21 January 2014

QBI researchers at UQ have discovered how a species of squid perceive distance, providing an evolutionary solution to a problem divers regularly encounter in featureless waters.

Sepioteuthis lessoniana, the bigfin reef squid

QBI researchers at UQ have discovered how a species of squid perceive distance, providing an evolutionary solution to a problem divers regularly encounter in featureless waters.

The scientists studied how Sepioteuthis lessoniana, more commonly known as the bigfin reef squid, overcome blurred vision due to a retinal bump caused by their brains pushing against their eyes.

PhD student Wen-Sung Chung discovered that these squid use a characteristic bobbing motion to not only balance-out their visual impairment, but precisely target prey.

“This remarkable range-finding mechanism allows for hunting, defence, and object size identification in an environment where depth perception is otherwise very difficult,” Mr Chung said.

“When the blue surgeonfish ‘Dory’ first saw the whale in the movie Finding Nemo, she was confused by its distance in the featureless mid-water world of the open ocean and assumed it was small and nearby, not huge and far away. This is exactly the problem that the unusual optics of squid eyes has solved ,” he said.

“The tools for distance judgment animals use on land, such as stereopsis and parallax are of no use at any distance underwater even in the clearest waters.

“This new proposed range finding mechanism in squid could help Dory to resolve this.”

As the shape of this soft-bodied creature can be easily deformed during catching and handling, observations of the distorted eye were previously overlooked.

To prove the retinal bumps were not the result of post-mortem damage from capture, non-invasive methods were applied to investigate an intact specimen, including a miniature high resolution ultrasound scanner.

Mr Chung also built a device for infra-red retinoscopy, to observe the eye shape of free-swimming squid.

It then took a year to develop a subtle range-finding model in which image blur can be used for coding distance to guide squid to enter the strike zone where their tentacles can reach.

“This discovery is another case of ‘matched filtering’, where animals only need a simple cue to know when and where to strike without complex neural computation,” Mr Chung said.

“Using matched filtering is particularly important for invertebrates, most of which do not have a big enough brain to deal with something complex.

“Therefore, they develop different strategies such as defocussing or polarised vision to remove ‘distractive factors’ to reduce load on their nervous system.”

Japanese researchers are currently using a similar concept to design a new form of robotic vision using a sweeping sensor, which is similar to the squid’s head bobbing mechanism.

“After we made this discovery, we studied other species and found that six others – another squid and five types of cuttlefish – also show retinal bumps,” he said.

The retinal bump has not been found in any deep sea squid studied so far, illustrating that the range of light conditions at different depths drive squid to develop different eye designs.

Mr Chung’s search for squid specimens also extended internationally, and famously he found himself involved in another world first – capturing the first ever footage of live giant squid in their natural habitat.

The results of the research, which was funded by the Australian Research Council and Asian Office of Aerospace Research and Development, are published in the journal Current Biology with a front page cover on 21 January 2014.

Media: Mikaeli Costello, Director Advancement and Communications, Queensland Brain Institute, +61 401 580 685 or mikaeli.costello@uq.edu.au; Professor Justin Marshall, Queensland Brain Institute, +61 7 3345 1397, +61 423 024 162, or justin.marshall@uq.edu.au