Blue appendages and temperature acclimation increase survival during acute heat stress in the upside-down jellyfish, Cassiopea xamachana

Background: 
Ocean warming, stemming from human-induced climate change, poses a significant threat to marine ecosystems. The increase in marine heatwaves is already causing degradation in vulnerable ecosystems like coral reefs and kelp forests. Marine ectotherms (cold-blooded animals) are particularly sensitive to temperature changes due to their physiological dependence on thermal environments. They now face population declines, phenological shifts, and range alterations due to ocean warming. Understanding the biological and ecological factors shaping their responses to ocean warming is crucial. Ectotherms can adapt their physiological characteristics in reaction to environmental shifts using phenotypic plasticity. Among the behavioral modifications inherent in phenotypic plasticity are alterations in structural coloration. Many marine invertebrates display varied colors, generated through structural coloration, pigments, or chromoproteins. Research on color in marine invertebrates has primarily focused on common colors like reds, greens, and oranges, with limited study on blue colors due to their rarity. The upside-down jellyfish, Cassiopea sp., showcases notable blue coloration attributed to a chromoprotein called Cassio Blue. Despite its prevalence, the physiological significance of this blue colouration remains unidentified. 

The authors of this preprint have investigated the responses of jellyfish (medusae), the experimental animal of this study, to heat stress, collected from two sites in the United States – near a shore and bay area. The two locations varied significantly in daily temperatures, however, the mean temperatures were comparable. The authors specifically examined how environmental history, appendage color, and temperature acclimation influence the jellyfish Cassiopea xamachana’ responses to heat stress. This study establishes the first connection between color phenotype and thermal stress resilience in Cassiopea, highlighting the importance of chromoproteins in thermal adaptation.

Key findings:
Daily temperature changes have no effect on bell pulsation rates or lethal temperatures during acute heat stress.

Although both jellyfish collection sites had similar average daily temperatures (30.1°C in the Bay and 29.9°C in the Atlantic), the Atlantic site exhibited significantly greater temperature variability (p < 0.001), with daily fluctuations averaging 4.55°C compared to 1.73°C at the Bay site. Despite these differences, there were no significant variations in bell pulsation rate changes between animals from the two sites during acute heat stress. In jellyfish, bell pulsation serves roles in oxygen exchange and temperature regulation, with pulsation rates increasing as temperatures rise and declining rapidly at lethal levels. In Cassiopea, bell pulsation rates may reflect metabolic rates, offering a measurable, whole-organism phenotype for assessing responses to acute heat stress. There were no significant differences in average lethal temperatures between Cassiopea xamachana taken from the Atlantic (39.81°C) and Bay (39.52°C) collection sites. Interestingly, despite surviving up to 40°C, no bleaching (symbiont loss) was observed in jellyfish from either collection site, suggesting that despite differences in daily temperature range and variability between collection sites, C. xamachana from both locations exhibited similar physiological responses to acute heat stress.

Bell pulsation and lethal temperature for survival are influenced by acclimation to temperature.
To investigate the acclimation of C. xamachana to elevated temperatures, 100 jellyfish were collected from the Atlantic site and divided into groups maintained at either ambient (26°C) or elevated (32°C) temperatures for 30 days. Interestingly, there were no significant differences in bell diameter or symbiont density between the two temperature groups, despite expectations of acclimation-related changes. Following acclimation, 40 jellyfish underwent heat stress trials, with bell pulsation rates steadily increasing until approaching lethal levels. Acclimation temperature significantly affected the change in bell pulsation rates during heat stress, with jellyfish acclimated at elevated temperatures exhibiting higher survival rates to higher temperatures compared to those acclimated at ambient temperatures. This suggests that C. xamachana can acclimate to prolonged elevated temperatures, providing beneficial effects in coping with acute heat stress.

C. xamachana survive acute heat stress due to their appendage color.

Given the lack of significant differences observed between jellyfish collected from two sites, this study then aimed to explore additional factors influencing C. xamachana physiology during heat stress, particularly focusing on the association between phenotype and survival in increased temperatures. The analysis categorized individuals based on the predominant color of their oral appendages as either “blue” or “brown” and examined their heat stress responses in this context. Interestingly, jellyfish with blue appendages consistently survived to significantly higher temperatures compared to those without, indicating a clear association between appendage color and survival. This effect was consistent across experiments and collection sites, suggesting the importance of appendage color in determining survival under heat stress conditions. Additionally, appendage color was found to be significantly associated with survival among jellyfish subjected to different temperature acclimation regimes.

What I like about this preprint

I particularly appreciate this preprint for its concise and straightforward examination of heat stress tolerance in jellyfish. The capacity of jellyfish to endure heat stress likely contributes to their widespread distribution in tropical habitats worldwide. Additionally, this study delves into the intriguing occurrence of vibrant blue coloration, particularly in the oral appendages of Cassiopea, the function of which remains mysterious. It’s commendable that the researchers investigated the influence of both external environmental factors and internal physiological traits on the thermal resilience of Cassiopea xamachana instead of solely focusing on one aspect.

Future directions:
While this study was elegant, I look forward to future investigations exploring heat tolerance adaptations in jellyfish from habitats with greater temperature variations, such as those farther from the coasts and in different latitudes. Additionally, examining the performance of blue appendages compared to other colors across a wide range of temperature fluctuations would be intriguing.

References:
1. Lang, B. J., Donelson, J. M., Bairos‐Novak, K. R., Wheeler, C. R., Caballes, C. F., Uthicke, S., & Pratchett, M. S. (2023). Impacts of ocean warming on echinoderms: A meta‐analysis. Ecology and Evolution, 13(8), e10307.
2. Filbee-Dexter, K., Wernberg, T., Grace, S.P. et al. Marine heatwaves and the collapse of marginal North Atlantic kelp forests. Sci Rep 10, 13388 (2020). https://doi.org/10.1038/s41598-020-70273-x
3. Umbers, K. D. (2013). On the perception, production and function of blue colouration in animals. Journal of Zoology, 289(4), 229-242.

Tags: acclimation, bell pulsation, thermal tolerance

doi: https://doi.org/10.1242/prelights.37238

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