Evolution of calcium-carbonate skeletons in the Hydractiniidae

Biomineralization has mostly been studied in the class Anthozoa (Phylum Cnidaria), but very little is known about the evolution of the calcified skeleton in the class Hydrozoa or about the processes leading to its formation. The evolution of the calcified skeleton is here investigated in the hydrozoan family Hydractiniidae. A phylogenetic analysis of ribosomal, mitochondrial, and nuclear-protein-coding DNA sequences supported two independent origins of the calcified skeleton within the Hydractiniidae and indicates a case of parallel evolution, as suspected in the Anthozoa. Neither of the two origins of skeleton in the Hydractiniidae has led to either speciose or numerically abundant species, in contrast with other skeletonized hydrozoan families. Finally, we show that the origin of calcified skeletons in the Hydractiniidae is significantly correlated with the distribution of species with calcium carbonate granules within a polyp's gastrodermal cells. This suggests that the presence of these granules precedes the origin of a full skeleton.