Getting to the Heart of the Matter in Aquatic Ecosystem Stability

For top predator fish species, having a big heart is not the secret key to their ability to forage across different habitats, a recent study by researchers in the Department of Integrative Biology suggests.

In an aquatic environment, the title of “top predator” goes to species at the top of the food chain. Top predators have the ability to forage in different habitats, a phenomenon that helps provide ecological stability.  But why some species are able to do this and not others has remained a mystery.

One popular theory is that top predators may be more mobile. Profs. Frédéric Laberge and Kevin McCann hypothesized that this could be due to a larger relative heart size in these organisms.  Bigger hearts would provide a greater supply of oxygen and nutrients to the fish’s tissues, which in turn would enhance their ability to hunt and catch prey across habitats. 

The researchers tested their theory by examining the hearts of around 300 fish, from 16 different species, collected from shallow and open water in Georgian Bay. They also used carbon and nitrogen isotope analysis to determine where the fish had been feeding. Isotopes are chemical signatures that can be used to distinguish the source of an animal’s diet.

What they found was that the hearts of fish occupying a high trophic position were larger than species that occupy a lower position in the food chain. However, the species that were feeding across different habitats (that is, in both shallow and open water) did not have larger hearts.

“Top predators are generally characterized by a large body size and high mobility, but increased foraging across lake habitats does not appear to translate to a larger relative heart size,” says Laberge.

What is most interesting about this study, however, is how it ties in with other work that Dr. Laberge recently completed on the brains of the same fish of Georgian Bay. He and colleagues found that, unlike heart size, the fish’s brain size was positively correlated with its ability to forage in different habitats.

“The combination of these findings is exciting because they suggest that behavioural capacity is more important than physical capacity,” explains Laberge. “It seems as though it is not the ability to cover more territory or swim faster that is most important, but the ability to make better decisions.”

When it comes to visualizing a top predator, the image of an aggressive, large-bodied (with the heart to match), and perhaps brutish animal usually comes to mind. How then, does intelligence come into play when it comes to utilizing energy resources from different ecological niches?   In fish, at least, brain power may be the key to understanding complex food webs, and perhaps even the evolutionary trends that have created such systems.

This work was completed in collaboration with Sophy Ding, Joe Scott, and Nicholas Edmunds, as well as the University of Windsor GLIER Stable Isotope Lab. Funding was provided by the Natural Sciences and Engineering Research Council and the Canada First Research Excellence Fund.

Read the full article in the Canadian Journal of Fisheries and Aquatic Sciences.

See also the related article on fish brain size in Ecology Letters.

Read about other CBS Research Highlights.