This blog post is provided by Marina J. Vergotti, Juan Pablo D’Olivo, and Diego K. Kersting, and tells the #StoryBehindThePaper for the paper “Reconstruction of long-term sublethal effects of warming on a temperate coral in a climate change hotspot”, which was recently published in the Journal of Animal Ecology. In their study, Vergotti and colleagues analysed the skeletons of the Mediterranean coral C. caespitosa and found that local oceanographic context is highly variable and plays a significant role in predicting population resilience to warming and stress events.

The Mediterranean Sea is heating up, and fast. Warming three times faster than other Seas worldwide, it has become what is commonly known as a climate change hot-spot. As temperatures rise, marine heatwaves now occur almost every summer, straining marine ecosystems, in particular foundational species – critical for the construction of habitats and for maintaining biodiversity and ecological balance. One such species, Cladocora caespitosa, is a coral endemic to the Mediterranean Sea, unique as the only shallow water coral in the Mediterranean with the ability to build reefs, making it a key habitat forming species. Nowadays, C. caespitosa populations are commonly found as dispersed small colonies. Nevertheless, there still remain some locations – such as the Columbretes Islands, in the north-western Mediterranean – where they form extensive reef-like formations. In view of environmental change, such sites provide an invaluable potential for understanding these benthic ecosystems.

Unfortunately, C. caespitosa has been repetitively impacted by marine heatwaves over the last three decades, resulting in widespread mass morality events.  As a result, it was included in the IUCN red list of endangered species in 2015, and is now protected in a number of Mediterranean countries. Nevertheless, it is still relatively understudied compared to its tropical counterparts and much remains to be understood regarding temporal changes and the sublethal responses of this coral to climate change.

Analysing the skeletons of C. caespitosa is a way to learn about the past. These corals respond to seasonal fluctuations by depositing annual growth bands, in a fashion akin to trees. Tools such as X‑rays can reveal the growth patterns, allowing us to reconstruct the coral’s life-history and understand how they responded to environmental change, including past thermal stress events. Such information is vital, as it is only by understanding the past that we will be able to foresee the future responses of those organisms to environmental changes, and effectively protect them.

In this study, we analysed the skeletons of C. caespitosa from three north-western Mediterranean sites – Cap de Creus, Montgrí, and the Columbretes Islands – to understand the long-term effects of warming on coral growth. We found that while there were no indications of stress at the regional level, the story was quite different at the local level. In the Columbretes Islands, evidence pointed towards a decline in calcification rates, possibly the result of repetitive thermal stress events. Several skeletons also had skeletal stress marks corresponding to years in which marine heatwaves occurred. Nevertheless, the response of growth to temperature was not uniform across locations, indicating that the local oceanographic context and higher nutrient concentration play a major role in modulating the resilience of this corals to stress events.

Read the paper

Read the full paper here: https://doi.org/10.1111/1365-2656.14225