Mineral formation in the primary polyps of pocilloporoid corals

In reef-building corals, larval settlement and its rapid calcification provides a unique opportunity to study

the bio-calcium carbonate formation mechanism involving skeleton morphological changes. Here we

investigate the mineral formation of primary polyps, just after settlement, in two species of the pocilloporoid

corals: Stylophora pistillata (Esper, 1797) and Pocillopora acuta (Lamarck, 1816). We show that the

initial mineral phase is nascent Mg-Calcite, with rod-like morphology in P. acuta, and dumbbell morphology

in S. pistillata. These structures constitute the first layer of the basal plate which is comparable to

Rapid Accretion Deposits (Centers of Calcification, CoC) in adult coral skeleton. We found also that the

rod-like/dumbbell Mg-Calcite structures in subsequent growth step will merge into larger aggregates

by deposition of aragonite needles. Our results suggest that a biologically controlled mineralization of initial

skeletal deposits occurs in three steps: first, vesicles filled with divalent ions are formed intracellularly.

These vesicles are then transferred to the calcification site, forming nascent Mg-Calcite rod/

pristine dumbbell structures. During the third step, aragonite crystals develop between these structures

forming spherulite-like aggregates.