A new study led by the ICM reveals that these episodes can lead to changes at the molecular level that affect the fish’s immune response, its reaction to external stimuli and its metabolism.
A new study by the Institut de Ciències del Mar (ICM) of Barcelona carried out in collaboration with the Yellow Sea Fisheries Research Institute (China) reveals that marine heatwaves, increasingly intense, frequent and long-lasting due to global warming, cause permanent changes in fish gene expression that affect their immune response, their reaction to external stimuli and their metabolism.
These changes in fish gene expression due to increased water temperature are caused by alterations in DNA methylation levels and can persist over time.
To carry out the study, published in the specialized journal Molecular Ecology, the researchers simulated a heatwave in the laboratory by increasing the water temperature by 3.6°C in a series of tanks containing several newborn European basses (Dicentrarchus labrax).
These conditions were maintained only during the first two months of the fish's life, then they were normalized. As a result, the researchers found a number of changes at the molecular level in European basses which persisted three years later despite the fact that their external appearance was normal.
These changes were detected in tissues with different embryonic origins, including the brain, which comes from the ectoderm; muscle and testicles, which come from the mesoderm; and the liver, which is formed from the endoderm. As explains Francesc Piferrer, ICM researcher and head of the project, "this indicates that the affectation took place in a generalized way during the early development of the fish, which implies that the changes may be permanent".
"Everything depends on when the heatwave occurs during a fish's life; if it occurs during the early stages of development it can leave permanent marks as we observe in European bass and could be transmitted to the offspring," details in this regard the first author of the study, Dafni Anastasiadi, who is currently working at The New Zealand Institute for Plant and Food Research.
These marks, which are induced by an environmental change and are common in tissues of different embryonic origin, are epigenetic because they are superimposed on the DNA sequence, i.e. they are found above their genetic information at specific locations in the genome and, if they are common in all tissues, they are called metastable epialleles.
This is the first time that metastable epialleles have been described in fish. According to researchers, they could be used to monitor the impacts of global warming on marine life, as they allow the reconstruction of the life history of individuals. Knowing these metastable epialleles, it is enough to take a DNA sample from a fish to know whether or not it was exposed to a heatwave or abnormal environmental conditions during its early stages of life.
All of this highlights the importance of knowing the sublethal effects of marine heatwaves on fish, something that was unknown until now. "Since epigenetic mechanisms are present in all animals, this could serve to monitor not only a species such as the European bass, but also other species, whether fish or not, that respond to environmental disturbances," concludes Piferrer, who assures that the identification of this type of epigenetic markings common in groups of species could be a key tool for evaluating the effects of global warming on a large scale.