Effects of substrate on the physiological characteristics and intestinal microbiota of Echiura worm (Urechis unicinctus) juveniles

Abstract The effects of environmental factors on the physiological characteristics of Echiura worm Urechis unicinctus have been widely studied, however, the influences of substrate on this species are still unclear. Differences in the growth performance, physiological response, and intestinal microbiota of U. unicinctus juveniles between the sandy and muddy substrates were evaluated in the present study. The results indicated that the weight gain was significantly higher in U. unicinctus juveniles with the sand bottom compared to the mud group, while the survival rate was obviously lower. The activities of intestinal digestive enzymes and the contents of triglyceride, glucose, and total cholesterol in the coelomic fluids had a significant increase in the individuals with the sandy bottom compared to the mud group. The oxidative stress and intestinal inflammatory were induced by the sandy bottom, meanwhile, the antioxidant system and goblet cells were activated to protect the U. unicinctus juveniles. Focused on the intestinal microbiota of U. unicinctus juveniles, Sphingomonadales, Burkholderiales, and Pesudomonadales were more abundant in the mud group. In contrast, Alteromonadales and Rhodobacterales were enriched in the sand group. Moreover, various correlations were observed between the intestinal bacteria and physiological indices, such as Acinetobacter with the lipoprotein- and hepatic-lipase activities, Shewanella with the superoxide dismutase activities, and Cupriavidus with the malondialdehyde contents.