The impact of hypoxia and heat stress on Corella spp. 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 subtidal sessile invertebrates 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, such as heat, but how low oxygen (hypoxia) and heat combined may influence heart rate and heartbeat reversal is unknown. We predicted that 1) heart rate and duration between heartbeat reversals will increase after exposure to either heat, hypoxia, or both stressors simultaneously, and 2) individuals exposed to both stressors would have increased responses compared to those exposed to a single stressor. Corella spp. were collected and acclimated to lab conditions for 16-24 hours (~13 °C, ~92 % saturation), then exposed to warm (~18 °C), hypoxic (~31 % saturation), or warm and hypoxic (~17 °C and ~31 % saturation) conditions for six hours. Heart rate and time between heartbeat reversals were measured after collection, acclimation to lab conditions, exposure to a stressor, and recovery from the stressor. There was a positive correlation between time between heartbeat reversals and heart rate when tunicates were measured immediately after collection, but not after lab acclimation. Heart rate increased with exposure to heat, hypoxia, or heat and hypoxia, with the greatest increase after exposure to both stressors. The positive correlation between time between heartbeat reversals and heart rate was maintained when tunicates were exposed to hypoxia, but time between heartbeat reversals did not increase with heat or heat and hypoxic conditions. Our findings show that temperature and oxygen availability influence Corella heart rate and time between heartbeat reversals differently, suggesting that tunicates may be able to optimize oxygen and nutrient movement when exposed to abiotic stressors.