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The environmental conditions inside the gut of Calanus hyperboreus and C. glacialis were measured with microelectrodes. An acidic potential hydrogen (pH) gradient was present in the gut of C. hyperboreus , and the lowest pH recorded was 5.40. The gut pH of a starved copepod decreased by 0.53 after the copepod resumed feeding for a few hours, indicating the secretion of acidic digestive fluid. A copepod feeding on Thalassiosira weissflogii (diatom) had slightly lower pH than that feeding on Rhodomonas salina (cryptophyte). Oxygen was undersaturated in the gut of both C. hyperboreus and C. glacialis , with a steep gradient from the anal opening to the metasome region. The central metasome region was completely anoxic. Food remains in the gut led to a lower oxygen level, and a diatom diet induced a stronger oxygen gradient than a cryptophyte diet. The acidic and suboxic–anoxic environments of the copepod gut may support iron dissolution and anaerobic microbial activities that otherwise are not favored in the well‐buffered and oxygenated ambient ocean.
Tang et al. (Mon,) studied this question.