Three digestive movements in Hydra regulated by the diffuse nerve net in the body column

Abstract

The mammalian digestive tract undergoes various digestive movements such as peristalsis and segmentation movement. How those digestive movements and the underlying mechanisms appeared in evolution remains unraveled. A widely accepted view has been that, early in evolution, the digestive process was static based upon diffusion, and later it became dynamic involving digestive movements. Here, we report digestive movements which occur in Hydra, a member of the phylum Cnidaria. We find that the body column of Hydra undergoes a series of movements when fed with Artemia. Comparison of the movements to those in mammals showed similarities in appearance to esophageal reflex, segmentation movement, and defecation reflex. When nerve cells were eliminated, polyps showed only a weak segmentation movement, demonstrating that the diffuse nerve net in the body column of Hydra primarily regulates the movements just as the netlike enteric nervous system does in mammals. Elimination of both secretory gland cells and nerve cells resulted in the complete loss of movement, suggesting that the gland cells are involved in the weak movement. Overall, these observations suggest that the digestive process in Hydra is dynamic and that the diffuse nerve net regulates the digestive movements as a primitive form of enteric nervous system.