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Front Physiol
2022 Jan 01;13:1077376. doi: 10.3389/fphys.2022.1077376.
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Temperatures leading to heat escape responses in Antarctic marine ectotherms match acute thermal limits.
Morley SA
,
Chu JWF
,
Peck LS
,
Bates AE
.
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Thermal tolerance windows are key indicators of the range of temperatures tolerated by animals and therefore, a measure of resilience to climate change. In the ocean, where ectotherms are immersed, body temperatures are tightly coupled to environmental temperature and species have few options for thermoregulation. However, mobile species do have the ability to orientate towards optimal temperatures and move away from sub-optimal or dangerous temperatures. Escape responses are one such locomotory behavior, which typically manifests as a series of violent flicking movements that move individuals out of dangerous environments. We tested 11 species of Antarctic marine ectotherms, from one of the most stable shallow water marine environments, with an annual temperature range of -2°C to +2°C, that are vulnerable to small degrees of warming. Three species, the clam Laternula elliptica, the sea cucumber Cucumaria georgiana, and the brittlestar Ophionotus victoriae, showed no, or virtually no, escape response to temperature. Escape responses from a further eight species had a median response temperature of 11.2 (interquartile range, 10°C-15.7°C), which is well above current environmental temperatures but close to the range for acute lethal limits of Antarctic marine ectotherms (CTmax range, 17.2°C-26.6°C). This highlights that both acute tolerance limits and escape responses, fall outside current environmental temperatures, but also those predicted for 100s of years in the Southern Ocean. In a warmer Southern Ocean Antarctic fauna may not have the capacity to use temperature to select optimal thermal conditions, which leaves adaptation as a primary mechanism for their persistence.
FIGURE 1. Theoretical thermal reaction norm for performance of a trait, highlighting the expected locomotory response of mobile marine ectotherms to increasing thermal exposure. Exposure to extreme temperatures above a threshold (orange line), for sufficient duration, will result in animals entering heat coma at CTmax, losing the ability to escape lethal temperatures. Short duration exposure to temperatures above this threshold, or extended exposure to temperatures close to this threshold, are expected to illicit escape responses that will move individuals to cooler temperatures. Individuals exposed to temperatures above their optimum range are expected to also move to cooler temperatures and relocate within their optimal temperature range, where they will remain.
FIGURE 2.
(A) Temperatures at which escape responses were recorded for each of the 11 species tested. Boxes are median values with upper and lower interquartile ranges. The lack of a box plot indicates that no escape response was recorded for C. georgiana and L. ellipitca. O. victoriae and Amphipod_B only responded at one temperature. Silhouettes represent the taxon of each species. Species with the same lowercase letters indicate that escape responses were not significantly different, Dunn’s non-parametric post-hoc test. (B) Escape temperatures plotted against CTmax (at a warming rate of 1°C h−1, data available for eight of the 11 species in this study from Peck et al., 2009). Median ± interquartile range (missing interquartile ranges indicate small sample sizes, or responses at a single temperature, as detailed above).
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