Growth characteristics and growth rate estimation of Japanese precious corals

The petrographic method has been a popular technique for estimating the ages and growth rates of corals based on growth ring density in their axial skeleton. The organic matrix staining (OMS) method, a modified method of petrographic method, has also been used for measuring age and growth rate of the Mediterranean red coral (Corallium rubrum) by staining the organic matrix (OM) in the calcite skeleton. Since the OMS method is based on the concentration of OM in the coral skeleton, this method may not be suitable for coral species with low OM content. In the presnt study, growth characteristics and growth rates of three Japanese precious corals (JPCs; Paracorallium japonicum, Corallium elatius and Corallium konojoi) were determined based on the principles of the petrographic method using a high resolution VHX-1000 digital microscope, termed as VHX-1000 hereafter, without staining the OM in the axial skeleton. Compared to the OM-stained cross-sections (slabs), growth rings in un-stained slabs of the Japanese red coral (P. japonicum) were more clearly visible with the VHX-1000. This may be due to low concentration of OM in the coral skeleton. Growth rate of JPCs differ significantly depending on coral species, habitat, and environmental conditions. Diametric and linear growth rates of the Japanese red coral (P. japonicum) were slower (0.20 ± 0.08–0.27 ± 0.01 and 2.22 ± 0.82–6.66 ± 5.52 mm yr− 1, respectively) than the Japanese pink (C. elatius; 0.30 ± 0.04 and 2.76 ± 2.09 mm yr− 1, respectively) and white (C. konojoi; 0.44 ± 0.04 and 7.60 ± 0.75 mm yr− 1, respectively) corals. In addition, the diametric growth rate of the Japanese red coral (P. japonicum) is slower (0.24 ± 0.05–0.44 ± 0.04 mm yr− 1) than the Mediterranean red coral (C. rubrum; 0.20–0.62 ± 0.19 mm yr− 1).

► Growth rate of Japanese precious corals depends on coral species, habitat and environmental conditions.
► Diametric and linear growth rates of red coral (P. Japonicum) were slower than pink (C. elatius) and white (C. konojoi) corals.
► Diametric growth rate of the Japanese red coral (P. japonicum) is slower than the Mediterranean red coral (C. rubrum).
► Without staining the organic matrix, the petrographic method using a VHX-1000 digital microscope is more suitable for studying Japanese precious corals.