Thus, these species present the jellyfish form throughout the year. Precisely, the jellyfish blooms detected in recent weeks in various parts of the Mediterranean coast (as in Almería, target= ”_blank”>Murcia, target=”_blank”> Girona and Menorca) correspond to the oceanic species Pelagia noctiluca, one of the most common in the Mediterranean. Individuals of this species can live for a few years and reproduce throughout the year, with a reproductive peak in spring and another in autumn. Therefore, it is common to find this type of jellyfish in the open sea at any time of the year.
The reason why oceanic jellyfish come to shore is related to weather issues. When the spring is rainy, the rivers discharge fresh water on the coast. The difference in salinity with respect to oceanic waters acts as a "barrier", making it difficult for oceanic jellyfish to be pushed to the coast. However, when spring is dry and rainfall is scarce, this difference between coastal and oceanic waters is not generated, making it easier for currents and wind to drag jellyfish from the open sea to the coast.
In summary, species of oceanic jellyfish, such as Pelagia noctiluca, are found throughout the year in the open sea, and in greater numbers when a reproductive peak occurs, and, depending on local climatic conditions, they can reach the coast in large numbers, leaving us phenomena such as those that have been reported in recent days.
On the other hand, climate change favors jellyfish. The increase in sea temperature accelerates metabolism, allows them to eat more and also alters reproductive cycles. For this reason, jellyfish are being observed at unusual times, both in open ocean and coastal species.
Studying the presence of jellyfish is important throughout the year, which is why we encourage the community to contribute their observations to the project Jellyfish Alert, also in winter!
Photo by Pepín Valdes.