Even as people recognize the intelligence of many animal species, we lag behind when it comes to fish. Expressionless—at least to our eyes—and living in an alien environment, their smarts have gone largely unnoticed, even by scientists.

That, however, is finally changing. Findings of complex fish cognition, from counting ability to tool use and even emotion, are becoming commonplace. And to those feats, add another: fish possess a sophisticated form of memory considered foundational to the rich inner experiences of other animals, including Homo sapiens.

Called episodic memory, it’s the ability to recall the what, where and when of events, categorizing the raw material of experience into some sort of coherency. Not long ago, episodic memory was thought unique to humans and the brainiest animals—yet as described in a study published in the journal Animal Cognition, zebrafish have it, too.

If zebrafish, a so-called model organism from whom findings are extrapolated to other species, have episodic memories, they’re likely a regular feature of aquatic life. “I would not be surprised if many other fish also demonstrated this type of memory,” says Trevor Hamilton, the study’s lead author and a neuroscientist at Canada’s MacEwan University.

Hamilton’s team presented individual fish with one of two different plastic figures in different locations within aquaria with walls painted either yellow or blue. The researchers could manipulate these variables such that, if their fish remembered episodically, they’d react differently to familiar objects in unfamiliar contexts. And indeed they did.

Though other research, on cleaner wrasses, has also suggested episodic memory in fish, Hamilton’s testing was especially rigorous. “Showing it scientifically is a little difficult,” says Culum Brown, a fish cognition specialist at Australia’s Macquarie University who was not involved with the research. “These authors have done a very good job.”

To Hamilton, the findings have immediate implications for using zebrafish in research on learning and memory. The ramifications may not end there. Until very recently, the sorts of conversations prompted by a deeper awareness of animal intelligence, about welfare concerns and moral responsibilities and the sheer neatness of living on a planet populated by other intelligences, rarely involved fish.

That’s now changing, and studies like Hamilton’s add to the case for regarding fish anew. Fish don’t exist purely in the moment; they swim through their memories, too. The more we learn about them, the more we seem to have in common. —Brandon Keim (Twitter / Facebook) | 7 September 2016

Source: Hamilton et al. “Episodic-like memory in zebrafish.” Animal Cognition. 2016.

Image: Tohru Murakami / Flickr