Morphological analyses of the adult and juvenile stages of the stalked jellyfish Craterolophus convolvulus (Johnston, 1835) (Cnidaria: Staurozoa: Stauromedusae: Craterolophidae) using micro-CT

Abstract

Investigations on the internal anatomy of stalked jellyfishes (Staurozoa) are crucial for species descriptions and systematic studies on this taxon. Contrast-enhanced X-ray micro computed tomography (micro-CT) has increased the opportunities for morphological descriptions of soft-bodied invertebrates in the last decade, but only few studies have focused on cnidarian taxa so far. We performed a micro-CT-based morphological study on the staurozoan Craterolophus convolvulus (Johnston, 1835) including the juvenile stage of the species for the first time. Virtual sections allowed for the analysis of specific anatomical details and revealed stage specific differences. For example, the peduncle length was longer in relation to the calyx length, the arrangement of arms in pairs was more pronounced, and the number of secondary tentacles and adhesive pads was lower in the juvenile specimen. Longitudinal virtual sections revealed the internal septa (claustra) with triangular lower edges in the adult and semicircular lower edges in the juvenile specimen. The primary tentacles, only present in the juvenile stage, had distinct distal swellings and adhesive pads at their stem bases. For both stages, our results revealed an internal canal system at the tip of the arms previously described as “intertentacular lobules”. Virtual sections also clarified the organization of gonads in densely folded gonad bands, which form sac-like pouches with small pathways. Even though our micro-CT data allowed for the discrimination of different tissues such as epidermis, gastrodermis, mesoglea, gonads, and muscles, their resolution was not sufficient to depict micro-anatomical details of gametes and nematocysts, which can be relevant for morphological studies on stauromedusae. Thus, micro-CT cannot replace histological standard techniques, but allow for a better spatial understanding of anatomical details and morphological features, facilitating taxonomic and phylogenetic studies on Staurozoa and other soft-bodied cnidarians.