New research published this month in the Journal of the Royal Society Interface investigates radiance-camouflage in the midwater squid Galiteuthis.
The deep sea is the Earth’s largest habitat by volume and is rich in species. Light drives the environment that ocean organisms experience and, as a result, there are many specialized adaptations.
The mid-water squid Galiteuthis has a transparent body except for its eyes. It has photophores, or light organs, under its eyes. In the three dimensional environment of the ocean’s midwaters, the shadows of Galiteuthis eyes would be visible to predators but for the presence of these light organs.
Photophores are prominent bioluminescent organs that are thought to function in counter-illumination. Counter-illumination is a clever adaptation, in which photophores emit light to match both the intensity and colour of downwelling sunlight to erase any shadows cast by Galiteuthis eyes, hiding their eyes from upward-looking predators.
The Galiteuthis photophore organ consists of laminated, fibre-like cells with dense protein layers surrounded by cytoplasm. The structure of these photophore cells is similar to that of ‘Bragg fibres’, which are synthetic waveguides that guide light by multiple reflections off concentric, alternating layers and as a result confine light to a central core. In addition, the cells found in the Galiteuthis photophore have been suggested to function as ‘light guides’.
Dr Allison Sweeney and her colleague Dr Amanda Holt, using a combination of methods from electron microscopy of photophore tissue to computational modelling of light transmission through Galiteuthis photophores, found that these cells are indeed capable of guiding light.
However, the results show that efficient light guides are not observed frequently. Instead the Galiteuthis squid relies on imperfections of modern Bragg fibres to optimise radiance camouflaging from predators who hunt using downwelling light. There is also variation in both the Galiteuthis light guide shapes and in the efficiency of guiding light, which result in ‘leaky’ light guides.
There may be a possible adaptive reason for this. The radiance distribution is variable at the depths where Galiteuthis lives and these ‘leaky’ light guides are able to reproduce this variation in the distribution of light in its three dimensional environment. In comparison to the fully ordered Bragg fibre, these light guides have evolved to be very nearly, but not quite, perfect. The findings of this study present an exciting opportunity for material scientists to consider the potential of irregularities in synthetic materials to increase performance for applications in optics.
Dr Allison Sweeney is an associate professor at the University of Pennsylvania who works at the interface of Biology and Physics. She is interested in how sea creatures have evolved light structures to manipulate light in their environment to aid organismal camouflage.