The coral trout & Octopus: An Unlikely Alliance

The coral trout & Octopus: An Unlikely Alliance

The coral trout & Octopus: An Unlikely Alliance: The coral trout is a grumpy-looking member of the grouper family, and it appears to confront a reef octopus out of its lair.

The coral trout & Octopus: An Unlikely Alliance

The coral trout & Octopus: An Unlikely Alliance

Soft-bodied octopuses are vulnerable out in the open, but the coral trout is mainly a fish-eater, so the octopus seems unperturbed. The grouper appears interested in a small fish or crustacean hidden under a branching coral, but it is too big to get at it.

It swims slowly back and forth, and then manoeuvres so that its head points down at the crevice in which the target is hiding. It shakes its head back and forth, pointing deliberately at the potential meal.

The octopus responds, its soft and highly flexible arms enabling it to probe the crack that the large fish is unable to enter, and flushes out the prey, which one of them grabs.

The coral trout & Octopus: An Unlikely Alliance

It is surprising behaviour. Both animals must have learned it. Octopuses are known for their intelligence, but most fish generally are not. This fish, however, has learned to point. It uses a sign language, dubbed the ‘headstand signal’, to reach across the vertebrate-invertebrate divide and encourage another species to help it hunt.

Until now, this kind of gesturing behaviour has been associated mainly with apes and birds in the crow family, such as ravens, but several species of groupers are now known to ‘point’.

In the Red Sea, for example, roving coral groupers team up with giant moray eels, but not all eels play along, so the gesturing grouper must also learn which individuals are worth asking for help and then return to them time and again.

Fish have small brains for their body size compared, say, to primates, so the big question for scientists is how fish can carry out complicated tasks like these with such limited brainpower.

However, although the animals cooperate, it is thought that individual interests ultimately drive the behaviour, a way to gain an advantage on the highly competitive tropical coral reef. 

While these two characters demonstrate a new understanding of the interdependence of marine creatures within a defined habitat like a tropical coral reef, another ‘first-time-for-television’ story explores the connection between two very different worlds, one above the waves, the other below.

It occurs in October on a small island in the Indian Ocean. Like the grouper and octopus, it has probably been going on unnoticed for many years. Only now can its story be told.