Adaptation to hypoxic stress involves amino acid metabolism: A case in sea cucumber

Low dissolved oxygen (LO) in seawater negatively affects aquatic animals , has received considerable attention. However, there is still much to learn about how echinoderms, which are keystone species in benthic ecosystems, respond to hypoxic stress. Here, we detected differentially expressed metabolites (DEMs) in sea cucumber (Apositchopus japonicus) between normoxic conditions (NC group) and hypoxic conditions (2 mg L-1) for 3 and 7 days (i.e., LO3 and LO7 groups). A total of 243, 298 , 178 DEMs were found in the NC versus LO3, NC vs. LO7, and LO3 vs. LO7 comparisons, respectively. Amino acids were the most abundant class of DEMs, and "biosynthesis of amino acids" was an enriched pathway in all three comparisons. Most of the enriched metabolite sets under hypoxic stress were related to metabolism. As the duration of the hypoxia treatment extended, the metabolism-related process maintained an upward trend, and signaling pathways maintained a downward trend. Thus, metabolism-related processes are affected in hypoxia-stressed sea cucumber, and amino acid metabolism is the most important process for adaption to hypoxic conditions, potentially function in osmotic regulation and energy regulation. Our results shed light on the adaptative strategies of sea cucumber to challenging environ-mental conditions.