How cuttlefish dazzle prey with a moving skin pattern

But first, we take to the depths of the ocean to examine the remarkable hunting technique of the broadclub cuttlefish. These cunning cephalopods are frequently featured on marine life documentaries because of their remarkable ability to camouflage themselves, stupefying their crab prey in the process. It has remained a bit of a mystery - but Matteo Santon at the University of Bristol thinks he’s cracked the shell…

Matteo - What is unique about this species is that it uses a very special hunting technique. When it's approaching prey, for example, crabs, what this cuttlefish does is that it changes the appearance of its head to a homogeneous white colour, then it's stretching six of its eight arms forward into a tight cone, spreading, stretching laterally the remaining two arms with a flat surface pointing towards prey. And at that point, the cuttlefish starts passing highly contrasting dark stripes in a downward direction across the head and the arms, and it's approaching prey until the strike. And we were particularly interested in trying to understand why this species is doing this. And there have been a few theories that were put up by natural history documentary filming, and that was mostly saying that this was some sort of mesmerising or hypnotising display that was somehow thought to help the cuttlefish catch prey. In particular, as a visual ecologist, we were interested in trying to understand how this actually worked.

Chris - Does it reserve this particular technique specifically for certain types of prey, or is this its modus operandi, regardless of what it's hunting?

Matteo - So the cuttlefish is actually quite peculiar because it can use this one hunting display that we call the passing stripe display, but it can also use up to three other types. We are still trying to understand whether there's actually a way that cuttlefish chooses between this display, but what I can tell until now is that it's just using them interchangeably.

Chris - And what effect does that seem to have on the prey when they experience this? Obviously it works because the animals use it, and if it didn't work, they'd go hungry, wouldn't they? So what effect does it have on the prey? What effect does it appear to have on the prey items?

Matteo - It's interesting because prey items like crabs, they're very sensitive to approaching cues, to sort of looming objects that come towards them, and they would respond normally by running away, by escaping. Whereas what we can observe when the cuttlefish is hunting with this display is that crabs don't seem to do anything. They just wait until the cuttlefish goes and grabs them. And that's something quite interesting because to us this hunting display appears very conspicuous. And that's one of the reasons why I thought there's maybe something more going on here.

Chris - So what do you think is going on and how did you try and test it?

Matteo - We've been using three different approaches to try to understand this question. And the first one was working with crabs in lab. And what we've been doing is that we've been feathering crabs on a sort of treadmill, like the ones that humans use, or very similar, made of styrofoam. And the in front of a LCD monitor. And what we then did is that we started to display to crabs expanding predator stimuli, which could have either moving stripes passing on them, very similar, trying to represent what the cuttlefish is doing, or stimuli without moving stripes, static stripes, or stimuli without any stripes at all. And then we had a second approach, which was based on actually going to the field, using scuba diving as a method, and try to really film this type of hunting display in the wild, and try to describe them in detail and try to understand whether there was something in the way that this cuttlefish was doing the display that could tell us something that was particularly relevant on the function of the display.

Chris - And what do you think it is? How do you think it's working?

Matteo - What we found out is that crabs seem to be less sensitive to respond to expanding stimuli when they feature moving stripes, which is sort of hinting us that cuttlefish may be using this just to prevent the crab from seeing the approaching cuttlefish. And we also found, while being in the wild and filming cuttlefish, is that somehow we saw that the faster is the approach of the cuttlefish towards prey, then the faster the passing stripes had been moved across the head and the body, which somehow is telling us that there is some sort of link, clear link, between the strength of the approach of the cuttlefish and also the strength of the motion that is generated by the moving stripes. And we think that this is in fact telling us a little bit how this hunting display works. We think that in this display, threatening motion cues of the predator, of the approaching cuttlefish, are actually overwhelmed by the strong non-threatening downward movement of the stripes that a cuttlefish is producing.

Chris - I was going to say, do we, based on what we understand about the way that a crab's visual system works, do we have any clues as to the effect that this moving stimulus might be having on them? Is it a distraction? Is it that they're so fascinated by it that it blurs and sort of obfuscates the approach of a predator and their interest in the stimulus motivates them to watch it more? Is that how it works?

Matteo - What is important to understand here is that crabs, they don't quite see very well, so their world is very blurred and often not in colour. And what their visual system is really fine-tuned to detect is expanding predator motion cues, which is something that animals in general get scared quite a lot if they see something approaching them very quickly. So their visual system is really finely tuned to detect a radially expanding approaching motion. And instead, what the approach of this cuttlefish with a passing stripe does is that the visual system of crab would not detect a radially expanding predator approach, but rather a strong unilateral downward motion, which is generated by the stripes, and therefore not really respond at all to predator. So we actually think that the crab is not really detecting the predator approach and is somehow invisible to the visual system of crab, and therefore it doesn't respond to it. And somehow this will give cuttlefish a hunting advantage and enough time to go close enough and then catch prey.