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Do Bigfin reef squid listen out for predatory whales?
Octopus and squid can hear.
The discovery resolves a century-long debate over whether cephalopods, the group of sea creatures that includes octopus, squid, cuttlefish and nautiluses, can hear sounds underwater.
Compared to fish, octopus and squid do not appear to hear particularly well.
But the fact they can hear raises the possibility that these intelligent animals may use sound to catch prey, communicate with one another or listen out for predators.
The question of whether cephalopods can perceive sound has been controversial since the early 20th Century. Some experiments suggested that blind octopus seemed able to locate the sounds produced by boats or by tapping on the outside of a tank.
But most cephalopods lack a gas-filled chamber, such as the swim bladders that fish can use to hear. That suggested they could not detect the pressure wave component of sound.
However, sensory physiologist Hong Young Yan of the Taiwan National Academy of Science in Taipei, Taiwan suspected that octopus and squid might use another organ called the statocyst to register sound.
An octopus's hearing is tuned to life on the seafloor
The statocyst is a sac-like structure containing a mineralised mass and sensitive hairs.
Fish also use it to detect sounds, and in previous research, Yan showed that prawns can use their statocysts to hear. "So we extended our work from prawns to cephalopods," says Yan.
Yan's team tested the auditory capabilities of two species, the Common octopus Octopus vulgaris and the squid Sepioteuthis lessoniana, often called the Bigfin reef squid.
They discovered that the octopus can hear sounds between 400Hz and 1000Hz. The squid can hear an wider range of sound from 400Hz to 1500Hz, they report in Comparative Biochemistry and Physiology, Part A.
"That indicates that squid have a better hearing capability than the octopus," says Yan. "Interestingly though, both species hear best at a frequency of 600Hz."
Yan's team had to overcome particular technical challenges to investigate the cephalopods' hearing ability. The usual way to prove that an organism can hear is to measure how its nervous system electrically responds to sound. But that can involve directly attaching electrodes to exposed nerves, an invasive procedure that could harm delicate cephalopods.
So Yan invented a non-invasive method, which involves placing electrodes on an animal's body to measure the electrical activity in its brain. In this way, he could measure within just a couple of hours whether the brain of an octopus or squid responds to sound.
Avoiding being eaten
The discovery could open up a new understanding of cephalopod behaviour.
"The key question which I would like to investigate is what kind of sounds are they listening to?" says Yan. "Perhaps they listen to sound to evade predators and can eavesdrop to sounds made by their prey. Or, perhaps they even could make sounds to communicate among themselves."
For example, because octopus or squid do not possess gas-filled chambers within their bodies, they cannot amplify sounds, limiting their hearing ability. But they can hear as well as invertebrates such as prawns, although less well than many species of fish and the toothed whales which often eat them.
A Bigfin reef squid undergoes a non-invasive hearing test
"Squid are heavily preyed upon by toothed whales including dolphins. So perhaps their hearing would aid them to avoid the pinging sounds made by dolphins," says Yan.
He says the different abilities of the octopus and squid also reflect the environment they live in.
The common octopus dwells on the seabed, which is covered by large rocks, boulders, coral reef and other features. In water, sounds above 1000Hz have a wavelength less than 1.5m. Such sound waves cannot in turn pass objects greater than 1.5m in size, and they get deflected, which could explain why the octopus doesn't need to hear them.
Squid live in the open water where there are fewer obstacles, and therefore sounds pass uninterrupted over a greater frequency range.