Three-dimensional morphological analysis of the dynamic digestive system in the green brittle star

Brittle stars (Echinodermata: Ophiuroidea) digest a great diversity of food in their stomach, which widely lies in the central disk. As for a possible digestive activity, the green brittle star Ophiarachna incrassata (Lamarck, 1816) is known to show a dynamic movement at the disk. This phenomenon would deeply involve the morphological structure of the stomach. However, past anatomical studies have shown the digestive system in two dimensions after wide incision of the body wall anchoring the stomach. This methodology restrains us from understanding how the stomach actually shapes inside a brittle star. We aim to visualize the morphology of brittle stars’ digestive system in a non-destructive and three-dimensional way, with a comparison between a relaxed specimen and a specimen fixed at the very moment of the disk’s movement. Employing X-ray micro-computed tomography (micro-CT) and introducing an instant freezing method with cryogenic ethanol, we found the stomach wholly transformed during the movement. We here brought transparency to the in vivo position of gut contents to hint the mechanism and digestive function of the movement. Our outcome spotlights a dynamic digestive process in echinoderms and a widely applicable method for probing into its relation with body structure.