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When exposed to an increase in the water temperature of their habitat, zebrafish, three-spined stickleback and flounder adapt more quickly than goldsinny wrasse, which dwells in deeper waters.

That's according to research led by Jérémy De Bonville, a lecturer and Ph.D. candidate in Université de Montréal's Department of Biological Sciences under the supervision of professor Sandra Binning.

His results, based largely on observations made during internships in Scandinavia, were recently in the Journal of Thermal Biology.

From the sea to the lab

Thermal acclimation refers to an organism's ability to adjust to heat exposure. To study it in fish, De Bonville began by observing zebrafish (Danio rerio) in a laboratory during an internship at the Norwegian University of Science and Technology.

He then continued his research at the Kristineberg Marine Research Center in Sweden, where he studied specimens of three species he had caught himself: three-spined stickleback (Gasterosteus aculeatus), goldsinny wrasse (Ctenolabrus rupestris) and European flounder (Platichthys flesus).

"These species come from different environments," De Bonville said. "The first two live in shallow coastal waters, at a depth of one or two meters, while the goldsinny wrasse inhabits deeper waters where temperatures are more stable."

Measuring maximum thermal tolerance

To assess the heat tolerance of the fish, De Bonville and his Scandinavian colleagues placed them in aquariums and gradually raised the water temperature "by 0.3°C per minute, until the fish lost its balance and turned on its side," said De Bonville. "We would then return it to cooler water and it would recover its normal state within around 30 minutes."

Using this method, the researchers were able to determine the maximum temperature the fish could withstand and how its heat tolerance evolved over time. "Our hypothesis was that a fish accustomed to 20°C water could tolerate a maximum of, say, 35°C, but if it acclimated to 25°C water, then its maximum tolerance might go up and reach 38°C," De Bonville said.

The research team then exposed individuals of each species to 5°C temperature increases and measured their maximum thermal tolerance after three, six and 24 hours, and then after four, 10 and 21 days.

Different strategies for different habitats

The results show that fish from shallow waters, such as the three-spined stickleback and zebrafish, adapt more quickly to temperature changes. "The heat tolerance of these two species changed after just three hours," De Bonville said. "The stickleback achieved the highest tolerance: 34.2°C, which was 1.4°C higher than its initial level."

The goldsinny wrasse acclimated after six hours and increased its thermal tolerance by 2.8°C to 31.7°C, but did not achieve a steady state after 10 days.

The flounder was the slowest to acclimate, taking four days to increase its tolerance by 1.2°C, also to 31.7°C.

These last two species also use other strategies to cope with rising water temperatures. "Flounder can dig into the sand to escape heat, while goldsinny wrasse prefer to move to colder, deeper waters," De Bonville reported. "These behaviors allow them to avoid water that's too warm rather than acclimating to it."

In the case of zebrafish, which are native to the rivers of India, the tolerance of both juveniles and adults changed rapidly, within three hours, and their acclimation was complete in just four days.

Valuable lessons

In light of these findings, can we conclude that fish may be able to adapt more easily to global warming than previously thought?

"We're not there yet," De Bonville cautioned. "Our results provide insights into the strategies some species might use, depending on their natural environment: some rely on rapid physiological acclimation, others on avoidance behaviors."

De Bonville also noted that other environmental factors, such as the presence of parasites, can also affect the ability of fish to acclimate.

He is currently working with master's student Andréa Serres at UdeM's biological research station in the Laurentians on her research project on pumpkinseed fish.

"We're looking at how parasites affect their ability to adapt to temperature changes," he said.

"The preliminary results are interesting: populations living in a parasite-free lake acclimate faster and can withstand higher temperatures than those in lakes where parasites are present."