Vision in the southern hemisphere lampreyMordacia mordax: Spatial distribution, spectral absorption characteristics, and optical sensitivity of a single class of retinal photoreceptor

The peak density of retinal photoreceptors in the southern hemisphere lamprey Mordacia mordax reaches 33,200 receptors/mm2 in the ventro-temporal retina.

• The photoreceptors have characteristics of both rods and cones, including a large refractile structure and a long cylindrical segment.
• Estimated peak spatial resolving power is 1.7 cycles per degree, corresponding to a minimum separable angle of 34'7''.
• Optical sensitivity is measured at 0.64 microm² steradian for white light and 1.38 microm² steradian for the preferred wavelength.
• The visual pigment in the outer segment is a rhodopsin with a maximum absorbance at 514 nm.
• The ellipsosome shows very low light absorptance, suggesting it does not function as a spectral filter.
• Adaptations in receptor packing and structure are associated with enhanced low light vision and improved photon capture.

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