The kids are alright – or are they? How warming oceans affect life stages differently

Written by Oscar Speed

Anthropogenic climate change is causing global rises in both air temperatures and sea surface temperatures (SSTs) through the pumping of CO2 and other greenhouse gasses into the atmosphere. This is no longer debated. Extreme Intergovernmental Panel on Climate Change (IPCC) predictions show potential SST increases of over 3°C by 2100. Though this may not sound like a lot, it will create unprecedented pressures on the marine environment and the fauna and flora that live there.

The vast majority of marine species are ectotherms, meaning they can’t regulate their internal body temperature like we can. Their internal thermostat ticks simultaneously with environmental changes, meaning increases or decreases in SSTs create mirrored temperature changes within their bodies. This may potentially result in catastrophic consequences to the physiological processes keeping them alive. Understanding a species’ thermal sensitivity is, therefore, increasingly relevant. This is particularly true for intertidal species (species living below the high-water mark and above low water) that are subject to both rising SSTs and air temperatures. Understanding a species’ thermal tolerance, however, is more complicated than you may think. Many species that have a high tolerance to short-term (acute) heat exposure are often more sensitive to sustained (chronic) exposure, which represents a trade-off between different methods of surviving extreme temperatures.

Marine invertebrates (species without a ‘backbone’ or a cartilaginous skeletal structure) and some vertebrates, like fish, have complex life cycles with multiple stages that are all subject to stage-specific thermal pressures. Individuals from different life stages may have adaptations that represent aspects of this acute vs chronic thermal tolerance trade-off. Different life stages of the same species may, therefore, be more or less at risk from climate change.

A team centred at the University of Plymouth’s Marine Biological and Ecological Research Centre analysed the potential variation in different life stages of the common intertidal snail Littorina obtusata. This important grazer, being an intertidal species, is specifically vulnerable to global temperature increases. The researchers hypothesized that different life stages would exhibit specific thermal strategies, hence having varying tolerance to changing temperatures. 

Common periwinkle (Littorina obtusata)

By incorporating the intensity and duration of heat-induced stress, they measured early embryo (technically known as early ‘veliger’), mid-stage veliger and adult sensitivity to temperature change and the upper critical thermal limit (CTmax). Critical thermal limits are generally considered to be temperature thresholds, that, when surpassed, result in death. The aforementioned L. obtusata study found that both veliger stages were more resistant to temperature changes than their adult counterparts but had an overall lower CTmax than adults. These observations support the hypothesis that there is a trade-off between resistance to acute and chronic thermal stress. Moreover, they demonstrate that tolerance may vary intraspecifically (within a species), depending on the life stage of an organism.

Findings like these are exciting and incredibly important for the field of thermal biology and for furthering our understanding of the complexities of climate change impacts. An overwhelming majority of research around the effects of climate change on marine life focuses exclusively on adults or larvae. The aforementioned study illustrates how this approach leads to the potential dismissal of differences between life stages. It also warns about projections which concentrate solely on the adult life stage.

Similarly, a large meta-analysis of thermal tolerance was completed last year by a team at the Alfred-Wegener Institute (AWI), in Germany. The scientists developed the idea discussed by the Plymouth team and demonstrated the same principle also applies to fish. The figure below, adapted from their paper, shows how the thermal window (the range of temperature that individuals can survive in) is significantly larger for adult and juvenile fish than spawners or embryos. This phenomenon occurs across the globe, but the disparity between life stages is most pronounced in the tropics. 

Thermal windows of fish across latitudes and life stages-adapted from Dhalke et al. (2020)

The importance of this work cannot be understated. It indicates that even in higher-level animals, complex lifecycles result in a vast range of thermal tolerances. Many studies which discuss the thermal tolerance of fish only consider one life stage at a time. The danger of this is that these findings may not accurately predict how, for example, increases in SSTs, will affect the species as a whole. What is true for an individual in its adult life stage may not apply to a juvenile, and these discrepancies must be accounted for. The mentioned studies point to the importance of considering variability and vulnerability across an animal’s life span. They also highlight the need to incorporate these findings into forecasts to gain a deeper understanding of this complex issue.

Pacific salmon spawning in Alaska

This international work from the University of Plymouth and the AWI sheds new light on one of the most important conservation issues of our time. It highlights the need to respond to this new knowledge and embrace the complex lifecycles that define the marine environment and render its species unique in a climate change context. These relatively new findings can help direct conservation efforts, making them focused on and relevant to specific life stages. Only when climate change effects are considered in this way, can the most effective protection be afforded.

Oscar is a current Master’s student of Marine Biology at the University of Plymouth where he is furthering his interests in combining fields of ecology and physiology. A self-professed ‘beach bum,’ his free time doesn’t venture too far from marine biology; spending most of his time at the beach, knee-deep in rockpools or amongst the waves. Find him on Linkedin here (https://www.linkedin.com/in/oscar-speed-a2a191169/) or email him directly if you want to learn any more about marine biology (ojspeed01@gmail.com). 


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