Impact of disease and detachment on growth and survivorship of sea fans Gorgonia ventalina

The importance of diseases of sea fans (Gorgonia spp.) is widely recognized; however, previous studies have not always distinguished between causes of mortality and differential effects of disease on different size clases of sea fans. In this study, mortality and net growth of healthy, purple-spotted and diseased Gorgonia ventalina colonies of small, medium and large size were estimated for 1 yr. Detachment occurred in all size and health categories. Mortality due to disease was exclusively observed in small colonies. Colony growth was influenced by health state and size. While medium and large colonies of all health states exhibited negligible net growth, small diseased colonies exhibited negative net growth. In this group, disease was an important cause of tissue loss, while in all 3 health states the main cause was fracture. Most healthy colonies that became diseased passed through the purple-spotted state. The rate at which purpled colonies returned to being healthy was double the rate at which they advanced to diseased, which then rarely recovered. Disease did not significantly impact growth or survivorship of medium and large colonies, but its effect on smaller fans is significant for populations because it reduces the transition to sizes that are reproductively active.