Meeting Abstract
108.3 Sunday, Jan. 6 Visual acuity in deep-sea fish and mollusks GAGNON, YL*; JOHNSEN, S; Duke University; Duke University 12.yakir@gmail.com
The ocean can be a challenging environment for visually active animals. Downwelling light is absorbed by the water and decreases exponentially with depth. At epipelagic depths (0-200 m), targets reflect ambient light and create extended scenes. The most suitable lenses at these depths maintain high image contrast at the viewer’s cutoff resolution (the highest spatial frequency that can still be registered by the viewer’s retina). At mesopelagic (200-1000 m) and bathypelagic depths (>1000 m), bioluminescence is more common and the ambient light is many orders of magnitude dimmer than at shallower depths. The visual scene becomes dominated by point source targets requiring a different type of lens. Scenes become more binary (with less gray levels) and low contrast at the cutoff resolution does not necessarily affect the image quality. We looked at the optical characteristics of the lenses of 24 different species of deep-sea fish and pelagic molluscs. The lenses’ radii, focal lengths, and focal capabilities were measured. Collimated light (550 nm) was focused on a camera CCD by adjusting the paraxial distance of the lens (suspended in buffer). We imaged the lens’ point spread function (PSF) (quantifying the amount of blur introduced by the lens). The PSF was used to calculate image contrast of targets with varying spatial frequencies. These results were compared to known cutoff frequencies of the investigated species, their depth, and biology. The heteropod, Pterotrachea coronata, had distinct elongated and narrow PSFs matching the linear array of photoreceptors in its retina. The Hatchetfishes, Argyropelecus aculeatus and Sternoptyx diaphana, had the highest angular resolution and smallest full PSF width at half maximum (FWHM) of all the examined species. This matches well the predictions that these fish need high resolution for viewing silhouettes against the downwelling light at mesopelagic depths.
