Adhesive organs with complex neuromuscular system: Organization of rhopalioids in Haliclystus auricula James-Clark, 1863 (Cnidaria: Staurozoa)

Staurozoa is a small group of marine stalked jellyfish, some of which have specialized attachment organs — rhopalioids, or so-called anchors. The adhesive function of these organs was mentioned in numerous studies; however, the mechanism of their temporary attachment is still unknown. Moreover, it is assumed that rhopalioids may be homologous to rhopalia of scypho- and cubozoans and provide sensory and integrative functions. Nevertheless, nervous elements associated with rhopalioids are poorly investigated. Thus, we focused on morphological features of rhopalioids in staurozoan Haliclystus auricula James-Clark, 1863 using histological and semithin sections, and also confocal laser scanning microscopy. We described histological organization of rhopalioids and observed four types of epidermal gland cell, which presumably provide the attachment and reattachment to the substrate. Supposedly, the musculature of rhopalioids can also play a role in the attachment and reattachment. We have studied organization of the nervous system in rhopalioids, which includes FMRFamide-, tubulin- and neurotensin-positive nerve clusters and FMRFamide-positive presumptive sensory cells. Based on our results, we assume that rhopalioids, besides the complex attachment, may act like sensory organs and play a role of integrative centres.