Fish cannibalism is rare in nature, study finds


X-ray image of an adult female Bahamas mosquito where a fish she had eaten can be seen inside her, revealing a case of cannibalism. Credit: Brian Langerhans

Mosquitoes and guppies, although known to be cannibalistic in captivity, are extremely unlikely to be cannibalistic in wild settings, and the rare instances of cannibalism in these fish are likely due to heavy competition for food. The conclusions, of a new Ecology and evolution conducted by US and UK researchers, could have implications not only for fish hobbyists and scientists who use mosquitoes as models for ecological and evolutionary studies, but could also help explain the causes and frequency of cannibalism in fish. other animals.

Cannibalism, attacking and eating other individuals of your own species, is a particular behavior, which features prominently in human mythology and fiction. But how common is this in nature, and why would organisms resort to such an extreme course of action just to get a meal?

Brian Langerhans, associate professor of biology at North Carolina State University, and Rüdiger Riesch, senior lecturer in evolutionary biology at Royal Holloway University in London, set out to find out by examining more than a decade of data collected from nearly of 12,000 fish of 17 species. in nature.

“This is data accumulated from several different projects over the years,” says Langerhans, lead author of the study. “To identify the mechanisms responsible for this type of phenomenon in nature, we needed very large samples. So we accumulated data from this work while doing other projects. »

“In captivity, mosquitoes and guppies will practice cannibalism commonly enough that there are protocols in place in research labs and aquaculture to quickly separate offspring from larger fish,” says Riesch, the author. corresponding to the work. Riesch started the project when he was a postdoctoral researcher at the Langerhans laboratory between 2010 and 2012.

“But when you look at the diet of fish in nature, you really don’t find a lot of evidence for it,” Riesch says. “We wanted to know if and why cannibalism occurs in nature.”

The research team examined the diets of 11,946 fish in the wild, using dissection or X-rays to determine what the fish had eaten. They found just 35 cases of cannibalism, in just three mosquito species – less than 0.30% occurrence.

Cannibalism was most common in populations with very high levels of competition for food; that is, populations devoid of major predators where the population densities of the fish studied were particularly high.

To experimentally test possible causes of cannibalism, the team studied 720 additional fish by creating “mesocosms,” large outer containers (6 feet in diameter) that recreated the fish’s natural environment but allowed researchers to control elements such as population density, predation risk and resource availability. The fish inside were observed for a week to determine what might be influencing the cannibalistic behaviors. The results of these experiments also indicated that population density and resource availability were the main drivers of cannibalism.

“Resource competition appears to be the strongest predictor of cannibalism,” Langerhans says. “We have also seen that a lack of predation has an indirect effect on cannibalism: the release from predation allows population density to skyrocket, which decreases resources. This same driving factor may be responsible for many cases of cannibalism in the animal kingdom in the wild.

The team was also able to rule out some potential causes of cannibalism.

“Cannibalism does not occur when larger fish encounter smaller fish more frequently,” Langerhans explains. “Furthermore, it was not simply large body size that explained which individuals cannibalized – females, who are larger, cannibalized significantly more than males, but this seems more related to their greater energy requirements for carrying live young than their actual size.”

The work has implications not only for hobbyists or those trying to save and repopulate endangered species, but also for researchers working in evolutionary biology and using the mosquitofish as an animal model.

“Cannibalism in these fish is a problem that biologists routinely deal with in laboratories and hatcheries, so it has been widely considered to be at least somewhat common in nature,” Langerhans said. “But we’ve shown here that’s really not the case.

“These fish are used as models for evolutionary work – quantifying the evolution of traits – in laboratories. Now that we know that cannibalism is not a common behavior in nature, we know that unnatural rates of cannibalism could alter traits in the lab in ways that affect the results and implications of the study, especially in behavioral evolution studies.

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More information:

Rüdiger Riesch et al, Resource competition explains rare wild cannibalism in live fish, Ecology and evolution (2022). DOI: 10.1002/ece3.8872

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Fish cannibalism rare in nature, study finds (2022, June 2)
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