Potential of X-ray micro-computed tomography for soft-bodied and gelatinous cnidarians with special emphasis on scyphozoan and cubozoan statoliths

Abstract

Taxonomic work on soft-bodied and gelatinous cnidarians is a challenging task because preservation often causes difficulties for storage and identification of the specimens. Moreover, examination of scyphozoan and cubozoan statoliths, which can probably provide information on systematics and the age of medusae, are difficult since the crystals dissolve when exposed to air or aqueous solutions. In order to overcome such limitations, we evaluated the benefits of a non-invasive approach using X-ray micro-computed tomography (micro-CT). Various cnidarian taxa (Scyphozoa, Cubozoa, Hydrozoa, Staurozoa) were investigated with particular focus on scyphozoan and cubozoan statoliths. We further evaluated the effect of different fixatives (e.g. ethanol, glutaraldehyde, Trumps) and contrast-enhancing agents (iodine or osmium tetroxide). The reconstruction of tomographic data revealed that osmium tetroxide staining significantly enhanced the contrast of diagnostic morphological features of soft tissues whereas iodine staining was less effective. The reconstruction of scyphozoan and cubozoan statocysts allowed statolith analyses in their natural arrangement as well as automated morphometric measurements in three dimensions. In conclusion, micro-CT allows a detailed reconstruction of taxonomically important characters of soft-tissues and provides a sophisticated tool for reconstructing the spatial arrangement of complex internal structures expanding the opportunities for future research on marine organisms.