The impact of hypoxia and heat stress on Corella inflata heart characteristics adds evidence to the functional role of heartbeat reversals

Marine habitats are experiencing changes in temperature and dissolved oxygen due to climate change, but how these changes are influencing the physiology of sessile invertebrates in the subtidal zone has not been well studied. Tunicates are a common subtidal invertebrate with a unique heart, in that their heartbeat can reverse directions. Tunicate heartbeat can be influenced by environmental stressors, but how combined low oxygen (hypoxia) and heat influence heart rate and heartbeat reversal is unknown. Corella inflata were collected and acclimated to lab conditions for 16-24 hours (∼13°C, ∼92% O2), then exposed to heat (∼18°C), hypoxic (∼31% O2), or heat and hypoxic (∼17°C and ∼31% O2) conditions for six hours. Heart rate and time between heartbeat reversals were measured after acclimation to lab conditions, exposure to a stressor, and recovery from the stressor. Heart rate increased with exposure to heat, hypoxia, and heat and hypoxia, with the greatest increase after exposure to both stressors. Time between heartbeat reversals increased with exposure to hypoxia and heat and hypoxia, but not heat. Our findings show that temperature and oxygen availability influence C. inflata heart rate and time between heartbeat reversals and suggests that tunicates may use heartbeat reversals to optimize oxygen and nutrient movement when exposed to stressors, which we propose as the Efficiency Hypothesis. Additionally, these findings provide insight into how tunicates may respond to changing ocean conditions in the future.