Meeting Abstract
The midwater squid Galiteuthis has prominent photophores on the ventral surfaces of its eyes. These photophores presumably function in counter-illumination, a common strategy for camouflage from predators looking up for shadows in the strongly downwelling radiance of this habitat. The Galiteuthis photophore organ consists of densely-packed, fiber-like cells with a complex semi-coaxial geometry of repeating, dense layers. The layers are made of densely-packed protein surrounding a core of active cytoplasmic and have previously been suggested to function as light guides. Here, we computationally modeled light transmission through Galiteuthis photophores using the finite difference time domain (FDTD) method. We found that the cells are capable of guiding light, but we also observed a surprising degree of variation in both Galiteuthis light guide geometries and in the relative efficiencies of these cells in guiding light. Many of the most common geometries in the eye result in “leaky” light guides, however efficient light guides are also observed less frequently. When we placed these light guides in the context of the radiance distribution where Galiteuthis lives, we gained insight into a possible adaptive reason for this inefficiency. At the depths where Galiteuthis is found (250 – 600 m), the angular distribution of light is surprisingly variable both with depth and the content of the water column above the animal. Our calculations show that the light guides in Galiteuthis subocular photophores are able to recapitulate this variation in angular distribution of light. We suggest that by selectively activating geometrically distinct populations of its light-guiding cells for bioluminescence at different depths, the animal would be able to reproduce the environmental angular distribution of light at all positions in its habitat.
