Cladocora caespitosa: a temperate coral growing in a climate change hotspot

The Mediterranean Sea is often referred to as a “climate change hotspot”. For people living around the Mediterranean basin, this is becoming increasingly obvious as storms become more violent and frequent, droughts become the new norm and crushing summer heat waves hit new records every year. But in the midst of these events, it might be easy to forget that the Mediterranean marine environment is just as vulnerable as its terrestrial counterpart. Indeed, with temperatures increasing three times faster than the rest of the globe, the Mediterranean Sea is prone to a topicalisation process that facilitates the settlement of potentially invasive tropical species and is responsible for extensive mortality events in benthic communities. As a result, irreversible shifts in species distribution are occurring, changing the composition of communities and their dynamics.

Cladocora caespitosa (commonly known as cushion coral) is a temperate coral endemic to the Mediterranean, and a good example of a species impacted by such climatic events. As the only coral in the region with the potential to build reefs, it is a key bioengineering species for Mediterranean benthic ecosystems, providing habitat and shelter to many species of algae, polychaete, crustaceans, sponges, and more. While it is more common during a dive to find a handful of football-sized colonies of this coral spread on the bottom, it is still possible to find places around the Mediterranean with extensive reef-like formations.

Colonies of Cladocora caespitosa growing in the Montgrí Natural Parc (Catalonia, Spain). The picture on the left shows a colony with partial mortality, while picture on the right shows a colony that completely died very recently. Pictures by Marina J. Vergotti

Its limited capacity to spread during reproduction, its slow life dynamic and the recurring mass mortality events affecting this species during the last three decades make it especially vulnerable to environmental stressors, including those linked to climate change. As a result, it has been a part of the IUCN red list of endangered species since 2015 and is protected in a number of Mediterranean countries. This fact has put Cladocora caespitosa at the centre of several research projects focusing on its distribution and long-term responses to increasingly frequent marine heat waves. However, it is still relatively understudied compared to its tropical relatives, and much still needs to be understood regarding the sub-lethal responses of this coral to climate change.

One of the ways corals reflect changes in their environment is by altering their growth patterns, which are recorded in their skeletons. Indeed, just as trees respond to seasonal cycles by depositing a pair of growth rings every year, so do corals by depositing in their skeleton a couple of growth bands of high and low density. These bands can be revealed using methods such as computer-tomography (CT) scanning or X-rays, and used to “go back in time” by reconstructing the coral’s life-history, which can, in turn, help understand changes in their environment. By reconstructing their past growth, it is also possible to extract information about past stress events, such as marine heat waves, that can result in anomalous growth patterns visible in the coral growth bands.

In MaCoBioS, we strive to study how Cladocora caespitosa responds to seawater temperature changes (such as marine heat waves) and seasonal variations of pH. For that, we study coral skeletons from several sites in the northwestern Mediterranean using X-rays to measure their growth bands and analyse how these environmental changes are affecting coral skeletal growth. The coral skeletons are studied further in the lab to analyse their chemical composition and the changes according to the seasons. The goal is to understand better the stress responses of this coral to marine heat waves, by checking for tolerance limits and for resilience patterns. We also want to get a better understanding of how the corals respond to seasonal changes in seawater properties, and how these responses affect this coral’s capacity to withstand stress events. This information is crucial for the development of effective management strategies to protect these species.

Written by Marina J. Vergotti, Freie Universität Berlin