Vision in Elasmobranchs
The location of the eyes upon the body dictates what part of the environment an organism sees and strongly influences how it perceives the world. We quantify the extent of the visual field in a variety of elasmobranch species that differ in head morphology and eye placement.
The sharks and rays are dark adapted and an electro-retinogram technique (ERG) is employed to measure the response of the photoreceptors to a narrow beam of light that rotates around the pupil. The data collected enable us to reconstruct the 3D visual field around the head of the animals and determine if they achieve binocular convergence. Additional tests enable us to determine the spectral sensitivity and the flicker fusion frequency for species that inhabit different environments.
This technique has been employed to quantify the visual fields of hammerhead sharks and their conventionally shaped carcharhinid sister taxa. Enhanced binocular vision has been proposed as a mechanism to explain the evolution of the hammerhead shark cephalofoil. The broadly spaced eyes confer a wider visual field and greater binocular overlap, but also create a large blind area immediately anterior to the snout of hammerheads. Thus, enhanced binocular vision is supported, but at the expense of a trade-off in the blind area.
The Horizontal visual field of an Atlantic Stingray (left) and a clearnose skate (right).