Can cleaner wrasse really recognise themselves in a mirror? Helmut Corneli / Alamy

The cleaner wrasse is only the size of a human finger but it has become the first fish ever to pass the mirror test – a classic experiment used to gauge self-awareness in animals.

Until now, only relatively intelligent animals – including apes, dolphins, elephants and magpies – have passed the test, which demonstrates whether an individual can recognise itself. But in 2016, two manta rays were filmed checking out their reflections in a mirror in a fish tank, suggesting that fish may be able to pass this test too. …