A marine biologist investigated whether deep-sea shrimp (Acanthephyra purpurea) collected from oxygen minimum zones (OMZs) have a higher tolerance to hypoxic conditions compared to those collected from well-oxygenated surface-mixed layers. A matched-pairs design was used, pairing 20 shrimp from OMZs with 20 shrimp of similar mass from surface-mixed layers. The researcher measured the critical oxygen partial pressure (PcritP_{crit}Pcrit in kPa) of each shrimp, which is the threshold oxygen level below which the shrimp can no longer maintain stable oxygen consumption. It was found that shrimp from OMZs had a significantly lower PcritP_{crit}Pcrit than those from surface-mixed layers.
Identify the independent variable in this study.
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