Ultrastructural description of surface receptors in the bioluminescent brittle star Amphiura filiformis (Müller, 1776)

Stäbchen are specific sensory receptors distributed along the integument of brittle stars (Ophiuroidea: Echinodermata). Found on the surface of spines and podia, these structures correspond to cuticular projections where one or more ciliated cellular processes end. Stäbchen are presumed to play a functional role in chemo- and/or mechanoreception. Structurally, the stäbchen of the bioluminescent brittle star Amphiura filiformis can be categorised into two types: (i) long-cilium cylindrical stäbchen, and (ii) short-cilium conical stäbchen. Electron microscopy analyses revealed a close and intricate association of the spine stäbchen with underlying luminous cells, known as photocytes. In vivo luminometry experiments have demonstrated that A. filiformis is sensitive to mechanical stimuli and to seawater containing injured conspecifics, responding by emitting blue luminescent flashes. These combined results suggest that spine stäbchen could be involved in initiating the light emission process.