Skeletal growth dynamics linked to trace-element composition in the scleractinian coral Pocillopora damicornis

The micro- and ultra-structural skeletal growth dynamics of the scleractinian coral Pocillopora damicornis (Linnaeus 1758) was studied with pulsed 86Sr-labeling and high spatial resolution NanoSIMS isotopic imaging. Average extension rates for the two basic ultra-structural components of the skeleton, Rapid Accretion Deposits (RAD) and Thickening Deposits (TD), were compared between corallite wall, spines and dissepiments. The RAD, forming the basal part of the dissepiment, were found to form extremely fast compared with RAD and TD in other parts of the skeleton. Trace element compositions (i.e., Mg/Ca and Sr/Ca ratios) obtained for each ultra-structural component reveal the full range of chemical variation on the scale of the individual corallite. The Mg/Ca ratio was found to vary about a factor of 6, from ∼2.2 mmol/mol in slow growing ornamental spines to ∼13 mmol/mol in the fast forming dissepiment RAD. A positive relationship between Mg/Ca and inferred average extension rate was observed. Sr/Ca, on the other hand, although it varies substantially in the range between ∼6.5 and ∼9.5 mmol/mol, does not show any relationship with inferred average extension rate, nor does is correlate with the Mg/Ca ratio. Rayleigh fractionation models are not capable of explaining the trace element variations. The results presented in this study imply that the coral is capable of controlling its biomineralization activity with great temporal and spatial precision. Spatial heterogeneity in coral tissue activity should be carefully investigated in the development of biomineralization models for scleractinian corals.