Sharks have long been described as having ‘poor’ vision. They are cone monochromats and anatomical estimates suggest they have low spatial resolution. However, there are no direct behavioural measurements of spatial resolution or contrast sensitivity. This study estimates contrast sensitivity and spatial resolution of two species of benthic sharks, the Port Jackson shark, Heterodontus portusjacksoni, and the brown-banded bamboo shark, Chiloscyllium punctatum, by recording eye movements in response to optokinetic stimuli. Both species tracked moving low spatial frequency gratings with weak but consistent eye movements. Eye movements ceased at 0.38 cycles per degree, even for high contrasts, suggesting low spatial resolution. However, at lower spatial frequencies, eye movements were elicited by low contrast gratings, 1.3% and 2.9% contrast in H. portusjacksoni and C. punctatum, respectively. Contrast sensitivity was higher than in other vertebrates with a similar spatial resolving power, which may reflect an adaptation to the relatively low contrast encountered in aquatic environments. Optokinetic gain was consistently low and neither species stabilised the gratings on their retina. To check whether restraining the animals affected their optokinetic responses, we also analysed eye movements in free-swimming C. punctatum. We found no eye movements that could compensate for body rotations, suggesting that vision may pass through phases of stabilisation and blur during swimming. As C. punctatum is a sedentary benthic species, gaze stabilisation during swimming may not be essential. Our results suggest that vision in sharks is not ‘poor’ as previously suggested, but optimised for contrast detection rather than spatial resolution.
The authors declare no competing or financial interests.
L.A.R. carried out the experiments, participated in data analysis, conception and design of the study, and drafted the manuscript. N.S.H. and S.P.C. participated in the conception and design of the study, interpretation of the data, and helped draft the manuscript. J.M.H. designed the software used for tracking eye movements, participated in data analysis, conception and design of the study, and interpretation of the data, and helped draft the manuscript.
We would like to acknowledge the financial support of Western Australia's State Government to N.S.H. and S.P.C. Animal acquisition and experimental design was supported by grants from the Sea World Research and Rescue Foundation. J.M.H. was supported by an Australian Research Council Future Fellowship.
- Received September 14, 2015.
- Accepted October 11, 2016.
- © 2016. Published by The Company of Biologists Ltd