The role of sclerites in the defense against pathogens of the sea fan Gorgonia ventalina (Octocorallia)

• Effectiveness of sclerites from diseases and healthy fans to delay fungal penetration into fan mesoglea
• Effectiveness of sclerites from diseases and healthy fans extracted with acetone to delay fungal penetration into fan mesoglea
• Healthy and diseased sclerites similarly effective at delaying fungal penetration
• Extracted sclerites less effective at delaying fungal penetration

Sessile organisms such as octocorals cannot avoid threats such as predation, parasitism or fungal infections through evasive responses. Instead, they rely on strategies that deter or reduce the impact of these threats. One such strategy is the development of hard structures such as sclerites, which are minute calcium carbonate skeletal elements located on top of the ectoderm and within the mesoglea and gastrodermal tube walls. Sclerites have multiple roles, including structural support of the colony and protection from predators. The role of sclerites as a physical barrier that deters fungal infection, however, is unknown. This study examines the potential role of sea fan sclerites as protection from fungal infection. To accomplish this, four different fungi isolated from healthy sea fans were inoculated into 5 mL tubes filled with culture media and with sclerites from healthy and diseased fans placed on top. To determine whether chemical compounds in the sclerites impact infection, sclerites from healthy and diseased fans were extracted for several days with acetone prior to inoculation. Results showed that the sclerite layer delayed fungal infection from reaching the agar when compared to controls (tubes with agar but no sclerites). There was no difference between tubes with healthy and diseased sclerites, but there were differences among sclerites extracted with acetone. This study suggests that, in addition to the roles in structural support and predator deterrence, sclerites play a role as physical and chemical barriers against to fungal infection.