The ocean covers 71% of the surface of Earth, and makes up 95% of all the space available to host life on Earth. Oceans are life-support systems for Earth, as well as a global commons that provide free ecosystem goods and services, including food for humans and the oxygen in the atmosphere.
Our oceans also regulate the global climate – they influence rainfall, droughts and floods, determine weather and influence global temperatures. Oceans also capture and store carbon from the atmosphere, and an estimated 83% of the global carbon cycle is circulated through the seas. As our climate is changing due to increased carbon emissions, energy and heat are building up in the ocean. This results in more extreme weather events and can result in changing currents and sea-level rise in the future.
Perhaps of greatest concern is the impact of climate change on the chemistry of the ocean. The increased absorption of carbon is increasing the acidity of the ocean, impacting oceanic habitats and destroying coral reefs.
A lot has been written about how warmer oceans will impact life at sea: animals will begin to migrate to cooler water; sea turtle breeding beaches are threatened by sea levels rising; coral bleaching will increase; currents will shift, and the poles will melt. Not a lot is known about how climate change is busy impacting different life stages of marine organisms.
What Is All This Doing To Life In The Ocean?
Most marine species are ectotherms, meaning their internal sources of heat are small or negligible in controlling body temperature, and they rely on environmental heat sources (commonly known as “cold-blooded” animals). This means they fall victim to changes in oceanic temperatures as these temperature changes in the water are mimicked in their bodies. This can have significant impacts on processes within the bodies of these creatures – processes that are critical for keeping them alive. It is therefore important that researchers understand the thermal sensitivity of marine life, as well as how it is influenced by changing oceanic temperatures.
Intertidal species (living between the high and low water marks) are arguably the most affected by climate change as they are exposed to increased ocean and atmospheric temperatures as the tides change. The thermal tolerance between species also differs and can be difficult to study: some species are able to tolerate significant temperature changes for short periods of time (acute temperature changes) however when exposed to sustained temperature changes (chronic), they struggle to adapt.
Studying Life Stages Of Invertebrates
Marine invertebrates, such as snails, jellyfish and sponges, have very complex life cycles with several specific life stages, each dependent on stage-specific environments. Individuals from the different life stages are specifically adapted to the environment at that stage of life, including temperatures, and therefore different life stages of the same species may be facing different levels of risk from increasing oceanic temperatures.
A team of researchers from the University of Plymouth’s Marine Biological and Ecological Research Center investigated the potential variation in different life stages of an intertidal snail, the common flat periwinkle (Littorina obtusata). This species is suspected to be particularly sensitive to climate change, as it is exposed daily to oceanic and atmospheric temperatures. The researchers hypothesized in a paper published in the Journal of Experimental Biology that each life stage of the snail would exhibit specific thermal strategies, therefore it would have a varying tolerance to changing temperatures, depending on the life stage.
To test their hypothesis, the researchers measured early embryo (known as early “veliger” stage), mid-stage veliger and adult sensitivity to temperature changes, as well as the upper critical thermal limit (temperature thresholds that, when surpassed, result in death). The study found that snails at both the early and mid-stage veliger stages were more resistant to temperature changes than the adult snails, although the earlier stages had a lower upper critical thermal limit than adults.
These observations confirmed the hypothesis, as there is a trade-off between resistance to acute and chronic thermal stress (meaning the organism can adapt to either acute or chronic thermal stress, but not both). The results also confirm that thermal tolerance varies on an intraspecies (within the same species) level, depending on the life stage of the individual.
Studying The Life Stages Of Fish
Inspired by the research done on L. obtusata, a team at the Alfred-Wegener Institute were able to demonstrate the same principle also applies to fish. A large meta-analysis of thermal tolerance in fish showed that the thermal window (the range of temperature that individuals can survive in) is much larger for adult and juvenile fish than embryos or freshly hatched fish. This means that earlier life stages in fish will be more significantly impacted by temperature fluctuations. The study determined that this phenomenon occurs for fish across the globe, however it is most pronounced in the tropics. This indicates that even higher level animals (and not just invertebrates) have complex life cycles with a range of thermal tolerances, depending on the life stage.
What Does This Mean?
Many studies that investigate the thermal tolerance of fish only focus on one life stage at a time. This means that the studies cannot accurately predict how increases and fluctuations in temperatures might affect the entire species, as what applies to an adult may not apply to a juvenile or embryo.
The results of the study on L. obtusata as well as the meta-analysis on fish are important for the field of thermal biology, as well as for furthering the understanding of the vast impacts of climate change. Most studies into climate change impacts on organisms focus solely on either adult or larval stages, however these studies show that a warming ocean will have differing impacts on the different life stages of organisms.
It should also encourage more research into the impacts of climate change on early life stages, as it is not as well understood as climate change impacts on the adult life stage. More research into this topic can help direct conservation efforts to focus these efforts on specific life stages – such as the breeding and spawning grounds of species. This might be the only way in which the impact of climate change on marine organisms may be reduced, and marine life be protected.