Steady-state and dynamic properties of photomechanical light and dark adaptation in the eye of the shrimp Eualus gaimardii (crustacea, natantia)

A reflectometric method was used to study dark and light dark adaptation in shrimps (Eualus) by recording the reflectometric response to alternating periods of light‐off and light‐on that varied in intensity. We found that 1) During light‐off and light‐on periods the response slowly attained an upper and lower asymptotic level, respectively, followed by a steady‐state phase of these levels. The steady‐state level correlated strongly with the position of the distal screening pigment. 2) Steady‐state response plotted against log intensity defined an intensity‐response function representing the dynamic range of the distal pigment and consequently that of the effective aperture. The threshold where the pigment departed from its fully light‐ and dark‐adapted position was 10−5.5 and 10−9.5 W/cm2, respectively. Over this interval the intensity‐response function was linear and the increase in effective aperture equalled about .5 log unit gain in image brightness per log unit change in intensity. 3) The intensity‐response function showed cyclic shifts along the intensity continuum caused by endogeneous factors. 4) The duration of light adaptation was constant while rate of change increased linearly as a function of log intensity. Dark adaptation had similar properties at low intensities, thereafter its rate of change was constant while the duration increased linearly. These findings are consistent with the idea that the distal pigment is controlled by two hormones, the dark‐adapting hormone being subordinated to the light‐adapting one. Both the size of the dynamic intensity range and the temporal properties of light and dark adaptation showed that the photomechanical mechanism is well adapted to cope with twilight viewing conditions.