As climate change causes the world's waters to warm, many aquatic animals are finding themselves in new locations because, for many, moving away is the only viable option to cool down. If they stay in hot enough water, fish lose their ability to stay upright, leaving them unable to find prey and making them easy targets for predators. But what happens to fish before they can no longer stay upright? Generally, when the water gets too hot, fish get more agitated and look for a cooler place to stay. Ian Bouyoucos, Alyssa Weinrauch, Ken Jefferies and Gary Anderson of the University of Manitoba, Canada, think that there may be other signs – besides becoming agitated – that scientists could use to see if fish are stressed because of the temperature of the waters that they live in. To answer this question, the researchers turned to the pacific spiny dogfish (Squalus suckleyi) to help them.
First, the team needed to find out what temperatures the sharks could withstand before they started showing signs of being unable to move properly. After putting the sharks in individual tanks and letting them recover from the move, the researchers slowly began increasing the temperature. The sharks started to become agitated at 23.2°C and started to show muscle spasms at 24.6°C, suggesting that this was the highest temperature that they could tolerate and still move. A week later, the team then began the arduous task of measuring the blood pressure of the sharks. After the sharks were fitted with blood pressure monitors, the researchers placed the dogfish in individual tanks before slowly warming up the water. Bouyoucos and colleagues were able to use the blood pressure measurements to estimate the shark's heart rate. As the water got hotter, the shark's heart rate more than doubled from ∼26 to ∼54 beats min−1, reaching this peak at 22.6°C; a temperature that is lower than both the maximum and the temperature at which they start getting agitated. This means that the warmer waters are affecting the dogfish before they start showing any outward signs. That then led to another question: are there any other signs that the sharks are stressed before they start becoming agitated?
Bouyoucos and colleagues took blood and tissue samples from the sharks at their normal temperature and again at the temperature when their heart rate was at its highest. They found that levels of lactic acid increased in the blood and the pH inside the blood cells was decreased – both signs that the animal is stressed. Additionally, the team found that the levels of the enzyme citrate synthase were higher in the muscle. This enzyme increases when the mitochondria are needed to make more energy for the cells to use, suggesting that these dogfish require more energy to keep their body functioning. If the muscles and blood were showing signs of stress at this temperature, surely there must be something that tells the shark that the water is too hot to stay in.
The researchers next turned to a protein called heat shock protein 70. The levels of this protein get higher when the animal is too hot in an attempt to protect the tissues. The team found that the RNA levels of this protein were higher in the gills, hearts and brain tissue of dogfish in warm waters. This suggests that these organs are extremely important for telling the animal when it's time to move to a new environment. This also gives scientists a new tool to use for determining how climate change will affect where species can live.
