31 March 2023

Fish Recognize Themselves

Welcome back. There are animals other than humans that can recognize themselves in a mirror (e.g., great apes, Asian elephants, dolphins, horses, magpies); yet implications for self-awareness--that they may actually be aware of the self--are uncertain and controversial.

I thought you would be interested in a potential breakthrough. A recent study with “cleaner fish,” which are on the list of animals known to be able to recognize themselves in mirrors, provides compelling evidence that the fish achieves self-recognition via a mental image of the self.

Study Elements
Researchers affiliated with Japan’s Osaka Metropolitan University and Switzerland’s University of Neuch√Ętel conducted a series of experiments with bluestreak cleaner wrasse, Labroides dimidiatus, one of several species of cleaner fish found on coral reefs from Eastern Africa and the Red Sea to French Polynesia.

Bluestreak cleaner wrasse, Labroides dimidiatus (from commons.wikimedia.org/wiki/File:Bluestreak_cleaner_wrasse_%28Labroides_dimidiatus%29_%2840460962202%29.jpg).
Cleaner fish, which had never been exposed to a mirror, were kept individually in separate tanks, visually isolated from one another. At one point, the fish were anesthetized and photographed outside of the tank. Using the photographs and open-source software, the researchers prepared four composite photographs for testing: self-face and self-body, unfamiliar face and unfamiliar body, self-face and unfamiliar body and unfamiliar face and self-body.

A. Faces of cleaner fish, B. Composite photographs: SS, self face with self body; UU, unknown face with unknown body; SU, self face with unknown body; US, unknown face with self body (figure 2, www.pnas.org/doi/10.1073/pnas.2208420120). 

Principal Tests

Response to Composite Photographs

Before exposing the fish to a mirror, they were presented the four composite photographs. All fish acted aggressively toward photographs of both themselves and unfamiliar strangers. After exposure to a mirror, the fish acted aggressively toward photographs of the three unfamiliar face or body but not toward the photograph of themselves, suggesting they recognized the motionless self-face/self-body photographs as the self.

Frequency of cleaner fish aggressive behavior toward composite photographs of self (SS), unfamiliar fish (UU), self-face/unfamiliar body (SU) and unfamiliar face/self-body (US) before and after mirror-self recognition (MSR) (from figure 3, www.pnas.org/doi/10.1073/pnas.2208420120).

Response to Mark on Throat
The researchers marked the fish subcutaneously on their throat with a brown-colored visual implant elastomer. The fish were monitored while the mirror was covered to ensure the marking did not affect their behavior. When the mirror was uncovered and the fish observed the mark in their reflection, they displayed throat-scraping behaviors on the tank substrate. (It has been speculated that the mark may resemble a parasite, which cleaner fish have a strong motivation to remove.)

Response to Mark on Photograph
Expanding on the mirror-mark test, the researchers tested fish, which had been exposed to a mirror but not to a mirror-observed mark. They exposed the fish to self-photographs with a brown mark on the throat.

The fish were first presented unmarked self-photographs for 5 minutes, then marked self-photographs for 1 hour. Six of eight fish tested displayed throat-scraping behavior after observing a self-photograph with a mark. In contrast, no fish scraped their throat when observing an unmarked self-photograph or a marked photograph of a familiar fish.

Wrap Up
The researchers noted that advanced cognitive abilities and self-awareness in non-primate animals remain controversial, particularly in fish. Still, they judge that their study provides robust evidence that, like humans, cleaner fish achieve mirror self-recognition via an internal sense of self.

Since the target animal is a fish, the finding suggests that nearly all social vertebrates may also have this higher sense of self. Thanks for stopping by.

Study of cleaner fish self-recognition in Proceedings of the National Academy of Sciences: www.pnas.org/doi/10.1073/pnas.2208420120
Article on study on EurekAlert! website: www.eurekalert.org/news-releases/979347

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