A description of the skeletal development pattern of the temperate coral Caryophyllia smithi based on internal growth lines

Scanning electron microscopy was used to study the microstructure and internal growth lines of the temperate ahermatypic coral Caryophyllia smithi (Cnidaria: Anthozoa). The arrangement of internal growth lines in combination with the orientation of aragonite crystals were used to describe the pattern of skeletal development in C. smithi. The observed pattern was verified using observations from another study on skeletal development in C. smithi, but which was based only on external observations of skeletons of living and dead specimens in different stages of development. The pattern of skeletal development in C. smithi is suggested to be subdivided into three stages, based on the deposition of specific skeletal elements during the development of the skeleton. In the first 'juvenile' stage, various primary skeletal elements are formed: the basal plate, the septa and a primary and secondary septotheca. The second 'half full grown' stage is characterized by development of apparently only an extended basal attachment, which enlarges the attachment area to the substratum. In the third 'full grown' stage, the edge zone of the polyp tissue is retracted from the extended basal attachment to the calice, and upward growth of the coral predominates followed by lateral thickening of the septotheca, costae, septa and columella. Internal growth lines were found throughout the skeleton and may be composed of, or resulting from the presence of organic matter. The septotheca and the extended basal attachment, which are important for a strong skeletal structure and a strong attachment to the substratum, respectively, are probably thickened faster than the other skeletal elements. Attachment scars were found in areas where attachment of the coral tissue to the skeleton is critical.