Functional thermal limits are determined by rate of warming during simulated marine heatwaves

Abstract

Marine heatwaves (MHWs) are increasing in both intensity and frequency against a backdrop of gradual warming associated with climate change. In the context of MHWs, animals are likely to experience sub-lethal rather than lethal effects, defining long-term limits to survival and/or impacting individual and population fitness. We investigated how functional sub-lethal limits track critical thresholds and how this relationship changes with warming rate. To this end, we monitored basic functioning, specifically the ability to right, feed and assimilate energy, as well as oxygen consumption rate in the common Antarctic sea urchin Sterechinus neumayeri. Water temperature in experimental systems was increased at rates of 1, 0.5 and 0.3°C d-1, in line with the characteristics of MHW events previously experienced at the site where the study urchins were collected on the Antarctica Peninsula. Functioning was assessed during the simulation of MHWs, and sub-lethal limits were determined when the rate of functional degradation changed as temperature increased. Results suggest that thermal sensitivity varies between the key biological functions measured, with the ability to right having the highest thermal threshold. Functions deteriorated at lower temperatures when warming was more rapid (1°C d-1), contrary to lethal critical thresholds, which were reached at lower temperatures when warming was slower (0.3°C d-1). MHWs and their impacts extend far beyond Antarctica, and in this context, our analyses indicate that the onset rate of MHWs is critical in determining the ability of an organism to tolerate short-term elevated temperatures.

Temperatures leading to heat escape responses in Antarctic marine ectotherms match acute thermal limits

“…This range also corresponds to the estimated long-term temperature limits for species in this assemblage (1°C–6°C over 2–5–months, Peck et al, 2009 ). Upper temperature limits in Antarctic marine ecototherms vary between physiological processes and with the rate of warming ( de Leij et al, 2022 ). The species and individuals exhibiting an escape response, in the current study, did not respond until a median temperature of 11.2°C (10°C–15.7°C).…”

“…It is not known whether acute escape responses and acute thermal limits are mechanistically linked. In fact there is growing evidence that multiple mechanisms underlie thermal limits for different physiological processes ( Lefevre et al, 2021 ; de Leij et al, 2022 ; Jorgensen et al, 2022 ). Behavioural and biochemical protective responses to temperature rely on an individual’s ability to sense and respond to suboptimal temperature exposure.…”

“…A key question given extreme heat events which are increasing in magnitude and frequency in the ocean, is, what mechanisms lead to survival and persistence? Different physiologies have different thermal limits, and functions critical for survival, such a righting, can have a higher thermal limit than less time critical functions such as feeding ( Pörtner, 2002 ; de Leij et al, 2022 ). Thermal limits can also vary with the rate at which they are warmed ( de Leij, et al, 2002 ).…”

“…Different physiologies have different thermal limits, and functions critical for survival, such a righting, can have a higher thermal limit than less time critical functions such as feeding ( Pörtner, 2002 ; de Leij et al, 2022 ). Thermal limits can also vary with the rate at which they are warmed ( de Leij, et al, 2002 ). At extreme temperatures, approaching those that are high enough to result in heat coma, escape response behaviours may be a last resort mechanism.…”

“…Polar marine ectotherms are characterised as being highly vulnerable, with a low capacity to survive acute warming ( Peck et al, 2009 ), coupled with generally poor acclimation capacities and long generation times that reduce opportunities for adaptation ( Peck et al, 2014 ). While some of the physiological mechanisms underlying the vulnerability of Antarctic marine ectotherms are well understood ( Pörtner, 2002 ; Peck et al, 2009 ; Peck et al, 2014 ; de Leij et al, 2022 ), others such as behavioural responses to environmental cues have been less well studied. A previous study by Bates et al (2010) indicated that movement in a thermal gradient was limited and suggested that this poor thermal behavioural response may be a result of their evolutionary environment in the Antarctic.…”