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Remember in Forrest Gump when Bubba is droning on about all the different ways you can prepare shrimp? Well, you could do a pretty good parody of that scene by reciting the myriad ways fishes engage in cannibalism.
Cannibalism, of course, refers to any instance of an animal consuming a member of its own species. And while that probably calls to mind adult fish duking it out, typically a larger fish is gobbling a younger one.
When too many eggs of a female Egyptian mouth-brooder die, she just up and swallows the rest. Plenty of fish eat their smaller and less developed siblings, but Dorada larvae will attack and devour their big brothers and sisters. And male razorfish cozy up to females in what looks like an attempt to mate, but when the ladies let the eggs loose, the males gobble up the eggs and bolt.
Scientists have been studying cannibalism in fish for nearly as long as they’ve been studying fish, says Larissa Strictar Pereira, a biologist at Brazil’s Universidade Estadual de Maringá. And in general, the behavior has been considered rather common. According to a review published in 1991, “Finding examples of cannibalism is not difficult, and it may be more interesting to look for taxa in which the behaviour does not take place.”
But if all the cool fishes are eating each other, why couldn’t Pereira find any evidence of cannibalism in the upper Paraná River floodplain, which encompasses areas of Brazil, Paraguay, and Argentina, and the field site for her research into river habitats? The contradiction prompted her to conduct a review of all the papers published about fish cannibalism to see if she could find out just how widespread it truly is.
In the end, Pereira says she and her coauthors expected to find many more instances of cannibalism than they did. While the team catalogued more than 1,000 publications reporting the behavior in both fresh and saltwater fishes, they found that cannibalism has only been observed in approximately one percent of the 30,000 fish species known worldwide.
While she says this line of inquiry led her to many fascinating and horrifying insights, like the fact that some parents will eat their offspring even when there’s plenty of other food present, the search also got her interested in investigating another theory presented in the literature—the idea that cannibalism might correlate with latitude.
When Pereira sat down with all her data, she found that fishes do actually get more bitey in the mouthy the farther north you go. The correlation was less strong for the southern hemisphere, though that might be because there’s significantly more ocean down south and generally fewer stark differences between habitats and the species they contain in these latitudes.
And this jives with what was found in earlier research—tropical fishes seem to engage in less cannibalism than temperate fishes. Pereira and her coauthors suggest that this is because tropical waters tend to contain more biodiversity. And where there are plenty of other fish in the sea, evolution should favor eating other species before your own.
Stay tuned though, because Pereira is just getting started.
“I gathered a huge amount of data with this research and I still have some questions to answer,” she says, “and my curiosity won’t leave me alone if I don’t do so.”