New insights into ophiuroid coelomocytes: from their cell diversity to their putative function in the transfer of coelenterazine in the bioluminescent brittle star Amphiura filiformis

The brittle star Amphiura filiformis is a keystone species within the burrowing fauna assemblage in the fine, muddy sediments along the European coasts. With high regeneration and bioluminescence capabilities, this species emerged as a model species for many studies. However, only a few studies have investigated the cellular components of their coelomic fluid, which are involved in functions such as immunity and metabolite transport. The present study provides a detailed morphological description, including the cell proportions and concentrations, of the different perivisceral coelomocytes found in the brittle star A. filiformis.
For this purpose, we here present a simple method to collect coelomic circulating cells from small and non-fleshy echinoderms presenting a limited volume of perivisceral fluid, using a needle to inject artificial seawater in the perivisceral coelom followed by an immediate recovery of the fluid containing coelomocytes. Based on morphological criteria obtained with optical and scanning electron microscopes, six different cell types could be distinguished. Morphological similarities with coelomocytes in other species of brittle stars and other echinoderm classes are discussed. Moreover, the involvement of this coelomic fluid in the transfer of the luminous substrate, coelenterazine, from the stomach content to photogenic sites is investigated, adding a step towards the understanding of this brittle star bioluminescence.