Effects of low water temperature on growth and magnesium carbonate concentrations in the cold-water gorgonian Primnoa pacifica

The large cold-water coral species found near Japan, Primnoa pacifica (Kinoshita, 1907), was investigated to estimate the effects of water temperature on its growth rate and magnesium concentration in coral sclerites. The coral inhabits water of approximately 0.6–0.7 °C, the lowest temperature recorded for this genus. Axial growth rate, estimated by counting annual growth rings, was about 0.24 mm yr₋₁. Compared to the growth rate of Primnoa corals in the NE Pacific and Atlantic Oceans at water temperatures of 3.7–12.1 °C, the growth rate was as low as might be expected considering the extremely low temperature. The carbonate content of sclerites was analyzed using an X-ray powder diffractometer (XRD). Analysis showed an average MgCO₃ concentration of 10.15 ± 1.37 mol % and an Mg/Ca mol ratio of 0.113 ± 0.017. These values were higher than those predicted from temperature and Mg/Ca relationships of other octocoral data. These results suggest that axial growth rate and magnesium concentration of NW Pacific cold-water gorgonians inhabiting extremely low temperature waters were not influenced by temperature in a way similar to those in warmer waters.