Colony and polyp biometry and size structure in the orange coral Astroides calycularis (Scleractinia: Dendrophylliidae).

Coral polyps inside a colony may differ in reproductive activity and ecological function even while sharing the same genetic identity. Although polyps are the basic units of coral colonies, their size, biometry and size structure have rarely been studied. This study investigated, for the first time, colony and polyp biometric relationships and intra-colony polyp population size structure in the Mediterranean endemic Astroides calycularis (Pallas, 1766). Biometric parameters for 160 colonies and 4162 polyps were measured with consideration of polyp position inside the colony (central or peripheral). The positive allometric relationship between polyp width and length, resulting in a progressively circular oral disc as polyp size increases, may relate to the low-sedimentation characteristics of the habitat of this species. The smaller size of peripheral polyps compared to central ones suggests that polyp budding occurs preferentially at the outskirts of the colonies, possibly increasing the competitive advantage for space utilization. Larger colonies had polyps with smaller size than small and medium colonies, due to an over-representation of the size class containing polyp size at sexual maturity. It is proposed that large colonies may invest energy in increasing polyp size up to the size at sexual maturity, rather than increasing the size of already mature polyps.