As air and water temperatures increase, some of us may be able to tolerate the changes better than others.
Ectotherms, or organisms that rely on their external environment to regulate their body temperature -- unlike humans and other mammals -- will face the full effect of temperature rises, unmitigated by their bodies' own processes. This fact makes many ectotherm species, such as fish, more vulnerable to future climate changes.
But simply knowing these species may be at greater risk, or more likely to succumb to temperature increases, isn't enough. Researchers are working to better understand both types of species' thresholds of tolerance as well as their potential to adapt. For clues on how to do this, some have turned to the most fundamental organ: the heart.
Early this month, two separate researchers, Anthony Hickey from the University of Aukland and Bryan Neff, a professor at the University of Western Ontario, conducted studies looking at the responses of two different types of fish to heat stress. While Hickey looked at common wrasses, small energetic fish that spend the bulk of their time darting between coral reefs and dock posts, Neff looked at chinook salmon populations during their time in the Big Qualicum River on the southern portion of Canada's Vancouver Island.
Both researchers found degrees of tolerance in their respective species, but not all responses were equal. Populations of wrasse found in warmer waters naturally showed less ability to withstand increases in temperature before heart failure compared to their cooler-water counterparts. Neff's salmon populations, meanwhile, showed that some fish with naturally lower resting heart rates were able to tolerate greater increases. Neff also found that salmon may have a genetic leg up in adapting to warming temperatures, as larger egg size seemed to correlate to higher temperature tolerance.
While both studied the effects of heat stress, each researcher did so from a different perspective.
Hickey was perplexed by the common justification behind ectotherm heart failure. He explained that many believe that as water temperature increases, its dissolved oxygen content decreases -- meaning there is less oxygen for fish to breathe, eventually leading to heart failure. But this, he said, discounts other principles, like the fact that oxygen becomes more diffuse, and thus easier to access, in warmer waters.
"If this thinking was true, it would mean the same problems for terrestrial animals," said Hickey, "we realized the thinking just wasn't quite right."
Wrasse in Fagatele Bay National Marine Sanctuary. (Photo: NOAA Office of National Marine Sanctuaries)
Just 'stamp collecting' or something more?
Coming from a biomedical background, Hickey wanted to know if there was a prelude to heart failure that could better show the cause of failure.
"What we really needed to know was how much of this oxygen were they able to take in and how efficiently could it be used at higher temperatures," said Hickey, "so we looked at the energy currency of the heart, mitochondria."
Mitochondria are tiny organelles that create and deliver energy to cells and are extremely abundant in the heart. They also regulate the death of cells, called apoptosis. Hickey realized that most likely, these cells would show signs of failure much before total heart failure, so they could provide a more fine-tuned glimpse of the effects of heat stress.
First experimenting with New Zealand's common wrasse, affectionately called the spotty, Hickey and his team monitored the hearts of fish as they ramped up the temperature they were exposed to. What they found was that indeed, the mitochondria became less efficient as the temperature increased, eventually reaching collapse. When enough mitrochonria died, heart failure followed.
"Finally, we could see it was not the amount of oxygen available, or its form; it was in fact how the existing structures in the fish could use it," said Hickey. Following up on his study with the spotty, Hickey also looked at two other species of wrasse, the luna wrasse, a more tropical fish and the banded wrasse. Interestingly, the tropical luna wrasse showed the least ability to withstand temperature increases, with the spotty and banded wrasse having wider ranges of tolerance.
"Even though we now know this information for the wrasse, our real issue going forward will be how we know if we are getting generalizable information we can apply to help understand other species' tolerances, or whether we are simply stamp collecting," said Hickey. "The wrasse as a subject has not undergone the effects of heavy industry as other species have, and because they are not of real commercial importance, they don't really make people sit up in their chairs when they hear something may happen to these fish in the future."
That's when Neff's majestic Pacific chinook salmon enter the picture. Not only are the salmon of great commercial importance, but their migration patterns make them a major dietary component of a slew of mammalian species, including humans. On top of this, recent years have seen a massive decline in natural populations, causing alarm and prompting more research efforts into protecting what remains.
Next up for study: deep sea species
Neff explained that overfishing, pollution, poor stream management and damming have all led to the salmon's current state.
"Anywhere from 90 to 98 percent of the natural runs are gone," said Neff. "Though we don't think climate change will be the worst these fish face," he said, "it may be the final straw."
Neff said salmon are more adapted to handle temperature increases naturally, as part of their migration from warm freshwater streams to cold ocean waters and back. This means the salmon have a wider range of tolerance than Hickey's wrasses to start with, but also, as Neff pointed out, the athletic nature of salmon makes them better adapted to temperature increases because they use oxygen more effectively.
"Salmon's hearts are built to swim long distances, to undergo hard work for long periods of time, whereas the wrasse uses more short bursts of energy," said Neff. This means the wrasse uses more energy, and hence more oxygen.
Because salmon have such marathon hearts, they also have lower resting heart rates.
"The lower the resting heart rate, the more room the heart has to do work before reaching its maximum," said Neff. "This gives the salmon more of a scale to work with in adjusting its heart workload."
Neff wondered if within salmon populations, natural selection was already generating climate-adapted salmon, with lower-than-normal heart rates. He was right, but this wasn't the only front salmon were adapting on. Salmon born from larger-than-average-sized eggs also proved better at withstanding temperature changes.
"The good news is that both are heritable genes," said Neff, which means they could become dominant all on their own if they prove crucial to future survival.
Neff is optimistic about his findings, but also, like Hickey, is quick to point out the holes still left in the larger picture.
"These kind of studies should alert stakeholders to the importance of gaining rules of thumb for different species going forward," said Neff. "We will never have complete data, and we can't study every species of fish in existence, but we know they all will respond differently and we have to think about that."
"Much more work needs to be done to understand how plastic, or flexible, ectotherms in general are to temperature change," added Hickey, "but maybe more importantly, a pattern must be established. If we look out to deep-sea species like tuna and see the same processes taking place as, say, the wrasse, the issue would become a much more worrying one and strike more public interest."
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.
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