Comparative analysis of morphology and trace element composition in the skeletons of congeneric coral species exhibiting distinct trophic strategies

Scleractinian corals build highly diverse skeletons whose morphology and chemistry reflect their evolutionary history, trophic strategy, and habitat. The solitary genus Balanophyllia represents an ideal model system because it includes closely related species with contrasting trophic strategies. The zooxanthellate B. europaea, unlike the azooxanthellate B. regia and B. elegans, shows higher growth rates, bulk density, and intra-skeletal organic matrix content, suggesting that symbiotic algae play a key role in shaping skeletal phenotype. In this study, skeletal morphology was further characterized through growth-band measurements from ESEM images. Additionally, skeletal elemental composition was analyzed using inductively coupled plasma–optical emission spectrometry (ICP-OES) on aragonitic skeletons of the three species. Growth-band profiles showed that B. europaea forms more regular and evenly spaced increments than the two azooxanthellate species, consistent with calcification being more tightly coupled to the diurnal photosynthetic cycle. ICP-OES analyses revealed clear interspecific differences in trace-element concentrations, with compositional trends distinguishing the zooxanthellate species from the azooxanthellate ones. These results suggest that trophic strategy influences both skeletal morphology and trace-element incorporation. By integrating growth-band analyses with elemental composition data, this preliminary study refines the characterization of Balanophyllia species and provides new insights into the biomineralization processes in scleractinian corals.