Polyp longevity in a precious gorgonian coral: hints toward a demographic approach to polyp dynamics

In this paper, we investigated the age distribution and dynamics of polyps in the slow-growing and long-lived gorgonian Corallium rubrum (the Mediterranean red coral), applying an a posteriori demographic approach by considering each colony as a population of polyps. In the Mediterranean red coral, new polyps emerge from the coenenchyme in different regions of the colony and their budding rate depends on the age of branches. The age of polyps, branches and colonies were estimated using the organic-matter-staining dating method on thin sections of the colony skeleton. The median age and maximum life span of polyps were 4 and 12 years suggesting the presence of senescence processes: thus a colony renews several times its polyps during life cycle. Polyps were divided into annual age classes, and their mortality rates calculated. The polyp age distribution was then used to construct a mortality table and an algebraic transition matrix based on the age at death of 234 polyps. Finally, the polyp budding rate of a young, unbranched colony was calculated, and polyp temporal dynamics simulated. These findings represent the first steps for developing demographic models able to describe polyp dynamics of old and highly branched colonies.