Retinal topography and spectral sensitivity of the Port Jackson shark (Heterodontus portusjacksoni). Issue 17 (11th April 2020)

• Record Type: Journal Article
• Authors: Peel, Lauren R.; Collin, Shaun P.; Hart, Nathan S.
• Is Part Of: Journal of comparative neurology. Volume 528:Issue 17(2020)
• Page Start: 2831
• Page End: 2847
• Publication Date: 2020-04-11
• Subjects: elasmobranch -- flicker photometry -- photoreceptors -- retina -- RRID:SCR_002526 -- visual acuity -- visual ecology; Comparative neurobiology -- Periodicals; Neurology -- Periodicals; 616
• DOI: 10.1002/cne.24911
• Languages: English
• ISSNs: 0021-9967

Abstract

Abstract: In this study, we investigated the visual system of the Port Jackson shark Heterodontus portusjacksoni, a shallow‐dwelling benthic species and generalist predator endemic to the temperate coastal waters around southern Australia. Measurements of retinal spectral sensitivity in juvenile sharks, made using single flash and heterochromatic flicker photometry under conditions of dark‐ or light‐adaptation, indicated a peak sensitivity at around 500 nm, with no evidence of a spectral shift with increasing levels of light adaptation. Histological sections of the retina revealed a heavily rod dominated retina containing only a few small cell profiles in the photoreceptor layer that might represent a sparse cone population or may be immature rods. Assessment of retinal topography in juvenile sharks indicated the presence of a distinct specialisation for increased visual spatial acuity in the form of a horizontal streak of higher rod photoreceptor (~80, 000 rods mm −2 ) and ganglion cell (~1, 800 cells mm −2 ) densities across the horizontal meridian of the eye. This specialization would be adaptive for panoramic sampling of the part of the visual field corresponding to the substrate‐water interface and remove the need for H. portusjacksoni to move its eyes extensively when resting on the sea floor. The estimated upper limit of spatial resolving power in juvenile H. portusjacksoni was 3.14 cycles deg −1, which is at the lower end of values measured in elasmobranchs. Taken together, these results suggest that the retina of H. portusjacksoni is well adapted for nocturnal vision. Abstract : Histological sections and electroretinography suggest the presence of a heavily rod dominated (and possibly rod‐only) retina in Port Jackson sharks. Mapping of the distribution of retinal neurons reveals a specialisation for higher visual acuity in a narrow horizontal band that corresponds to the part of the visual field covering the water‐substrate interface. Such a specialized retina would be well‐adapted to low‐light (scotopic) vision and a benthic lifestyle, and may assist the shark in finding food and avoiding predators.