[en] Amphiura filiformis is a common burrowing brittle star from muddy environments along the European shores. This species emits a blue light at the level of the arm spine tips when mechanically stimulated. Although the putative luciferase of A. filiformis was recently discovered, the ultrastructure of the luminous cells, i.e. the photocytes, remain unknown for this species as well as for most other bioluminescent echinoderms. In this study, we describe the morphology of the arm and spines in the brittle star A. filiformis and compare it to the luciferase expression pattern revealed by immunohistochemistry. Luciferase is expressed at the base of the spine and at the arm tips within well-defined photocyte clusters. Additionally, we investigated the ultrastructure of spine tissues before and after chemical stimulation of light emission in order to identify photocytes and improve our comprehension of the photogenesis phenomenon in brittle stars. Type II granular cells, found in the basal to mid-basal portion of the spine inner tissues and showing ultrastructural modifications during light-emission, are proposed as photocytes. Putative photocytes appear intimately associated with the spine nerve plexus as well as with mucus cells and presumptive pigment cells. Photocytes are characterised by the presence of specific vesicles with reticulated contents in their cytoplasm. Furthermore, the structure of the calcified spine ossicle has been investigated using microCT scanning.
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