Refractive state and accommodation in the eyes of free-swimming versus restrained juvenile lemon sharks (Negaprion brevirostris)

Optical measurements of the refractive state of the eyes of various shark species typically have depicted sharks as hyperopic (far-sighted) with little evidence of accommodation (i.e. the ability to change focus for visualizing objects at different distances from the eye). In this study, we used infrared video retinoscopy to measure the refractive state in juvenile lemon sharks (Negaprion brevirostris). This technique allows dynamic measurement of refractive state in free-swimming animals as they pass by an aquarium window. We found that unrestrained lemon sharks are focused emmetropically relative to a 1-m distant photorefractor for the lateral visual field. However, when restrained either right side up or upside down (the latter inducing tonic immobility), the sharks become increasingly hyperopic, an artifact also reported in some other vertebrates. In addition, unrestrained lemon sharks display small amplitude accommodative excursions. Thus, refractive state measurements on restrained sharks in general may not reflect the natural, resting state of the shark eye, but rather, an induced hyperopia and lack of accommodative function. Such an artifact may be present in other vertebrate species, underscoring the need to obtain measurements of refractive state in unrestrained animals.