Metabolic rate and tolerance to hypoxia in the harpacticoid copepod Amphiascoides atopus

Harpacticoid copepods are key components of the meiofauna in nearly all benthic ecosystems, commonly characterized by a low availability of dissolved oxygen (DO). Harpacticoids exhibit a functional loss of Hypoxia Inducible Factor (HIF), and their ability to tolerate and respond to hypoxic conditions has been studied in the tidal-pool genus Tigriopus , but not in harpacticoids inhabiting other ecosystems. To improve our understanding of the metabolic pathways used by harpacticoid copepods to survive in low-oxygen environments, we measured the metabolic rate and established the critical oxygen pressure ( P crit ) in Amphiascoides atopus using flow-through respirometry along a DO gradient. Our results indicate that A. atopus switched from oxyregulating to oxyconforming behavior at a P crit of 10.5 kPa. Below a DO concentration of 5.9 kPa, the metabolic rate of A. atopus was undetectable in 72 % of the cases, indicating a probable switch to anaerobic metabolism. Additionally, we performed a survival assay at experimental DO concentrations of 20.8 kPa,7.8 kPa, and 3.1 kPa, and anoxia at DO levels <0.92 kPa. Survival rate after 96 h was 100 % under the two hypoxic conditions and 25 % under anoxia. Finally, we compared the temperature-dependent metabolic rates between harpacticoids (w/o functional HIF) and calanoids (with functional HIF). The differences between the two groups were mainly driven by temperature, as studies in harpacticoids have generally been conducted at higher temperatures, as in the present study. • A. atopus regulate its metabolic rate in accordance to DO concentration in water. • The high tolerance to low DO levels explains the dominance of harpacticoids in hypoxic environments. • An alternative mechanism for responses to hypoxia is suggested in harpacticoids.