Contrast enhancement through structural variations in the rhabdoms of oplophorid shrimps.

Rhabdom morphology in oplophorid shrimps varies both interspecifically and also within the eye of an individual, particularly with respect to the relative sizes of distal and proximal rhabdoms. In this study we have combined published data on anatomical measurements, visual pigment absorption and underwater light distribution to model the absorptance of light from different sources by the rhabdoms in these shrimps. In Systellaspis debilis the violet-sensitive distal rhabdom is prominent and increases in size along a dorso-ventral gradient. The model shows that the dorsal ommatidia are efficient at detecting downwelling irradiance and photophore emissions. In more ventral regions of the eye, the enlarged distal rhabdom results in the ommatidia in this region being more sensitive to eyeshine from other shrimps and to their bioluminescent secretions. This will improve the contrast of such light sources against the background space light. Acanthephyra pelagica has virtually no distal rhabdom and the model shows that this arrangement results in greater sensitivity.