Microstructure of octocoral sclerites for diagnosis of taxonomic features

The octocoral family Xeniidae constitutes a major faunistic component of Indo-Pacific coral reefs, particularly in the Red Sea. The sclerites of several common genera, among which Xenia, are composed of minute platelets, or corpuscle-like forms, with a surface that appears almost smooth at light microscope magnification. Scanning electron microscopy (SEM) revealed the fine microstructure of the xeniid-sclerites, leading to the establishment of several new taxa, including the genus Ovabunda, whose spheroid sclerites are composed of aggregations of minute microscleres rather than the Xenia-style platelets. The present study applied for the first time high-resolution environmental SEM (ESEM) in order to examine the detailed structure of these octocoral sclerites. It determines whether and how the microstructural features in three type specimens of O. biseriata, O. faraunensis and O. impulsatilla play a role in determining their structural stability: namely, how the corpuscular microscleres adhere together and maintain the sclerites' spheroid shape. The study revealed the surface of the microscleres to be constructed of coarse granular crystals, and the adjacent microscleres to be interconnected by various microstructural means. It is suggested that these microstructural features are diagnostic for the genus. Future examination of type specimens of other Ovabunda species, as well as additional xeniid genera, will also enable a better evaluation of the taxonomic importance of high-resolution images in relation to other genera.