research

Vision in elasmobranchs

An anaesthetized lemon shark in an ERG rig.

The pupil of a lemon shark dilates as it is dark adapted over 30 minutes.

Although it has been suggested that the laterally displaced eyes of hammerhead sharks might confer a broader visual field and greater binocular overlap, it is more likely that the widely spaced eyes are detrimental. The broadly spaced eyes create a large blind spot immediately anterior to the snout of hammerheads. We are quantifying 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 used 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.

The horizontal visual field of an Atlantic stingray (left) and a clearnose skate (right).

	McComb, DM & SM Kajiura. 2008. Visual fields of four batoid fishes: a comparative study. Journal of Experimental Biology 211: 482-490.
	Nelson, PA, SM Kajiura & GS Losey. 2003. Exposure to solar radiation may increase ocular UV-filtering in the juvenile scalloped hammerhead shark, Sphyrna lewini. Marine Biology 142(1): 53-56.