What squid hear

Loligo pealei   Marine biologists are starting to get a good idea now of how squid hear and how they react to sounds in the ocean. It is only recently that scientists have come to accept that cephalopods have any auditory capability at all.
But new experiments show noises of varying loudness and frequency will elicit a range of behaviours in the animals - such as jetting or inking, and even a change of colour.
The research has been featured at the biennial Ocean Sciences Meeting.
It was presented by Aran Mooney from the Woods Hole Oceanographic Institution (WHOI) of Massachusetts, US.
He is interested in squid because they represent something of a keystone species in the ocean, sitting right at the heart of many food webs. If they are not the predator in those webs, they will almost certainly be the prey.
And that makes it important to know whether anything we may be doing in the oceans could be unsettling them.
"We produce a lot of noise through exploration of the oceans, scientific research, oil drilling, gas exploration and commercial shipping," Dr Mooney tells BBC News.
"A lot of that is low-frequency noise, which is what squid detect. And if we're influencing these animals then presumably we could change their behaviour."
Previous work at WHOI's Sensory Physiology and Sensory Ecology Lab had established that squid could hear sounds in the range of 50Hz to 500Hz; but they are best below 300Hz. It is the same sort of range as fish.
The squid use two closely spaced organs called statocysts to sense sound.
"I think of a statocyst as an inside-out tennis ball," explains Dr Mooney.
"It's got hairs on the inside and this little dense calcium stone that sits on those hair cells.
"What happens is that the sound wave actually moves the squid back and forth, and this dense object stays relatively still. It bends the hair cells and generates a nerve response to the brain."
The latest research has attempted to gauge exactly what sound means to the squid, and how they might use it.
In a tank in his lab, Dr Mooney plays noises of varying loudness and frequency to the animals, and watches for their response. He has been able to map how the different levels of sound will prompt the cephalopods into different behaviours.
"They react in about 10 milliseconds," he says. "That's really fast; it's essentially a reflex. That's really important in terms of behavioural responses because they're not thinking about processing it; they're not deciding whether they should react - they're just doing it.
And he adds: "The responses can be really dynamic. They can be a change in colour; they can be jetting (moving quickly) or inking responses. Squid are also very cool because you can look at a range of colour changes - is it a really startling colour change or a more subtle change?
"Squid can probably use their hearing to find their way around the environment - to sense the soundscape of the environment; for example, to find their way towards a reef or away from a reef, towards the surface or away from the surface."
The research to date has concentrated on the longfin species (Loligo pealei), an important fishery on the East Coast of the US. But Dr Mooney plans to extend his studies to other cephalopods, such as another Loligo species on the West Coast and the eponymous Humboldt squid, which represent the largest invertebrate fishery in the world. Already in the lab, work has started on cuttlefish.
"They will ink more and jet more," says Dr Mooney. "We're finding subtle differences between the species that I wouldn't necessarily have expected. You just don't know until you try it. Go to octopuses and you may get a different response again; and the same is true with the really big squid."
Getting a giant squid into the WHOI tank might prove problematic. These creatures are rarely sighted, let along caught; and certainly not alive. But ongoing research is trying to establish more details about the functioning of squid statocysts, and from that information it may be possible to make some useful predictions about auditory talents of the oceans' largest cephalopods.