Pathology of tissue loss in three key gorgonian species in the Mediterranean Sea

The Mediterranean is known for its marine biodiversity, especially gorgonian forests. Unfortunately, these are experiencing rapid declines due to climate change, manifested by repeated marine heat waves resulting in mass mortality events since the early 1990 s. To better understand why gorgonians are declining, more systematic approaches to investigate the exact causes are needed, and pathology may aid in this goal. We described gross and microscopic pathology of tissue loss in three key gorgonian species in the Mediterranean region, Paramuricea clavata, Eunicella cavolini, and Leptogorgia sarmentosa, that were all experiencing various degrees of acute to subacute tissue loss characterized by exposed axial skeleton sometimes partly colonized by epibionts and thinning of adjacent tissues. The most significant variety of lesions was seen in P. clavata followed by L. sarmentosa and E. cavolini. For all species, dissociation of gastrodermal cells was the dominant microscopic lesion followed by necrosis of the gastrodermis. Ciliates invading gastrodermis and associated with necrosis of polyps were seen only in E. cavolini. Epidermal tissue loss was seen only in L. sarmentosa, while P. clavata was distinguished by a prominent inflammatory response and unidentified dark round structures within the tentacle epidermis and gastrodermis with no host response. Further work to understand the cause of death in gorgonians is needed, particularly to elucidate the role of ciliates and environmental co-factors or infectious agents not visible on light microscopy, as well as applications of additional tools such as cytology.