Abstract

Ocular structures are adapted to light conditions, and in fossils they may be useful in reconstructing water depth. Maximum water depth (X) at which benthic podocopid ostracodes could have useful vision in downwelling sunlight is limited by the ability to distinguish light from dark, which is ultimately limited by statistical fluctuations ("noise") in the numbers of photons captured. Downwelling light is attenuated exponentially; therefore it can be shown that

where I0 is the intensity of sunlight just below the ocean's surface, Ar is the area of a rhabdom, t is the sampling time of the eye, is the minimum number of photons per sampling time for discrimination between light and dark, k is the vertical attenuation coefficient, v is the frequency of light, and h is Planck's constant. All quantities are known except t and ; t is about 100 milliseconds (ms) for similar organisms, and Gaussian statistics indicate that is about 4 ms. Regardless, t and are arguments of a logarithm, and the calculation is not very sensitive to their values.

Vision with sunlight in podocopid ostracodes should be, according to the above, limited to the upper 280m in clear ocean water and to about 85m in coastal water. The eye structures already are among the most efficient known, and modification could produce little improvement for vision in dimmer light. The method has application to fossil ostracodes and other groups with eye structures that permit the measurement of the aperture, focal length, and estimates of the rhabdom size.

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