Metabolism in Opaleye Fish is Linked to Diet and Temperature

People consume about the same amount whether it is hot or chilly outside. However, a fish’s appetite varies greatly depending on the temperature. Their metabolism is influenced by environmental factors because they are cold-blooded animals: Cold fish take their time to warm up, but when the temperature rises, they go into overdrive.

Researchers at UC Santa Barbara investigated opaleye fish under a variety of temperatures and diets to better understand how temperature affects our finned pals. Diet and temperature had an impact on fish physiology and metabolism, although the effects were different for each feature.

The findings, which were published in the Journal of Experimental Biology, further debunk the notion that an animal’s basic functions all react to temperature changes in the same way.

Opaleye were an excellent model for investigating this connection. From Point Conception to southern Baja California, the omnivorous fish can be found in kelp forests and reefs. Wild opaleye eats more algae in the warmer, southern region of their range than in the colder north, according to UCSB researchers in 2006. Other omnivorous fish follow the same pattern.

“We hypothesized they were doing this because it benefitted them,” said lead author Emily Hardison, a doctoral student in the Department of Ecology, Evolution, and Marine Biology.

The scientists monitored the fish under four different temperature and nutrition conditions to see if this was true. Half of the fish were fed only brine shrimp, while the other half were allowed to eat a combination of brine shrimp and Ulva algae, which opaleye eat in the wild.

The researchers then split each of these groups in two, putting half in 12° C water and the other half in 20° C water. In Santa Barbara, these temperatures correlate to the seasonal extremes that wild fish face.

The researchers tracked the fish’s growth over time and sprint speed as they were subjected to various treatments. They also took measurements of the fish’s baseline metabolic rates as well as their maximal metabolic rates when they were active.

To accomplish so, the researchers utilized a technique known as aquatic respirometry, in which they recorded each fish’s rate of oxygen consumption after activity and during rest.

We hypothesized they were doing this because it benefitted them. We found that diet could influence the temperature response that the fish had, but it wasn’t consistent across all these important measurements that we made.

Emily Hardison

The difference between the maximum and baseline rates represents the animal’s aerobic scope, which Hardison defined as the fish’s energetic capacity to flourish in its environment, including swimming, eating, digesting, finding a mate, and so on.

The researchers also performed tests to determine the fish’s thermal tolerance limitations, which included assessing the animal’s heart’s thermal tolerance. Previous research suggests that in fish, the heart may be the first organ to collapse as a result of heat stress.

“We found that diet could influence the temperature response that the fish had, but it wasn’t consistent across all these important measurements that we made,” Hardison said.

The researchers predicted fish on a mixed diet to have a greater basal metabolism since plant stuff required more energy to digest. Meanwhile, neither nutrition nor temperature had any effect on the fish’s sprint pace.

Diet had little effect on the animals’ growth rates, while temperature had a significant impact. “At 20 degrees, the fish ate so much, and they grew so much. Whereas at 12 they barely ate, and they did not grow,” said co-author Erika Eliason, an assistant professor of ecological and evolutionary physiology.

“All these different rates don’t scale in the exact same way,” she continued. “Growth does not scale in the same way as heart rate, which doesn’t scale in the same way as metabolism. They’re all influenced by temperature in different ways.”

Previously, scientists assumed that an animal’s operations were all optimized for the same temperature range. It made modeling a lot easier. “But biology’s complex,” said Hardison, “and so we think that the optimal range for different biological rates is going to be different.” While this isn’t the first study to question this notion, it is the first to look into how an animal’s diet affects the ideal temperature for certain functions.

Furthermore, the more herbivorous diet did not appear to provide any benefits. Omnivorous fish, for example, could not achieve the same rapid heart rates as their carnivorous counterparts. This shows that their hearts’ capacity may have been diminished as a result of the varied diet.

“That was a big surprise,” Hardison said. Since wild opaleye are more herbivorous in the warmer parts of their range, the researchers had thought this diet might help the fish’s hearts. On the contrary, there were only costs to eating more algae. There were no benefits to the traits that we measured.

“That suggested it’s a more complex story than what we originally thought,” Hardison continued, “and that there could be other ecological reasons why these fish are changing their diet with temperature.”

This is, indeed, a question that remains unanswered. The researchers want to investigate the digestion costs of a herbivorous diet, which could provide insight into the opaleye’s preferences.

The team is also looking into how a fish’s diet can affect how it reacts to heatwaves in the ocean. Animals require energy and nutrients to alter their physiology in order to respond to these events.

As a result, the researchers believe that diet plays a role in how rapidly animals adapt to new environments. “The intersection between nutrition and temperature is really understudied,” Hardison said.

In the context of climate change, it’s critical to understand the relationship between diet and environmental conditions. Not only is global warming rising water temperatures, but it is also altering the nutritional landscape. According to Eliason, climate change might affect food quality and quantity. Even if an animal’s diet does not vary in reaction to temperature, the resources accessible to them may change.

Temperature is simply one of the variables in play. Acidity, salinity, and even dissolved oxygen concentration in the water are all being affected by climate change. It’s still being researched how these interact with the diets and physiology of animals.

Former UC Davis student Tina Nguyen spent eight weeks in the Eliason lab as part of the UC LEADS program, and Krista Kraskura and Jacey Van Wert (both in the Eliason lab) also contributed to this work. Eliason’s Hellman Faculty Fellowship provided funding for the project.

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