A new paper published in the journal PLOS Biology has demonstrated that a species of fish meets the experimental criteria for self-awareness.

But the research itself deliberately calls into question one of the key tools used to assess such cognition, and an accompanying primer by the legendary Dutch primatologist Frans de Waal invites us to rethink the very notion of self-awareness itself.

At the heart of the research is the bluestreak cleaner wrasse (Labroides dimidiatus), a small fish that lives off the parasites and dead tissue of other fish species in a mutually beneficial arrangement.

The authors of the study, led by Alex Jordan from the Max Planck Institute for Ornithology (MPIO) in Germany, claim to have experimentally demonstrated that these helpful little critters react to their reflection in a mirror, a behaviour known as mirror self-recognition (MSR).

Specifically, the researchers claim that cleaner wrasse have passed a key trial called the “mark test”, used to establish self-awareness, by responding to, and attempting to remove, coloured marks on their bodies that can only be seen via the mirror. Self-awareness is perhaps the highest form of cognition: where consciousness is the awareness of the world around us, self-awareness is the awareness of that consciousness.

MSR was first developed by psychologist Gordon G Gallup in the 1970s and has become the standard test for cognition across species. And the mark test, Jordan and colleagues state, “is held as the benchmark behavioural assay for assessing whether an individual has the capacity for self-recognition”. The tests were originally designed for the study of primates, but have spread well beyond the taxon’s boundaries.

Only a select few species can pass the test: humans, chimpanzees, elephants, dolphins and one non-mammal species, the Eurasian magpie (Pica pica). Passing the mark test has, until now, been widely held to demonstrate self-awareness.

Jordan and his colleagues now add the bluestreak cleaner wrasse to this select list.

Cleaner wrasse are highly social organisms and feed by visually identifying parasites on the skin of client fish, so the researchers reasoned that they have the requisite sensory apparatus and associated cognitive ability to undergo the mark test.

The test has three phases: introduction of the mirror, the reflection in which is taken to be another member of the same species. This is often marked by attempts at social signalling or aggression.{%recommended 5565%}

Phase two sees the organism assess whether the reflection is actually that of the test subject by using idiosyncratic behaviour, such as swimming upside down. The third phase is characterised by observation and exploration of the test subject’s body in the mirror. When this third phase is achieved, the mark test can be undertaken.

The scientists injected a synthetic polymer known as elastomer under the skin on both sides of the fish. One side was clear, while the other was coloured to look like a parasite, thus trying to control for the irritation caused by the procedure.

What they observed was the fish posturing to allow them to more clearly observe the marks, followed by attempts to remove the coloured elastomer by engaging in scraping behaviour, rubbing the side of the body against a hard surface.

“The behaviours we observe,” says Jordan, “leave little doubt that this fish behaviourally fulfils all criteria of the mirror test as originally laid out.”

From this they posit only three possible conclusions: the behaviours observed have been misinterpreted and thus the fish haven’t passed the test; they’ve passed the test and are self-aware; or “that cleaner wrasse pass the mark test, but this does not mean they are self-aware”.

They dismiss position the first possibility and are enormously sceptical of the second, saying “this would require a seismic readjustment of our cognitive scala naturae,” (a reference to Plato’s Ladder of Being), and instead opt for the third conclusion.

So, what does this mean? Well, in part, it brings into question the MSR and mark tests themselves. Both become less and less justified the further one moves from apes and humans, with certainty decreasing as “taxonomic distance increases between the test species and the primate taxa for which the test was initially designed”.

But it also throws doubt on the often-assumed all-or-nothing model of self-awareness and cognition, which de Waal calls the “Big Bang theory of self-awareness”.

Rather than self-awareness appearing fully formed in the select few, he advocates a gradualist model, in which there are different levels across many species.

“What if self-awareness develops like an onion, building layer upon layer, rather than appearing all at once?” he asks.

Indeed, he too doubts that the bluestreak wrasse is self-aware in the sense that humans and chimps are. Rather, he argues that Jordan and his team’s work indicates an intermediary stage of self-awareness.

“My conclusion” he says, “is that these fish seem to operate at the level of monkeys, not apes.”