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Global climate change and, in particular, the warming of the oceans has caused the frequency and severity of marine heatwaves to increase every year, with serious consequences for the stability and resilience of coral populations. Researchers at the Biodiversity Research Institute (IRBio) of the University of Barcelona have analysed the impact of the sudden increase in temperatures on the early life stages of two key species in the Mediterranean: the red coral (Corallium rubrum) and the white gorgonian (Eunicella singularis).

The results of the study show that heat stress drastically reduced the survival of red coral larvae and also the dispersal ability of gorgonian larvae, which could have implications for the viability of adult populations in the Mediterranean, where they play an important role in supporting marine biodiversity.

“Although previous experimental studies found that adult colonies of the species studied are mostly resistant to heat stress, our results at early stages suggest that the persistence and connectivity of local populations may be severely compromised by an increase in the frequency and intensity of heat waves like the ones we have experienced this year,” explains Cristina Linares, professor at the Department of Evolutionary Biology, Ecology and Environmental Sciences of the Faculty of Biology of the UB and researcher at IRBio, who led the study together with Núria Viladrich, Marie Curie researcher at the UB and the University of Washington (Seattle, USA).

“Moreover — she adds — , since many corals play a structural role in increasing the diversity of marine ecosystems, changes in their reproductive processes could also lead to a radical loss of biodiversity, affecting hundreds of associated species, which may ultimately also threaten direct economic resources, such as fisheries or recreational activities such as diving.”

The paper, published in the journal Global Change Biology, is also authored by Jacqueline L. Padilla-Gamiño, researcher at the University of Washington.

Marine ecosystem engineers

The study focused on two species of octocorals which are endemic to the Mediterranean, and which play a key role in their environment, for both are considered “engineering species” of marine ecosystems. “They play an important structural and functional role because they form complex three-dimensional structures that generate spatial heterogeneity and provide a suitable habitat for hundreds of associated species, many of them with a high economic value, such as lobsters and many other commercial fish larvae that take refuge from predation around the three-dimensional structure of corals, gorgonians and also sponges,” explains Núria Viladrich, also member of the Marine Biodiversity Conservation Research Group (MEDRECOVER).

In fact, the communities formed by these species are known as “marine animal forests, since, like trees in terrestrial forests, they strongly increase the biodiversity of the ecosystem,” the researchers stress.

Experimental study with different temperatures

This complex marine ecosystem is increasingly influenced by climate change, as the Mediterranean Sea is one of the fastest warming regions in the world with an increase of 0.41°C per decade, a warming rate three to six times higher than that of the global oceans. In this context, the ability to predict the vulnerability and resilience of corals at different life stages during extreme events is, according to the researchers, “essential to understand the effect of global climate change on species distribution, estimate adaptation potential and design effective management strategies.”

To study the effects of this increase in temperature, the researchers collected ten white and ten red coral colonies in the Cap de Creus natural park (Girona) before the expected larval release season. They then monitored larval survival in an experimental chamber at different temperatures: 24°C, the temperature observed during the Mediterranean heatwaves of recent years; 26°C, the temperature expected to be reached in future heatwaves; and 20°C, the control temperature. In addition to larval survival, factors such as settlement and post-settlement survival rate, larval biomass and larval energy consumption were assessed.

At the end of the study, the researchers found that while temperature increases did not cause significant negative effects on white gorgonian larvae, the survival of red coral larvae was drastically reduced. “Specifically, thermal stress reduces the survival of red coral larvae, which, coupled with the species’ marked recruitment pulses, could compromise its ability to recover and cope with climate change,” the researchers say.

However, the white gorgonian showed a higher tolerance to heat, but also showed other potentially negative effects. “The larvae were highly resistant to increased temperature, with similar survival and settlement rates. But their larvae settled more quickly and thus closer to the native population, limiting their dispersal and genetic connectivity between populations.”

Red coral: a species at serious risk due to climate change

These results suggest that the white gorgonian could be a “winning species” under future climatic conditions in the Mediterranean Sea, as they reinforce previous studies showing the high thermal resilience of adult colonies of this gorgonian. However, the researchers warn that if heatwaves continue and severe conservation and management plans are not implemented, the persistence of red coral populations is at greater risk. “Their viability is further aggravated by the fact that red coral is a highly threatened species due to its use in the jewellry industry and, as a result, it has been and still is overexploited in several Mediterranean countries,” the researchers stress.

Moreover, the results will also help to better understand the behaviour and future of benthic communities in the Mediterranean Sea and, therefore, can contribute to developing management and conservation policies that best suit these communities. “Our study provides empirical data that can be used to project the population dynamics and demography of both octocoral species under future model-based global climate change scenarios. These simulations can be used to design measures to preserve these endemic species and their associated biodiversity,” says the team.

New explanations for the biological response to heat

The study also highlights that the biological causes of the effects of heat on larvae may be much more complex than previously thought. Previous studies indicated that larval size, the presence of larval symbionts (as in the case of white gorgonians) and brood quality (i.e. biomass and energy consumption) would explain the degree to which larvae are affected by heat stress. But, as the researchers state, “the results show that the biological responses could be much more complex because, unlike white gorgonians, red coral larvae do not have symbiont algae and therefore the increased mortality rates caused by increased temperature were not related to endogenous energy depletion.” This opens the door to future studies at the cellular and molecular level to better understand the possible causes of heat waves on larval survival.

Moreover, the results also showed that larval survival in heat stress was dependent on the day of release in the two species studied. “This highlights the importance of considering the day of larval release to better project the success and viability of future coral populations,” they conclude.

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