Authors: Colin Bruce Jack
A focusing eye acts as a high-performance retroreflector, potentially appearing millions of times brighter when illuminated from a distance than would a matte white surface of the same area. In particular an eye which either has a reflective tapetum near its focal plane, or is operating at f-number less than unity, returns around half of the light entering its pupil in a direction parallel to that in which it arrived.
To detect other animals by eyeshine in this way, however, the detector requires a light source, potentially giving its own position away. The ideal method is therefore to use a form of light which the other animal cannot see, or cannot discriminate from other light. Alternatively, emitting brief flashes of light whose duration is shorter than the time required for the other animal’s pupil to contract, or its eyelid to operate, optimises detection while giving minimal useful information about the detector’s position. Modulation of the source by the detecting animal will cause retroreflected light to vary in sync, potentially allowing the detector to distinguish retroreflected light from background illumination even when the latter is stronger.
In 2009 I pointed out that fluorescent rings around the eye of the triplefin blenny discovered by Michiels et al could allow it to detect other animals, such as potential prey or predators, in this way. The blenny’s rings fluoresce red when illuminated with blue light: only blue light normally penetrates to the depth where it lives. Use of red thus maximizes sensitivity, and red may be invisible to other animals living at depth which have not evolved to see this colour. Michiels et al continue to accumulate evidence supporting the theory. It had previously been suggested that fish living at great depth might emit light for similar reasons. However, little attention appears to have been paid to potential use of the mechanism by animals other than fish. In this paper I introduce circumstantial evidence that many other types of animal likely use it.
Comments: 7 Pages.
[v1] 2014-11-18 02:36:35
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